Ärid-zone hydrology investigations with sotope techniques
TRANSCRIPT
■.
ÄRID-ZONE HYDROLOGY INVESTIGATIONS WITH SOTOPE TECHNIQUES
PROCEEDINGS OF AN AD VISO R Y GROUP M EETING, V IE N N A , 6 - 9 NOVEMBER 1978
Ш
W ) I N T E R N A T I O N A L A T O M I C E N E R G Y A G E N C Y , V I E N N A , 1 9 8 0
ARID-ZONE HYDROLOGY: INVESTIGATIONS WITH ISOTOPE TECHNIQUES
STI/PUB/547
C O R R IG EN D U M
Paper IAEA-AG-158/17, by J.Ch. Fontes et al.
Page 237, authors’ namesF o r P. POUCHON rea d P. POUCHAN(The Contents List should be corrected accordingly)
Page 259, equation at top of page F o r 0.3 rea d 0.13
List of Participants
Page 265, under ‘International Atomic Energy Agency (IAEA)’A d d the fo llo w in g n a m e:
Gonfiantini, R. Section of Isotope Hydrology,(Scientific Secretary) Division of Research and Laboratories
A R ID -ZO N E H YD RO LO G Y: IN V EST IG A T IO N S W ITH ISOTOPE TECH N IQ U ES
The following States are Members of the International Atomic Energy Agency:
AFGHANISTANALBANIAALGERIAARGENTINAAUSTRALIAAUSTRIABANGLADESHBELGIUMBOLIVIABRAZILBULGARIABURMABYELORUSSIAN SOVIET
SOCIALIST REPUBLIC CANADA CHILE COLOMBIA COSTA RICA CUBA CYPRUSCZECHOSLOVAKIA DEMOCRATIC KAMPUCHEA DEMOCRATIC PEOPLE’S
REPUBLIC OF KOREA DENMARKDOMINICAN REPUBLICECUADOREGYPTEL SALVADORETHIOPIAFINLANDFRANCEGABONGERMAN DEMOCRATIC REPUBLICGERMANY, FEDERAL REPUBLIC OFGHANAGREECEGUATEMALAHAITI
HOLY SEEHUNGARYICELANDINDIAINDONESIAIRANIRAQIRELANDISRAELITALYIVORY COASTJAMAICAJAPANJORDANKENYAKOREA, REPUBLIC OF KUWAIT LEBANON LIBERIALIBYAN ARAB JAMAHIRIYALIECHTENSTEINLUXEMBOURGMADAGASCARMALAYSIAMALIMAURITIUS MEXICO MONACO MONGOLIA MOROCCO NETHERLANDS NEW ZEALAND NICARAGUA .NIGERNIGERIANORWAYPAKISTANPANAMAPARAGUAYPERU
PHILIPPINESPOLANDPORTUGALQATARROMANIASAUDI ARABIASENEGALSIERRA LEONESINGAPORESOUTH AFRICASPAINSRI LANKASUDANSWEDENSWITZERLANDSYRIAN ARAB REPUBLICTHAILANDTUNISIATURKEYUGANDAUKRAINIAN SOVIET SOCIALIST
REPUBLIC UNION OF SOVIET SOCIALIST
REPUBLICS UNITED ARAB EMIRATES UNITED KINGDOM OF GREAT
BRITAIN AND NORTHERN IRELAND
UNITED REPUBLIC OF CAMEROON
UNITED REPUBLIC OF TANZANIA
UNITED STATES OF AMERICA URUGUAY VENEZUELA VIET NAM YUGOSLAVIA ZAIRE ZAMBIA
The Agency’s Statute was approved on 23 October 1956 by the Conference on the Statute of the IAEA held at United Nations Headquarters, New York; it entered into force on 29 July 1957. The Headquarters of the Agency are situated in Vienna. Its principal objective is “to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world”.
© IAEA, 1980
Permission to reproduce or translate the information contained in this publication may be obtained by writing to the International Atomic Energy Agency, Wagramerstrasse 5, P.O. Box 100, A-1400 Vienna, Austria.
Printed by the IAEA in Austria November 1980
PA N EL PRO CEED IN G S SER IES
A R I D - Z O N E H Y D R O L O G Y :
I N V E S T I G A T I O N S
W I T H I S O T O P E T E C H N I Q U E S
PROCEEDINGS OF AN ADVISORY GROUP MEETING ON APPLICATION OF ISOTOPE TECHIQUES
IN ARID ZONES HYDROLOGY ORGANIZED BY THE
INTERNATIONAL ATOMIC ENERGY AGENCY AND HELD IN VIENNA
FROM 6 TO 9 NOVEMBER 1978
INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1980
ARID-ZONE HYDROLOGY: INVESTIGATIONS WITH ISOTOPE TECHNIQUES
IAEA, VIENNA, 1980 STI/PUB/547
ISBN 92-0-141180-4
FOREW ORD
The present publication includes the papers presented at the Advisory Group Meeting on the Application of Isotope Techniques to Arid-Zone Hydrology, which was held in Vienna from 6 to 9 November 1978. Twenty-eight scientists - twenty-two invited participants and six observers — representing fourteen countries and one international organization, attended the meeting which was chaired by Professor J.Ch. Fontes of the University of Paris-Sud.
It is frequently admitted that isotope hydrology has achieved the most significant results in arid zones. The most frequent applications of environmental isotope techniques are, on the basis of the papers presented at the meeting:
( 1 ) To investigate the occurrence and the mechanisms of modern recharge which, in extremely arid zones, seems to take place mainly through wadis rather than through direct infiltration of the rainfall; this latter mechanism seems to be significant only in less arid and in semi-arid areas.
(2) To assess the occurrence and the characteristics of past recharge and of palaeogroundwaters, which are anon-renewable resource.
(3) To provide evidence of interconnections between aquifers.
All these problems are quite complex and therefore the isotopic data should be interpreted together with all available hydrological, hydrogeological and hydrochemical data. It should be emphasized, however, that isotopes often provide a type of information which is not possible to obtain with other techniques. For instance, tritium occurrence is a definite proof of the presence of modern water in an aquifer, and stable isotopes can be used as conservative tracers to prove or disprove interconnections between aquifers and to follow groundwater flow patterns.
In conclusion, we believe that this book gives a sufficiently complete and up-to-date account of the progress made by environmental isotope techniques in arid-zone hydrology, as well as of the problems that still remain open and demand further thought and data. It is hoped therefore that hydrogeologists working in arid zones who are not familiar with the use of isotope techniques will find in this book ideas and suggestions on how to tackle some of the problems that they are facing.
CONTENTS
Precipitation, flood- and groundwaters of the Negev highlands: Anisotopic study of desert hydrology (IAEA-AG-158/1 ) ...................... 3M. L evin, J .R . Gat, A . Issar
Use of environmental isotopes in arid-zone hydrology (IAEA-AG-158/2)... 23T. D in g er
Interpretation of environmental isotopic groundwater data: Arid andsemi-arid zones (IAEA-AG-158/3) ............................................. 31M .A . G eyh
A geochemical and isotopic approach to recharge evaluation in semi-aridzones: Past and present (IAEA-AG-158/4)................................... 47W.M. E d m u n d s , N .R .G . Walton
An examination of recharge mound decay and fossil gradients in aridregional sedimentary basins (IAEA-AG-158/5)....... ....................... 69J.W . L lo y d
Environmental isotopes in North African groundwaters; and the Dahnasand-dune study, Saudi Arabia (IAEA-AG-158/6).............. ............. 77C. Sonntag, G. T hom a, K .O . M ü n n ich , T. D in çer, E . K litzsch
An injected gamma-tracer method for soil-moisture movementinvestigations in arid zones (IAEA-AG-158/7)............................... 85
■ A . R . Nair, S. V. Navada, S.M . R ao Aspects of the isotope hydrology of two sandstone aquifers in arid
Australia (IAEA-AG-158/8) ..................................................... 93P .L. A irey , G .E. Calf, R E . H artley , D. R o m an
Study of the leakage between two aquifers in Hermosillo, Mexico, usingenvironmental isotopes (IAEA-AG-158/9) ................................... 113B .R . P ayne, L. Q uijano, L. L a to rre D.
FIELD INVESTIGATIONS ON GROUNDWATER ORIGIN AND FLOW PATTERNS
Utilization of natural isotopes in the study of salination of the waters inthe Pajeú River Valley, northeast Brazil (IAEA-AG-158/10).............. 133E. Salati, E . M atsui, J .M . Leal, P. F ritz
G RO U N D W A TER R E C H A R G E ST U D IE S
Isotope investigations as a tool for regional hydrogeological studies inthe Libyan Arab Jamahiriya (IAEA-AG-158/11 ) ............................ 153D. S rd o c, A dela Sliepcevic, B. O belic, Nada H orvatincic, H. M oser,
W. S tich lerGroundwater flow patterns in the western Libyan Arab Jamahiriya
evaluated from isotopic data (IAEA-AG-158/12 )........................... 165 ‘0 . Salem , J .H . Visser, M. D ray, R. G onfiantini
Recharge of groundwaters in arid areas: Case of the Djeffara Plain inTripolitania, Libyan Arab Jamahiriya (IAEA-AG-158/13) ................ 181M. A llem m o z, Ph. Olive
Aspects of environmental isotope chemistry in groundwaters in EasternJordan (IAEA-AG-158/14) ...................................................... 193J. W. L lo y d
A conceptual hydrochemical model for alluvial aquifers on the SaudiArabian basement shield (IAEA-AG-158/15) ................................ 205/. W. L loyd , P. Fritz , D. Charlesw orth
Isotope methods as a tool for Quaternary studies in Saudi Arabia(IAEA-AG-158/16) ................................... ............................ 215H : H ötzl, C. Jo b , H. M oser, W. R auert, W. Stich ler, J .G . Z ötl
Environmental isotope study of groundwater systems in the Republic ofDjibouti (IAEA-AG-158/17) .................................................... 237J .C h . F o n tes , P. P o u ch o n , J .F . Saliege, G.M. Z up pi
List of Participants................................................................... 263
G RO U N D W ATER R E C H A R G E STU D IES
IAEA-AG-158/1
PREC IP ITAT IO N, FLOOD- A N D G RO U N D W ATERS OF THE N EG E V H IG H LANDS: A N ISOTO PIC STU D Y OF D ESER T H Y D R O LO G Y
M. LEVIN*, J.R. GAT**, A. ISSAR*♦Institute for Desert Research,Ben-Gurion University of the Negev,Sdeh-Boqer
**Isotope Research Department,Weizmann Institute of Science, Rehovot,Israel
Abstract
PRECIPITATION, FLOOD- AND GROUNDWATERS OF THE NEGEV HIGHLANDS:AN ISOTOPIC STUDY OF DESERT HYDROLOGY.
Precipitation in the Negev highlands was found to be surprisingly depleted of 180 and deuterium and generally characterized by “deuterium excess” values of d > 15%o. Isotopic compositions are relatively uniform over a wide area on any particular day, but differ appreciably from storm to storm. Thus, they are valuable tools for hydrographic analysis of flood-flows. Flood-flow samples, collected in Nahal-Zin and Nahal-Besor, were often even more depleted in heavy isotopes than the total rainstorm, indicating that run-off is generated selectively by high-intensity rains. The initial rush of the flood flushed away the surface salinity and saline accumulations in surface pools, but apparently does not involve subsurface salinity to any great extent. Recharge to groundwater appears to be accompanied by a slight evaporative enrichment of the isotopes, more so in the case of waters recharged from flood-flows. Environmental tritium can be used as an indicator of direct flood-water contributions to the aquifers.
INTRODUCTION
The Negev Desert covers about a million hectares, and forms part of the desert belt extending from the Sahara through the Sinai peninsula to the deserts of Trans-Jordan and Arabia. Northwards there is a rather rapid transition into the semi-arid areas of Israel with their more humid Mediterranean-type climate. The Negev highlands consist of a series of parallel anticlines with elevations ranging from 300 to 1000 m above sea-level. These mountains comprise mainly Senonian and Eocene chalks and Cenomanian-Turonian limestones. Older formations, such as the prominent Makhtesh Ramon, are exposed by erosion.
3
4 LEVIN et al.
T he m ountains are dissected b y wadis, w here rare floods discharge in to the
M editerranean Sea or in to the A rava V alley.In the highlands the rainfall varies from 5 0 to 1 5 0 m m annually; how ever,
th e average rainfall has little m eaning because o f its erratic nature and wide
variations in the rain ’s in tensity . O cto b er to M arch can be considered the rainy
p eriod , w hich is m ore o r less in phase w ith th e M editerranean clim ate. H ow ever, precipitation in the Negev is characterized by extrem e annual variations and the
influence o f m onsoon circu lation m ay be a com plicating feature.T h e M editerranean-type regional precipitation is characterized by the
“ L evan tine M eteoric W ater” relationship o f d = — 8 5 180 > 15%o [ 1 ], w hichhas been related to the air-sea exchange conditions associated w ith the w inter
cyclogenesis [2 ] w hich in turn gives rise to th e w in ter p recipitation . O n the
Israeli co ast the m ean w eighted iso top ic om position ( 1 9 5 9 —1 9 7 0 ) was about 5 180 = — 4.75% o, 5 d = - 2 2 .8 % c and the altitude effect has a m ean value o f ab ou t 1.1 X 1 0 ~3%o m -1 [3 ] . T he increasing aridity in the rain-shadow valleys to the east o f the m ountains expresses itself in an en richm ent o f heavy isotopes accom panied by a decrease in the “ d ” p aram eter. This trend does n o t, how ever, con tin u e in to the desert region proper. P recip itation at B eer Sheva, situated on
the b ord er o f the arid zone on the 2 0 0 -m m isoh yet, and also in the Sinai highlands, show s ap p roxim ately the norm al com p osition o f the coastal rains, w ith o u t an
appreciable evaporation e ffe ct, b u t also w ith n o indication o f an inland o r an
altitude e ffect. This has been n oted by G at and Issar [4 ] and explained in general
term s by the sporadic (n o t con tinu ou s) ap pearances o f desert show ers.
T h e few d ata published on p recipitation from the Negev in dicate an overw helm ing predom inance o f heavy isotop ic species [3 ] ; in the light o f ou r
exp erien ce w ith rain co llecto rs in this arid area w e suspect th at evap oration in
th e co lle cto r m ay have d istorted these data. S om e surprisingly depleted values from groundw aters in this area w ere tentatively related to recharge under m ore hum id con ditions, since such low 5 values in an arid zone w ere n o t then exp ected .
T h e p ro ject under discussion aim ed at characterizing the Negev p recipitation
and resu ltan t flood-flow s isotop ically , and in turn relating these to the located grou nd w ater sou rces, b oth shallow ones found in the alluvial wadi fill and larger
aquifers in th e chalk and lim estone units. A b e tte r understanding o f the
hyd rological regim e and recharge m echanism is being sought. C ollectors fo r rain
and flood-w aters w hich m inim ize the effe ct o f evaporation w ere developed fo r
this p roject. T h e study area is show n in F ig . 1.
1. IN ST R U M EN T A T IO N
Standard rain recorders do n o t p erform satisfactorily as rain collectors u nder arid conditions ow ing to losses o f sam ple parts by evaporation . M oreover,
IAEA-AG-158/1 5
FIG .l. Location map o f study area, indicating precipitation-sampling sites.
to ob tain a large enough sam ple fo r environm ental isotop e analysis from rain
am ounts o f on ly a few m illim etres larger-than-standard rain co llecto rs , in w hich
the evaporation effects are even m ore severe, m ust be used. F o r this reason a
special “ h e rm e tic” rain co lle cto r w as developed (F ig . 2 ) [5 ] , w ith a special
provision (th rou gh a m agnetic ca tch ) for h erm etic sealing o f the co llectio n flask
on ce it has been filled b y a m inim um am o u n t o f 1 m m rainfall. A n overflow
m echanism then enables successive rain am ounts to be sam pled, w ith each
additional sam pler acco m m o d atin g roughly 1 m m rainfall.
6 LEVIN et al.
8 0 0 rnm
FIG.2. Hermetic rain collector.
A com p arison o f the d ata fo r one sto rm , w hich yielded 4 .3 m m rain collected
by the h erm etic sam pler, w ith the con cu rren t collection o f to ta l rainfall by a
standard rain gauge, w hich was em ptied soon a fte r the passage o f the storm , gave
the follow ing results:
1st m m : 2n d m m : 3rd m m : 4 th m m : A verage:
5 180 = 5 180 = 5 180 =s 18o =5 180 =
- Ъ.21%о - 4П9%о - 5 . 5 6 %o— 1.27%o - 5 . 2 2 %o
T h e first three fractions w ere filled during a period o f 2 0 m inutes each . T w o
hours la te r the last sample was discharged in one m inute. T he to ta l daily rain
sam ple, co llected in a standard rain gauge, gave the value o f 5 180 = - 5 .45% o.
T h e h erm etic-typ e co lle cto r was also the basis fo r a flash-flood sam pler [6 ] , w hich has a 3-litre con tain er w hich is sealed h erm etically by a float as soon as it
fills (F ig . 3 ) . E a ch in stru m en t is buried in the w adi bed in a sim ply con stru cted
ch am b er and covered by a succession o f various sizes o f clean gravel to filter the
in-flow w aters. T he to p does n o t p rotru d e at all from the surrounding ground so as to escape d etection and to prevent the sam pler from being sw ept aw ay by the flood . D ifferent stages o f the flood , e x ce p t fo r the very sta rt o f flow , can
be selected by placing the con tainers a t d ifferent elevations above the flood plain.
IAEA-AG-158/1 7
METALLIC FILTER
SAMPLE INLET-... .o r-Zrr. - : О o'. ••1 • ■■CÜi ¿? ’¿>rf?¿r>C '
FLOAT
COLLECTING FL ASK(3 L ite r)
INSTRUMENT SUPPORT
FIG.3. Collector fo r flood-waters.
2 . P R E C IP IT A T IO N
Daily rain samples w ere co llected from the regular n etw ork stations o f the Israel M eteorological Service, utilizing rain gauges w hich are em ptied daily, thus
minimizing evaporative w ater losses. The stable isotop e d a ta 1 (T ab le I) represent
the 1 9 7 7 —7 8 rainy season samples from Revivim , Sdeh-B oqer, A vd at and M izpeh-
R am on , all situated in the Negev highlands, as w e ll as from the Sinai sea-shore
station at S adot and also from B eer Sheva in the n orth ern Negev (w here rainfall is som ew hat m ore abundant, the annual m ean being 2 0 0 m m ). T he d ata are
p lotted on a conventional ó о vs 5 180 diagram (F ig . 4 ) w ith tie lines indicating the syn ch ronous sam ples. On this diagram the d = 10%o M eteoric W ater Line and the M editerranean P recip itation Line are given as referen ce ; the isotop e data from the IA EA-W M O netw ork station o f Bet-D agan, w hich is situated in the semi-
arid coastal plain east o f T el Aviv w here the m ean annual precipitation is
5 3 5 m m , are given fo r com parison .The im m ense spread o f these individual values, from + 3% o to — 9 % o in 5 180 ,
is n o te d ; how ever, such a spread is n o t outstanding fo r individual rain events, and
alm ost as wide a sca tte r was rep orted by G at and D ansgaard fo r show ers at
A shdod [3 ] . T he data ob ey a m eteo ric w ater relationship rath er well and, w ith
1 Isotope analyses were performed mass-spectrometrically at the Isotope Laboratories, W IS, Rehovot (I. Mauravinsky, R. Silinikov and M. Feld). Data are given in 5 % o units relative to SMOW. Average reproducibility is ± 0 . 12%o for 180 and ± 1.0% o for deuterium.
T A B L E I. P R E C IP IT A T IO N IN T H E N E G E V (M A JO R R A IN D A Y S)
8 LEVIN et al.
Date(day-month-year)
Rain amount (mm)
6 180(%.) %ÍD
T.U.
Beer Sheva(300 m a l t . ) 1 4 - 1 2 - 7 7 1 3 . 9 - 6 . 7 2 - 3 0 . 5 21
2 1 - 1 2 - 7 7 2 . 4 - 5 . 7 6 - 3 0 . 6 272 2 - 1 2 - 7 7 1 1 . 2 - 6 . 7 7 - 2 9 . 3 372 3 - 1 2 - 7 7 4 . 4 - 7 . 2 2 - 2 5 . 6 39
2 - 0 1 - 7 8 1 . 2 - 5 . 5 1 - 1 3 . 2 -
2 3 - 0 2 - 7 8 1 9 . 2 - 3 . 6 8 - 1 8 . 1 _
1 3 - 0 3 - 7 8 1 0 . 0 - 5 . 1 7 - -
2 3 - 0 3 - 7 8 1 . 7 - 1 . 8 3 - .
weighted mean - 5 . 4 2 - -
Sadoth 2 9 - 1 0 - 7 7 3 . 2 - 0 . 7 6 + 9 . 3 32( s e a s h o r e) 2 - 1 1 - 7 7 3 . 9 - 1 . 6 1 + 11 . 9 18
1 1 - 1 1 - 7 7 3 . 8 + 0 . 42 - 1 8 . 0 .
6 / 8- 1 2 - 7 7 7 . 1 - 2 . 4 3 - 6 . 8 81 2 - 1 2 - 7 7 5 . 1 - 5 . 2 2 - 3 1 . 4 _
1 4 - 1 2 - 7 7 3 9 . 6 - 6 . 9 4 - 3 8 . 0 _
2 1 - 1 2 - 7 7 0 . 6 - 2 . 9 6 - 1 7 . 8 -
2 2 - 1 2 - 7 7 6 . 1 - 5 . 1 2 - 2 3 . 1 272 3 - 1 2 - 7 7 9 . 6 - 5 . 2 2 - 1 7 . 3 24
2 - 0 1 - 7 8 4 . 6 - 3 . 9 9 - 1 6 . 3 153 - 0 1 - 7 8 2 . 8 - 9 . 4 2 - 5 8 . 9 _
2 3 - 0 2 - 7 8 6 . 6 - 3 . 6 8 - 2 0 . 7 _
12/13- 02 - 78 U . 2 - 4 . 8 4 - -
2 9 - 0 3 - 7 8 4 . 1 - 5 . 3 7 -
weighted mean (up t o 2 3 . 0 2 ) - 5 . 1 1 - 2 . 4 3
Ki r yat Sde-Boqer 1 7 - 1 0 - 7 7 0 . 6 - 2 . 2 6 - 1 . 5 .
(S00 m a l t . ) 1 2 - 1 1 - 7 7 3 . 4 - 1 . 7 8 + 5 . 6 286 - 1 2 - 7 7 0 . 6 - 1 . 2 2 + 17 . 4 -
9 - 1 2 - 7 7 1 . 6 - 1 . 0 3 + 8 . 3 181 4 - 1 2 - 7 7 4 . 9 - 7 . 4 5 - 3 7 . 9 222 2 - 1 2 - 7 7 1 6 . 8 - 6 . 8 6 - 3 3 . 1 302 3 - 1 2 - 7 7 6 . 6 - 5 . 2 2 - 1 4 . 5 30
2 - 0 1 - 7 8 1 . 4 - 5 . 2 6 - 2 2 . 1 _
3 - 0 1 - 7 8 0 . 8 - 6 . 1 6 - 2 9 . 0 _
1 7 - 0 1 - 7 8 0 . 2 +3 . 43 + 9 . 8 _
1 9 - 0 2 - 7 8 1 . 0 - 2 . 9 2 - 3 6 . 9 _
2 3 - 0 2 - 7 8 2 . 0 - 3 . 9 0 - 1 8 . 0 _
1 2 - 0 3 - 7 8 1 . 1 - 3 . 9 0 _ _
1 3 - 0 3 - 7 8 2 . 4 - 3 . 1 3 . _
3 0 - 0 3 - 7 8 4 . 3 - 5 . 4 5 - 3 8 . 7 _
2 4 - 0 4 - 7 8 1 . 2 - 2 . 3 4 _ _
weighted mean - 6 . 7 8
Revivim 1 1 - 1 1 - 7 7 1 0 . 0 - 1 . 8 8 - 3 . 8 30( 300 m a l t . ) 9 - 1 2 - 7 7 1 . 2 - 0 . 5 5 + 8 . 5 -
1 2 - 1 2 - 7 7 0 . 8 - 0 . 4 6 + 1 . 0 -
1 4 - 1 2 - 7 7 2 3 . 0 - 8 . 5 1 - 4 6 . 8 1415 - 1 2 - 7 7 0 . 5 - 0 . 4 6 + 18 . 3 -
2 1 - 1 2 - 7 7 4 . 1 - 5 . 6 7 - 2 8 . 6 192 2 - 1 2 - 7 7 15. 1 - 6 . 7 7 - 2 5 . 8 312 3 - 1 2 - 7 7 9 . 4 - 6 . 5 b - 2 4 . 5 21
2 - 0 1 - 7 8 1 . 6 - 1 . 7 7 . _
1 9 - 0 2 - 7 8 1 . 8 - 3 . 7 7 . -
2 3 - 0 2 - 7 8 1 . 4 - 3 . 1 2 - -
1 2 - 0 3 - 7 8 0 . 5 - 4 . 5 4 - -
1 3 - 0 3 - 7 8 3 . 0 - 4 . 5 4 _ _
3 0 - 0 3 - 7 8 0 . 9 - 5 . 1 3 - -
w e i g h t e d mean
IAEA-AG-158/1 9
T A B L E I . ( c o n t .)
Date(day-month-year)
Rain amount (mm)
6 %(%„)
T.U.
Avdat 1 2 - 1 1 - 7 7 7 . 0 - 2 . 3 6 + 2 . 4 20( 650 m a l t . ) 3 - 1 2 - 7 7 0 . 5 5 - 7 . 9 8 - 6 9 . 0
9 - 1 2 - 7 7 0 . 9 5 - 1 . 3 6 + 4 . 41 4 - 1 2 - 7 7 3 . 0 - 6 . 9 7 - 3 7 . 4 361 5 - 1 2 - 7 7 1 . 0 - 3 . 4 0 + 3 . 6 _
2 2 - 1 2 - 7 7 1 5 . 4 - 6 . 2 1 - 3 1 . 1 4 72 3 - 1 2 - 7 7 8 . 0 - 4 . 2 9 - 2 8 . 0 221 9 - 0 2 - 7 8 1 . 1 - 2 . 2 9 - 4 0 . 0 _2 3 - 0 2 - 7 8 1 0 . 0 - 5 . 3 4 - 2 5 . 8 233 0 - 0 3 - 7 8 0 . 8 - 2 . 8 2 .
2 4 - 0 4 - 7 8 3 . 0 + 2 . 4 6 _ _
weighted mean - 4 . 5 0
Mizpeh-Ramon 2 1 - 1 0 - 7 7 1 . 1 5 - 3 . 9 9 - 1 6 . 3(900 ra a l t . ) 1 1 - 1 1 - 7 7 2 . 6 - 2 . 6 2 + 1 . 4 20
1 4 - 1 2 - 7 7 2 . 8 - 9 . 4 2 - 5 7 . 8 321 5 - 1 2 - 7 7 0 . 1 - 3 . 0 5 - 5 . 72 2 - 1 2 - 7 7 7 . 2 - 7 . 0 1 - 3 1 . 0 372 3 - 1 2 - 7 7 8 . 0 - 6 . 8 1 - 2 2 . 5 34
3 - 0 1 - 7 8 0 . 3 _
2 0 - 0 2 - 7 8 1 . 7 _ _
2 3 - 0 2 - 7 8 8 . 6 - 5 . 9 1 - 2 5 . 1 292 4 - 0 4 - 7 8 7 . 4 - 4 . 7 8 _
weighted mean - 6 . 0 6
few excep tio n s, all d ata fall w ithin the band o f m eteo ric w ater lines (lines o f
slope o f 8 in 5 180 , Sp space) o f 15%ó < d < 2 5 %o. One n otes the close correlation b etw een syn ch ronous sam ples, w ith the position o f d ata on the diagram apparently d ictated first b y th e p articu lar s torm and less by the location o f the station .Indeed th ere is n o con sisten t ord er betw een the highland station s regarding either
the daily data o r even the m on th ly and annual seasonal averages; a on e-year period is evidently insufficient fo r establishing a statistically significant m ean.The d ata as a w hole average around 5 180 = — 6 .5 %o, w hich is an u n exp ected ly low
value.In Fig . 5 the d ata are p lotted as a fu n ction o f the daily rain am oun t. A rath er
w eak “ am ou n t e ffe c t” can be seen — there are only isolated cases w ith
5 180 < - 4%o in the group o f sam ples w hich correspond to daily rain am ounts
o f less than 2 m m , w hereas, by co n tra st, the iso top ic values o f the rain events over4 m m , fall in betw een - 4 .5% c > 6 180 > - 7.0% o (w ith few e x cep tio n s, n otab ly
the rain event o f 1 4 D ecem b er). A slight inland effe ct is recognized in th at the
d ata from the coastal stations (S a d o t and A shdod, the la tte r given fo r com parison
purposes) are usually slightly enriched in 180 fo r any given rain in tensity . The “ am ou n t e ffe c t” can be m ore clearly recognized on the öjj vs 8 180 p lot (F ig . 4 ) .
10 LEVIN et al.
+ 2 0
0
-° - 2 0 oQ60
- 4 0
- 6 0
FIG.4. Stable, isotope data o f daily precipitation samples from the Negev during the 1977178 season. Samples collected simultaneously at different stations are identified by tie-lines. Mean values o f a ten-day rain collection at Bet Dagan in the Israel coastal area are shown fo r comparison purposes.
D ata from rain-deficient days (am ou n ts < 2 m m ) can be seen to occu p y pred om in an tly the area o f m ore enriched isotop e data and also the area below the m eteo ric w ater line o f d < 10%o. T he “ am ou n t e ffe c t” is m ainly the expression
o f the con tinu ou s depletion o f an air mass b y the preferential rain ou t o f the heavy
iso top ic species. The larger the rain the greater the depletion - how ever, the
p rocess is one w hich obeys th e m eteo ric w ater relationship o f A S p /A S 18 = 8.T he effe ct is fu rth er enhanced by the en richm ent o f the heavy isotop ic species
as a result o f evaporative w ater loss from droplets falling beneath the cloud base -
an effe ct w hich is n oted m ainly in the case o f slight rains w hich do n o t saturate
the n ear-surface air layers. This process has the additional effe ct o f moving the
iso top ic value aw ay from the m eteo ric w ater line in the d irection o f sm aller “ d ”
values. T he evaporation effect is usually the dom in an t p art o f the am oun t
effect in the low lands o f the sem i-arid zone. In the present set o f d ata this
effe ct seem s u nim portan t (m o st enriched values lie w ithin the band o f m eteo ric
8 l 8 0 ( % o )
0 - 8 - 6 - 4 - 2 0 + 2
' x d = l07oo METEORIC WATER LINEJ____ ° I_____I I I_____I_____I J_____I I 1
IAEA-AG-158/1 11
+4
+ 2
- 2
P "4 60OO
- 1 0
% »0- ^ X• XX
л :Л » . О
«• о x Iл Д .*
...•
/ Dec. 14
X _L
T Г
L e g e n d:
• Negev Mountain Stations oS a d o t лВ еег-Sheva
x Ashdod (Jan. 1968)
_L10 15 2 0 25
PRECIPITATION ( m m / d a y )
3 0 35
FIG.5. The “amount e f fe c t” in precipitation samples o f Negev stations, 1977/78 season. The abnormally depleted values o f 14 Dec. are marked by a tie-line.
w ater lines); possibly this is related to the low tem p eratu re on rain days at these
high elevations, w hich w ould minim ize any evaporative w ater loss. W hatever
the reason , we n ote a distinction betw een p recip itation in the desert highlands, com p ared w ith the low land areas, w ith isotop e com p osition o f the la tte r d om inated by the evap oration process, w hereas in the highlands the isotope d ata are characterized by a high “ d ” value.
T he tritium co n te n t o f the rain ranged from ab ou t 1 0 —5 0 T U .
3 . R U N -O F F FR O M IN D IV ID U A L STO RM E V E N T S
A num ber o f the flash-flood sam plers described above w ere distributed along
three m ajor w atersheds o f the central N egev. O ur aim w as to ch aracterize the
storm ru n -off in the wadis from the aspects o f salinity and isotop es and to identify
the sources o f w ater and salinity.F o r this purpose w e chose the Zin, H a ro ’a and Revivim w atersheds (F ig . 6 ).
T h e Zin drains in to the A rava V alley, w hereas the la tte r tw o are tributaries o f N ahal-B esor, w hich finally drains in to the M editerranean.
120
080-----
12 LEVIN et al.
180
FIG. 6. Detailed map o f the Zn and Besor catchment area indicating the location o f the sampling stations.
T hree o f the instrum ents described above w ere distributed in each o f these
w atersheds, and tw o m ore w ere located fu rth er dow nstream in N ahal-B esor. These sam plers responded to the initial stages o f any flood even t; w hen the occu rren ce o f a flood in the area was rep o rted , the site was visited by one o f us
(M .L .) and hand-sam pling o f the late stages o f the flood was carried o u t when
possible. I t m ust be appreciated th at som e areas, such as the p ool areas o f W adi Zin, are p ractically inaccesible during a flood.
IAEA-AG-158/1 13
The specific con d u ctiv ity was m easured soon after sampling a t the Sdeh-B oqer- In stitu te . Stable isotop e and chloride analysis w ere done on the sam ples later at
R eh ovot.
3 .1 . T he W adi Zin drainage system
A sch em atic outline o f the W adi Zin system is show n in Fig. 6 . T he headw aters
o f the drainage area originate from the A vdat to M izpeh-Ram on area, a t an elevation o f 6 0 0 to 9 0 0 m . O th er tributaries draw th eir w aters from low er elevations
to the n orth east. The m ain wadi enters the gorge o f E in A vd at, plunging in a
series o f spectacu lar w aterfalls in to p ools, w hich are up to 7 m deep. These pools
are perennial, being fed th rou gh ou t the year by a num ber o f ra th e r saline springs and seepages. F u rth e r dow nstream the main w adi is join ed by Wadi H avarim and o th er local tributaries and finally discharges in to N ahal A rava and the Dead Sea.
O ne flood sam pler was located in U pper W adi-Zin (u p stream o f the gorge
and pools o f E in A vd at), an oth er below the ju n ctio n o f W adi-Zin and Nahal H avarim , and a third close to the discharge area near Mashash. N ear the first
and last sam pler flood-level m onitors o f the H ydrological Service w ere also located .T he perennial seepages o f E in A vdat w ere sam pled (S ectio n 4 ) : their isotope
co n ten t varies considerably from place to place b u t is rath er steady in tim e, a
m ean value being 5 18 = - 5 .9 % ;; = - 2 8 %o \ salinities range from 1 2 0 0 to1 8 0 0 mg C l/ltr. The p ool w ater and the cascading w ater in the falls b ecom e enriched in the isotop ic species and salinity, owing to the evaporative w ater loss, m ainly during sum m er; typ ical values m easured w ere 5 180 = - 4% o; Sq = - 17%o.
3.1.1. The flood o f22/23 December 1977 (Fig. 7)
W idespread rain fell on 2 2 D ecem ber 1 9 7 7 and again on the 2 3 rd . Based
on daily rain sampling the isotop ic com positions ranged from 6 180 = — l % o to - 5% o , the early rain (m o re intense ? ) being the m ore depleted in heavy isotopes. I t is n otew orth y th at the rainfall on the second day was ch aracterized by a m arked
deuterium excess, d = + 29%o.Flo w started at the U pper Zin station late on 2 2 D ecem b er, increasing
slightly w ith the additional rain o f the 2 3 rd . F lo w in the w adi con tinu ed well in to 2 5 D ecem ber. D ow nstream the flood was m uch larger, b u t began only at
midnight o f the 2 2 n d , and in the h ydrograph there was evidence o f the arrival o f
a num ber o f flash-flood fronts during the 2 3 rd . F lo w ceased rath er suddenly.The first rush o f w aters as sampled b y the installed flash-flood sam plers was
quite depleted (F ig . 7 ) , m ore so than the value show n by the p recipitation samples
o f the w hole storm . T he pick-up o f the saline com p on en t during the first rush o f
w ater through the pool area was apparent. Assum ing com p lete m ixing o f the
p ool’s w aters, w hose volum e was estim ated to be ab ou t 1 0 0 0 m 3, and based on
M IZ P E A V DATRAIN GAUGE RAIN GAUGE-7 .0 14.6 — 22— -6.21 76.1
-6 .8 6.6 - 2 3 — -4.3 1 0.3
UPPER-Z IN HYDROGRAPH
L e g e n d :SA M P L IN G
S I T E
UPPER Z 1 N
2 2 - -7.4 10
2 3 - - -
2 4 - -6 .2 9.3
2 5 - -5.21 10.6
* 10.4*
+ 10.06 j
DATE: 22/12/77 • 23/12/77 24/12/77INTEGRATED
FLOW:ly ^ o c W 6.700m5 2.ÏOOm3
23-
WADI HAWARIM
-5 .6 39.7M A SH A SH -Z IN HYDROGRAPH
2 2 -
WADI Z 1 NM ID R ASH A
-6.9 522
-7 .0 267
LOW ER Z INMASH ASH
-7.9 117
* UTl «
DATE: 22/12/77 23/12/77 24/12/77INTEGRATED
FLOW: - 146.700 m3 53.300m*
F I G . 7 . 1B0 a n d s a l i n i t y d a t e f o r t h e f l o o d in N a h a l Z in , d u r i n g 2 2 —2 5 D e c . 1 9 7 7 . T h e d a t e s ( 2 2 , 2 3 , 2 4 , 2 5 ) a r e s h o w n o n o n e s i d e
o f t h e d a t a . S a m p l e s a r e f r o m t h e h e r m e t i c f l o o d s a m p l e r e x c e p t w h e n i d e n t i f i e d a s m a n u a l l y s a m p l e d b y t h e l e t t e r H .
IAEA-AG-158/1 1 5
M I Z P ERAIN GAUGE
- 5 . 9 14. 4 — 2 3 —
AVDAT RAIN GAUGE- 5 . 3 10.4
2 3 -
2 4 -
UP PE R Z 1 N
- 6 . 2 4 2 . 3
6 . 3 16 . 8
2 3 -
2 4
2 3 -
WADI Z 1 N MIDRASHA
- 5 . 3 8 3 4
- 6 . 0 2 4 7
LOWER Z I N MASHASH
- 5 . 6 152
FIG.8. Data fo r the flo od in Nahal Zin o f 2 3 -2 4 Feb. 1978. Legend and symbols as in Fig. 7.
the h ydrograph o f the U pper-Zin statio n , the pool would be ex p e cte d to flush ou t during the first day. Indeed, salinity was considerably reduced on the 2 3 rd at the Zin-M idrasha station .
3.1.2. The flood o f 23 February
A s show n in Fig . 8 , the developm ent o f this flood m ore o r less follow s the
p attern o f the D ecem b er event. T h e m o st n otab le fa ct is again the flushing ou t
o f the saline reservoir in the E in-A vd at gorge.
3 .2 . T he Besor-R evivim w atershed
T he sch em atic m ap o f the area, F ig . 6 , indicating sam pling p oin ts, show s the
follow ing distinctive features. In the H a ro ’a b ran ch , an earth dam was con stru cted
to in tercep t flood-w aters and only excess w aters spill over in to the B esor system :
1 6 LEVIN et al.
L e g e n d :
The Dec. I 4 event
The D e c . 2 2 e v e n t {
о Roin Gou д F lood Sa • Rain Gau ■ F l ood Sa
- 9 0
- 7 0
- 8 0
- 4 0
- 5 0 ~. О
- 3 0
- 10
- 2 0
0о
- 14 - 1 2 - 1 0 - 8 - 6
8 1 8 0 (%o)
F I G . 9 . S t a b l e i s o t o p e c o m p o s i t i o n o f f l o o d s o f 1 4 D e c . a n d 2 2 D e c . 1 9 7 7 in t h e R i v i v i m
b r a n c h o f N a h a l - B e s o r .
On the o th e r hand, on the Revivim bran ch w aste w aters o f the settlem ents in the M ashavei-Sadeh area, as well as storage p ools, could possibly be contam inating the flood flow.
3.2.1. The flood o f 14 December 1977 (Fig. 9)
Follow ing lighter rains on 12 D ecem b er, a heavy storm on 14 D ecem ber
dum ped an impressive 2 3 m m o f rain on R evivim , w ith m uch sm aller am ounts
m easured a t o th e r precipitation stations. This rain triggered a flood in the Revivim
bran ch o f N ahal B esor, but n o t in the o th e r tributaries. T h e d a y ’s p recipitation
was characterized by extrem ely depleted iso top ic values, averaging 6 180 = - 8 % j . The resultant flood show ed even m ore depleted isotop ic values, w ith a value o f
5 180 = - 12.2% o being a record low fo r this p art o f thw w orld.
The flood evidently did n o t pick up m uch salinity until far dow nstream at
the B esor-H alutza S tation . A t this stage the isotop e com p osition was quite a bit
closer to th a t o f the m ean rain value, w h ether as a result o f additional surface
ru n -off o r because o f the ad m ixtu re o f w ater from a shallow aquifer in the stream
bed at Bir-A jluz ( 5 180 = - 4 .4% o; § d = 1 8 .0 %o\ S = 1 8 0 0 mg C l-l tr - 1 ), is still u ndeterm ined.
IAEA-AG-158/1 17
A s already described in con n ection w ith the flood in N ahal-Z in, the rainfall on 2 2 and 2 3 D ecem b er was w idespread and rath er uniform in com p osition over
the w hole central Negev. In the Revivim b ran ch a flood was record ed on
2 2 D ecem b er (F ig . 1 0 ). T here is little th at is n o tew o rth y in this even t e x ce p t fo r
the effe ct o f the additional flow from the b ran ch originating in the B oq er m ountain .The rain o f the 2 2 n d caused flood flow s in the H azaz b ran ch o f this system
(F ig . 1 0 ) and flood sam plers in W adi H azaz and H a ro ’a station w ere filled, and
show ed rath er similar com p osition ( § 180 = — 8 .6% «). A pp aren tly the flood fron t
picked up a considerable am o u n t o f salinity, w hich reached 5 2 m g C l/ltr in the dow nstream sam pler. The flood ceased som e hours later, enabling the resetting
o f the sam pler in readiness fo r the n e x t rain even t, w hich occu rred on the
follow ing day (th e 2 3 rd ). R ain on the 2 3 rd w as m ore localized, and apparently
flood occu rred only in the H a ro ’a b ran ch , filling b oth the u pstream sam pler o f
W adi H a ro ’a and dow nstream sam pler a t H a ro ’a station . H ere again th e relative
high salinity in the initial rush o f w aters was n oted . L u ck ily the isotop e
com p osition o f the rain o f the 2 3 rd was quite d istinct from th a t o f the previous
day (F ig . 1 0 ) , so th at the ad m ixtu re o f w ater rem nants can be clearly discerned
from the previous d a y ’s flood in W adi H a ro ’a, resulting in the value o f
S 180 = - 7.4% o at H aro ’a statio n (th e isotop ic com p osition o f rain and floods o f
the 2 3 rd ap proxim ates 5 18 = — 5 .9 %o). T h e continuing flood w as then sampled
m anually, b oth in the trib u taries (u p stream ) and the dow nstream section o f the wadi, and a drastic red u ction o f salinity during the continuing flood and the
flushing o f the in-betw een storage was n oted .
The in tercep ted flood-w aters n ear the earth dam w ere sam pled the follow ing
day and give an in tegrated (m ean ) value o f this flood event o f 5 180 = - 7 .79% o;S = 1 0 .3 mg/Ur w hich lies on a m ixing line betw een ch aracteristic values o f the
tw o days’ floods. Q uite evidently the initial high salinity was diluted by the continuing influx o f fresher flood flow , and the m ean salinity o f the w hole event
was n o t particularly high.
4 . G R O U N D W A T ER S O F T H E N E G E V H IG H LA N D S
The lim ited set o f grou nd w ater d ata given here (T ab le II) represents samples
from the k arst E o ce n e lim estone w hich crop s o u t over wide areas o f the N egev as
well as from shallow wells in the alluvium. In addition, d ata o f few springs and
seepages w hich o ccu r in the area are included. M easurable tritiu m in m any o f
these samples shows th em to have been recharged recen tly .
T he shallow alluvial wells are, as a group, the freshest w aters and evidently
recharged d irectly from the surface flow . This group indeed has the highest
3.2.2. The flood o f 22—23 December 1977
00
HAROATRIBUTARIES WADI HAZAZ
no f lo o d — 22— -8 .7 12.4
- 5 .8 4 6 .6 — 23 morning
- 5 .9 4 .7 — 23 afternoon
- 9 - 8 ' 7 - 6
S l8 0 (%o)
- 5
h - 1*
HYPOTHETICAL
HOLDUP
2 2 -u_ h -
L .
HAROA STATION
-8 .5 51.7
-7 .4 21 .2
-5 .5 2 2 . 0
L e g e n d :SAMPLING
SITESl80 (%oyn9 l/t -Date
*23 a ffe rn o o n
HAROA STORAGE L A K E \
-7.8. 10 -2 4
F I G . 1 0 . I s o t o p e a n d s a l i n i t y d a t a f o r t h e f l o o d e v e n t o f 2 2 - 2 3 D e c , 1 9 7 7 in t h e W a d i H a z a z a n d H a r o a . S a m p l i n g s i t e s a r e i n d i c a t e d in F i g . 6 .
LEVIN
et al.
IAEA-AG-158/1 1 9
T A B L E I I . G R O U N D W A T E R S
Name o f sou rce: Type o r Resource sam p lingdate
Cl
(mg/l)
6 l8 o
(% .)ÖD
( ' о )
Зн
(TU)
B e 'e ro ta im w e ll in Eocene l’stone 20 -07 -77 475 -5 .9 3 -31.4 25±1near r i v e r bed 30 -01 -78 566 -5 .6 6
23 -03 -78 561 -5.41
E in -K u d e ira t sp r in g 20 -07 -77 493 -6 .28 -3 1 .3 0.8+0 .624 -10 -77 497 -6 .2 3 -33 .8 0 . 5 ± 0 .323 -03 -78 504 -6 .0 5
Qseima w e ll Eocene i’stone 20 -07 -77 1474 -5 .4 8 -3 1 .3 14+3and c la y m antle 24 -10 -77 1302 -4 .78 -2 8 .6 1.Ш
23-03 -78 504 -5 .1 0
B ir -M u ila j w e ll in a llu v iu m 20-07 -77 1738 -5 .5 8 -28 .1 8±0.423 -03 -78 1838 -5 .5 3
B ir -N itz a n a w e ll in Eocene I’stone 20 -07 -77 661 -5 .9 3 -2 6 .5 7±3near r i v e r bed 24 -10 -77 653 -4 .7 8 -3 2 .2
19-01-78 646 -5 .3 623 -03 -78 659 -5 .67
B ir -M a in w e ll in a llu v iu m 24-07 -77 176 -5 .6 8 -27 .1 23±321 -05 -78 238 -3 .84
Berot-Oded perched water ta b le 24 -07 -77 115 -5 .3 8 -2 7 .7 32+419 -05 -78 116 -5 .11
Bir-Atam ed w e ll in a llu v iu m 25-07 -77 175 -4 .7 8 -21 .4 19±5
B ir -B e id a w e ll in a llu v iu m 24-07 -77 75 -5 .6 3 -28 .1 42.5+1
E in -A qev sp r in g 18 -09 -77 1061 -5 .24 -27 .7 12+0.319 -12-77 1049 -5 .41 -26 .726 -03 -78 730 -5 .8 6
E in -Z iq seepage 18 -09 -77 412 -6 .2 8 -2 9 .3 29+0.5
E in -A v d a t ( l) seepage 26 -07 -77 -5 .8 8 -3 4 .6 4 .2+0 .52-09 -77 1249 -6 .2 8 -33 .7 1.3±0.79 -0 3 .7 8 1222 -5 .5 8
E in -A vd a t (2) seepage 26 -06 -77 1818 -5 .7 3 -22 .7 1.3+0 .62 -09 -77 1403 -6 .2 8 -2 5 .8
E in -A vd a t (3) seepage 26 -06 -77 1403 -5 .3 3 -22 .1 2.9+0 .6
B ir -A vd a t w e ll in Eocene l’stone 30 -08-77 863 -5 .9 4 -2 9 .6 4±4
E in -A vd a t b ig w a te r fa l l 26-07 -77 1079 -4 .5 0 -19 .4 9 . 5±0 .4sm a ll w a te r fa l l 26 -07 -77 1800 -3 .7 9 -16 .4 10.310.4
tritiu m values, approaching those o f the atm osp h eric levels. T he isotop e com p osition is enriched (along evaporation lines) com pared to the group o f
samples from the lim estone wells in w hich the in filtration o f rain w ater occu rs
in a m ore direct w ay. These w ater sources from the lim estone terrain form an in term ed iate group as far as salinity is con cern ed . T here is quite a variety o f
tritium values, ranging from relatively high values in the wells situated close to
2 0 LEVIN et al.
Sl8 0(% o )- 7 - 6 - 5 - 4
F I G . 1 1 . G r o u n d w a t e r s o f t h e N e g e v H i g h l a n d s - s t a b l e i s o t o p e d a t a . A = A v d a t a r e a .
the W adi (B e ero ta im , N itzana e tc .) to som e very low tritiu m values o f less than 1 T U , w hich indicate quite a delay from the recharge site to the discharge area. T he isotop ic com p osition is som ew hat less depleted in the heavy iso top ic species than the m ean p recipitation o f the 1 9 7 7 /7 8 season (and certainly less than the
flood s) (F ig . 1 1 ) b u t still quite a b it lighter in com p osition than th e equivalent
w ater sources o f the Ju d ean M ountains.
Springs and seepages o f the A vdat region (m arked A on F ig .l 1) are quite
varied in th eir stable iso top ic com p osition , low in tritium and also quite saline.
T hese are the w ater sources w hich con trib u te the salinity to the flood-w aters in N ahal-Zin.
It appears from these d ata th a t the tritiu m am oun t is a good in d icator o f
d irect flood -w ater con trib u tion to a well, and th at a lengthy underground ro u te , under the arid conditions o f the Negev, expresses itself im m ediately through
a d rastic red u ction o f the tritiu m co n ten t. Salinity rem ains a very individual m atte r , and n o simple general rule em erges from these observations.
IAEA-AG-158/1 2 1
W ater sources o f the Negev highlands w ere found to be surprisingly depleted in the heavy iso top ic species and on the w hole w ere characterized by the high
value o f the “ d ” p aram eter, w hich is typ ical o f the Levan t region. T h ere is
relative u n iform ity in the com p osition o f individual s torm events th rou gh ou t the
area and the “ d ” p aram eter is often a d istin ct p rop erty o f a single d a y ’s rain.This facilitates the use o f stable isotop e param eters fo r identifying com p on en ts o f
the flood hydrograph.Invariably the flood-flow was found to be even m ore depleted in heavy
isotopes than the w hole s to rm ’s average isotop e com p osition . Assum ing th at the
relationship o f the am o u n t effe ct holds also fo r parts o f a single event (as indeed
it did in the case o f the storm analysed in detail in section 1) then these data point
to the fa ct th at ru n -off is activated selectively by the high-intensity rains. Since
som e o f the extrem ely depleted floods (su ch as the one o f 1 4 D ec. 1 9 7 7 ) differ
considerably in th eir iso top ic com p osition from the com m on ly recognized
M editerranean M eteoric W aters, one should p roceed w arily in the id entification
o f the origin o f w ater sources solely on the evidence o f the stable isotop e
com position .T he data presented n o t on ly throw light on the role o f the high-intensity
rains in the in cep tion o f floods, b u t th ey also in dicate quite clearly the salinity
p attern o f flood-flow s in the desert en vironm ent: relatively high salinities (o f
the ord er o f 5 0 mg C l/ltr , com p ared w ith 5 — 10 m g /ltr in rain) acco m p an y the
initial rush o f flood -w ater; salinity originates apparently from the surface
accum ulations o f salts w hich are then rapidly flushed aw ay. As exem plified by
the flood o f 2 2 D ec. in the H a ro ’a basin (F ig . 10 ) a continual high salinity
th rou gh ou t the d uration o f a flood m ay result from the successive m obilization o f additional parts o f the w atershed later during the evolution o f the flood . In m any
o th e r cases the la ter stages o f the flood consisted o f very fresh w aters indeed, pointing to the fa ct th at the desert flood is a ra th e r shallow surface event w hich does
n o t involve the subsurface accu m u lation s o f salinity. H ow ever, higher salinities can result in areas, such as E in A vd at, w here seepage from saline grou nd w ater sources accu m u late large am ounts o f salinity over exten d ed periods on the surface; these are then flushed and carried aw ay by the flood-flow (Fig s 7 , 8 ) .
I t is an encouraging fa c t th a t, despite the sporadic n ature o f the p hen om enon, there is quite som e regularity in the iso top ic and salinity p attern o f a desert flood, enabling a quantitative understanding o f these im p ortan t features o f desert
hydrology.F ro m the rath er lim ited set o f grou nd w ater d ata w e learn th a t the w ater in
the wells o f the alluvial w adi fill, w hich und ou b tedly are recharged through the
in term ed iacy o f flood-flow s, are som ew hat enriched in the heavy isotop es relative
to the local p recip itation , and th erefo re considerably so com p ared w ith the flood-
flows (w h ich , as described above, are generally depleted by a few per mill relative
5. S U M M A R Y D IS C U S S IO N
2 2 LEVIN et al.
to the m ean isotop ic com p osition o f the p recip itation ). T he lim estone aquifers,
w hich seem to be recharged by m eans o f a m ore direct ro u te , are closer in
com p osition to the rain, but also in this case it w ould appear (F ig . 11) th at som e
evaporative en richm ent had accom panied the recharge process th at the isotop ic com p osition evolved, along low -slope evaporation lines.
T here appears at first sight a p arad oxical situation w here those groundw aters
w hich are derived from the very depleted flood-flow s are m ore enriched in the
heavy isotop ic species than others d irectly recharged by rain infiltration. Does
this signify th at the recharge from surface flows involves a delay o f the w aters
n ear th e surface b efore rech arge? O ur d ata are to o scan ty to answ er this
interesting point and fu rth er observation will be necessary to understand fully
the relationship betw een groundw aters, the p recip itation and the flood-w aters.
A C K N O W LED G EM EN T S
We are grateful to Mr. Shavit o f the Israel M eteorological Service for perm ission to use their rain stations. Special thanks are due to all th ose responsible fo r the rain stations. We also thank E . A dar, hydrologist o f the
W ater R esources U nit (W R U ) o f the In stitu te fo r D esert R esearch (ID R ) fo r valuable discussions on the lo catio n o f the rain and flood stations and for help in the field w ork. The au thors are indebted to L . Landsm an, geologist o f the
W R U o f the ID R , for his collab oration in the in stru m en t installation and sample collection . We acknow ledge gratefully the devoted analytical service by the s ta ff o f th e Stable Iso to p e and T ritiu m L ab o ra to ry at R eh ov ot. T hanks are due to Uri
G at fo r chloride analysis.
R E F E R E N C E S
[1 ] G A T, J .R ., CARM I, I., Evolution o f the isotopic composition o f atmospheric waters in the Mediterranean Sea area, J . Geophys. Res. 75 (1 9 7 0 ) 3039 .
[2 ] TREW A RTH A , G .T ., The World’s Problem Climates, Univ. o f Wisconsin Press, Madison (1 9 6 1 ) 3 3 4 pp.
[3] GAT, J .R ., DAN SGAARD, W., Stable isotope survey o f the freshwater occurrences in Israel and the Jordan R ift Valley, J . Hydrol. 16 (1 9 7 2 ) 177.
[4] GAT, J .R ., ISSA R , A., Desert isotope hydrology, water sources o f the Sinai Desert, Geochim. Cosmochim. A cta 38 (1 9 7 4 ) 1117.
[5 ] A DAR, E ., LEVIN , N., B A R Z IL A I, A., “ A multiple hermetic rain sample collector development” , in preparation.
[6 ] LEV IN , M., A DAR, M .E ., B A R Z IL A I, A., “ An herm etic sampler fo.' ephemeral Wadi floods” , in preparation.
U S E O F E N V I R O N M E N T A L IS O T O P E S
IN A R I D -Z O N E H Y D R O L O G Y
T . D IN Ç ERDeli H useyinpasa Cad. O zyurt,Istanbul, T urkey
Abstract
U SE O F EN VIRON M EN TAL ISO TO PES IN A RID-ZON E H Y D RO LO G Y .A fter aridity is defined some physical and hydrologie features common in arid lands are
described. An attem pt has been made to identify various groundwater recharge mechanisms in arid countries and to assess their relative im portance. The influence o f the arid climates on the isotopic com position o f the precipitation is discussed. The use o f environmental isotopes in run-off and precipitation infiltration and recharge is evaluated, and the im portance o f isotopic studies in investigating interrelations between aquifers is stressed. Finally, some examples o f isotope applications in surface water and its relation to groundwater are given.
IAEA-AG-158/2
IN T R O D U C T IO N
Arid conditions are said to exist in a region when the p oten tial évapotranspiration is larger than th è precipitation fo r th e m o st p art o f th e year. The
difference betw een annual p oten tial évapotranspiration and th e annual p recip itation determ ines the degree o f aridity in a given place. While in som e very arid
zones such as the A rabian Peninsula this d ifference exceed s 2 m annually, in
semi-arid regions the difference is usually less than 1 m as in central and eastern T u rk ey. L on g periods o f arid ity change th e face o f th e land drastically, w hich in tu rn has a d ifferent h yd rological response to th e atm osp h eric inputs and to the incom ing rad ian t energy. It is useful th erefo re to give a general description o f th e clim atic and hyd rological features o f arid lands b efore discussing th e use o f isotop e techniques in these areas.
In very arid cou ntries th e p recipitation is n o t high enough to sustain a perm an en t plant cover on soils. When top ograp h ic conditions are favourable to erosion the soil cover is rem oved and barren ro ck surfaces are exp osed to the
atm osp h ere. As a result th e ru n -off from such areas increases, especially in response
to short-d uration heavy p recip itation typ ical o f arid clim ates. It w ould be a
m iscon cep tion to th ink th a t these intense rains are ex trem ely rare phenom ena —
people living in arid cou ntries are quite fam iliar w ith them . The ru n -o ff p roduced
by these rains results in impressive flood waves w hich are absorbed b y highly
perm eable wadi alluvia, o r disappear in sink-holes in karst lim estone w ith negligible
evaporation losses. O ften a flood wave co m p letely disappears b efore reaching the
2 3
24 DINÇER
term inal area o f a wadi system consisting o f a m ud flat (sabkha, p laya), sand-dunes
or the sea. Thus, th ere is an in term itten t b u t som ew hat regular recharge to alluvial
and karst aquifers and to aquifer form ations in w hich alluvium has been deposited.
As a result o f soil erosion and the subsequent “ sifting” o f the sand by winds, the soils in arid zones tend to be sandy and in extrem e cases consist o f pure sand
in sand-dune regions and in larger sand seas. T h e h ydrological im plication o f this
change are th e higher in filtration rates and low er soil evaporation losses, b oth beneficial to the con servation o f w ater. T here are indications th a t even in very
arid cou ntries such as Saudi A rabia rain-w ater infiltrating in sand-dunes could con trib u te to th e rech arge o f th e aquifer form ation s underlying sand-dunes.
P O S S IB L E G R O U N D W A T ER R E C H A R G E M ECH AN ISM S IN A R ID Z O N ES
As one o f the m o st prom ising uses o f isotopes in arid zones is the stud y o f the rech arge to groundw ater, an a tte m p t has been m ade to identify recharge
conditions m ost likely to exist in arid zones.
1. R u n -off rech arge: This typ e o f recharge seems to be the m o st com m on in
arid zones even under e x trem ely arid con ditions. R u n -off recharge could be
subdivided in to :
1.1. Recharge to the wadi alluvium: S tream -flow m easurem ents in dicate th a t
recharge to alluvium is one o f th e m ost im p ortan t recharge m echanism s in arid zones. In m any cases th e aquifer form ations (fractu red crystalline ro ck s, sedim en tary form ations, volcan ic ro ck s) underlying o r bordering on th e alluvium are also recharged th rou gh th e alluvium w hich is th e first recipient o f flood w aters.
1.2. Recharge to karst limestone: It has been observed th a t w hen flood- w aters flow over such form ation s large quantities o f w ater disappear in caves and solution channels typ ical o f k arst lim estone.
1.3. T here are reasons to believe th a t w hen th e m ud flats, “ sabkhas” , are
flooded som e w ater reaches the grou nd w ater table by infiltration.
2 . G roundw ater recharge in v o lcan ic ro ck s: This typ e o f recharge o ccu rs by
the in filtration o f precipitation in to young basalts o r in to areas covered b y pum ice, w hich are devoid o f ru n -off channels.
3 . R echarge to grou nd w ater through the in filtration o f the p recip itation in
sand-dunes: The m ean grain size o f dune sand very often varies betw een 0 .1 5 and0 .4 m m . Sand-dunes w ith coarser grains let the rainw ater infiltrate a t considerable
IAEA-AG-158/2 2 5
depths ow ing to th eir low field cap acity . O bservations and exp erim en ts also show th at evap oration from sand decreases as th e m ean o r m edian grain size increases.
Thus, even in regions w ith an annual precipitation less thah 1 0 0 m m th ere are appreciable quantities o f soil m oisture in dune sand profiles. T hus, areas covered
by dunes should be considered as favourable areas fo r the rech arge o f groundw ater.
4 . R ech arge through perennial rivers and sw am ps: This typ e o f recharge
is observed ra th e r frequ en tly in sem i-arid zones. B u t even in very arid countries it is possible to have perennial rivers such as the Nile in E gyp t. This is also true
fo r lakes and sw am ps. A good exam p le is the O kavango swamp in B otsw ana, which regularly recharges th e shallow aquifers in the region o f the O kavango delta.
Som e o f the recharge m echanism s described above could, be studied successfully w ith conventional techniques. The recharge to the alluvial aquifers from
flood-flow s, fo r exam p le, can be estim ated b y stream -flow m easurem ents, o r by
studying the w idth o f the recession channel [1 ]. On th e o th er h and, th e study
o f the recharge through in filtration o f th e precipitation is m uch m ore difficult and requires the application o f isotop e techniques to reach quantitative estim ates
o f recharge.
ISO TO PIC CO M PO SITIO N O F T H E P R E C IP IT A T IO N IN T H E A R ID ZO N ES
In m any arid cou ntries p recip itation is n o t only seasonal but sh ort and heavy
show ers o ccu r in a p ractically dry atm osp h ere. This influences th e isotop ic
com p osition o f the p recipitation w hich clearly show s the effect o f th e evaporation in the atm osphere. The ra tio o f the standard deviation o f the deuterium values to th at o f the o x y g e n -18 (m o n th ly values) b ecom es less than th e m ean ratio
( 7 .0 3 ± 0 .1 0 ) fo r 9 0 W M O -IA EA n etw ork stations. F o r six typ ical arid-zone stations th e m ean ratio is 6 .3 1 ± 0 .2 4 . This departure from the w orld-w ide m ean is for isotop e values w eighted b y p recipitation and w ould certainly be larger if
unw eighted values would be used. It is perhaps useful to m en tion here th at the slopes quoted here are n o t related to the slope o f 8 obtained when tim e-averaged m ean values are used.
T he com parison o f th e stable isotop e co n te n t o f the groundw ater in alluvium
in Saudi A rabia and o f the precipitation show s th a t, as e x p e cte d , only heavy
p recipitation is recharging these aquifers. This sim ilarity can be used to estim ate the m ean isotop ic com p osition o f the precipitation b y exam ining a few samples
o f shallow grou nd w ater near th e wadi beds.F ro m the stable iso top ic com p osition o f old groundw ater it is seen th a t the
p recipitation in som e arid cou ntries, such as Saudi A rabia, had a d ifferent isotop ic com p osition in th e past com p ared w ith m odern p recipitation . N ot on ly is the
2 6 DINÇER
old grou nd w ater isotop ically lighter but it also has a deuterium excess significantly
less than th a t o f the local p recipitation . T he first poin t can be explained by a cooler clim ate, b u t it could well also be a result o f the changing p attern o f
atm osp h eric circu lation . N ow th a t we have a satisfactory am ou n t o f isotope
d ata on p recipitation it can be seen th at the so-called “ tem p eratu re e ffe ct” is
n o t, in n ature, as strong as it should be, especially at low latitudes. This also
applies to the deuterium excess w hich seem s to indicate w ater vapour p roduced
in m o re hum id conditions over the oceans under past clim atic con ditions, but
could as well be a result o f changing atm osp h eric circu lation p attern s, and m ost
probably a com b in ation o f m any clim atic facto rs. A t present w e do n o t have a
satisfactory exp lanation for the discrepancy o f the isotop ic com p osition of
m odern and old p recip ita tio n , as inferred from groundw ater d ata .
S tu d y o f th e ru n -o ff recharge to th e shallow and deep aq u ifers
G roundw ater rech arge th rou gh ru n -off is a visible and m easurable p henom enon. E ith e r stream -flow o r channel geo m etry m eth od s are o f great use in estim ating this type o f rech arge [ 1 ]. On the o th e r hand, it is n o t possible to have a q u an titative estim ate o f recharge using tritiu m co n cen tra tio n in shallow
groundw ater. Tritium and stable isotopes cou ld , how ever, be useful in studying
th e grou nd w ater m ovem ent b eyon d the wadi channel. In such studies one has to
be carefu l in considering th e fa c t th at in arid cou ntries irrigation is very often
m ade w ith o u t drainage, w hich results in recircu lation and som e change in the
chem istry and isotop ic com p osition o f groundw ater.Isotop e techniques are ex trem ely useful in studying interrelations betw een
aquifer system s and in determ ining, to g eth er w ith the chem istry o f w ater, grou nd w ater m ixtu re problem s. T here are num erous exam ples o f such applications [2, 3 ] and th ere is n o need to expand on this subject.
S T U D Y O F G R O U N D W A T ER R E C H A R G E B Y D IR E C T IN F IL T R A T IO N O F
T H E P R E C IP IT A T IO N
It is rath er surprising to observe th at sand-dunes in very arid cou ntries such
as Saudi A rabia con tain significant am oun ts o f m oisture despite low p recipitation
(b etw een 5 0 and 1 0 0 m m /a on th e average). This is also tru e fo r sandy desert
flats such as th e K alahari d esert, w hich show s th at p recipitation could be a
d irect source o f recharge to groundw ater.T he presence o f m oistu re in sand layers depends a great deal on the clim ate
and on th e size d istribution o f the sand. While n o m oisture can be found in sand- dunes w ith a m ean grain d iam eter o f 0 .2 m m in Saudi A rabia — e x c e p t during the
rainy days and the w eek follow ing — sand w ith a sim ilar grain size in the
IAEA-AG-158/2 2 7
F I G . l . T r i t i u m c o n t e n t o f t h e s a n d m o i s t u r e a t 1 1 4 k m o n t h e R i y a d h - D h a h r a n h i g h w a y .
( F r o m R e f . [ 4 \ j
n orth ern K alahari desert has, on an average, 7% m oistu re by volum e. This is the
result o f m u ch higher p recip itation in n orth ern K alahari ( 5 0 0 m m /a) com p ared w ith 7 0 m m /a in cen tral Saudi A rabia. T h e sampling o f sand-dunes in Saudi A rabia has show n th a t in th e dunes w here the m ean grain size is ab ou t 0 .3 m m , the w hole sand profile dow n to a d epth o f 7 m con tains m oisture w hich can be
as high as 7% by volum e, b u t in general is low er than the “ field ca p a c ity ” . I t is interesting, o f course, to stud y the m o vem en t o f this m oisture w hich could
eventually find its w ay to th e saturated zon e. M easurem ent o f the tritiu m co n ten t
o f th e sand m oistu re m ad e in 1 9 7 2 in D ahna sand-dunes has show n th a t the
tritiu m peak o f 1 9 6 3 —1 9 6 4 had m oved dow n to 4 m ( F i g .l ) [4 ] . R e ce n t m easurem ents given in an oth er p ap er [5 ] at this m eeting show th a t the peak is no longer
th ere ; in addition, th e m oistu re co n te n t o f the sand at the sam e lo catio n was m easured to be 1 .7 3 % by volum e com p ared w ith 4 .5 5 % in 1 9 7 2 . T h e low
m oistu re co n te n t o f th e sand prevented a go od sampling owing to th e auger hole caving in, and this m ay acco u n t fo r th e uniform isotop ic com p osition o f th e sand m oistu re, a t least p artly . I t is hoped th a t w ith m ore advanced sam pling techniques
developed by the H eidelberg E nviron m en tal Physics group such p roblem s will no
2 8 DINÇER
longer plague exp erim en tal results. In any case, th e study o f th e tritiu m m ovem en t in the sand m oistu re, eith er naturally o r artificially in jected , seems to be one
o f th e m ost prom ising applications o f isotop e techniques in arid zone h yd rology ,
especially when it is used to calibrate evaporation m odels from the sand.
In the sand-m oisture m ovem ent study in Saudi A rabia an oth er observation
o f in terest was the exten sive stable isotope en richm ent o f the sand m oisture as
given in R e f .[4 ] . This is in teresting because in the case w here the m oisture could
reach the saturated zone it would give an op p ortu n ity fo r identifying w ater
originating from sand-dunes. This applies n o t only to m odern w ater but also to .
old w ater and could explain th e low er deuterium excess o f th e old groundw ater
in th e Sahara and Saudi A rabia. The slope o f th e d eu teriu m /180 line o f the sand
m oisture is unusually low at ab ou t 2 . This is a startling result as it seem s to be m uch low er than any evap oration slope m easured by different isotop e w orkers.
A plausible explanation is th a t the slope o f the evaporation line is determ ined n ot only b y the relative hum idity o f the atm osp h ere but also by the isotop ic com p o
sition o f th e atm osp h eric m oistu re, w hich m ay be o f local (in ocean s and in large
lakes and sw am ps) o r o f non-local origin (sm all lakes, evap oration pans and sand
dunes). E vap oration from sand in arid cou ntries can n ot p rod u ce enough w ater
vap ou r and th e sand m oistu re, during evap oration , m ust have m olecu lar exchange
w ith an alien w ater vapour. If one assumes an iso top ic com p osition fo r the
atm o sp h eric m oisture in iso top ic equilibrium w ith th e local p recip itation , it is
possible to have evaporation slopes o f 2 and even low er. One should also consider
the fa ct th at evaporation from sand does n o t take place e x a ctly like the evaporation from open w ater — in sand we are dealing w ith a th ick diffusion layer (2 0 cm o f d ry sand layer a t the surface is equivalent to 1 m o f diffusion layer in the air)
and also a very large variation o f tem p eratu re in th at layer (F ig .2 ) . As a result, evaporation from sand is very inhibited and am ounts only to 10 m m from M ay to O cto b er when th e daily atm osp h eric tem p eratu res are above 3 0 °C .
S T U D Y O F T H E IN T E R R E L A T IO N S B E T W E E N A Q U IF E R S U SIN G S T A B L E
ISO TO PES
A lim ited num ber o f stable isotope m easurem ents usually gives a good idea
o f th e typ ical isotop ic com p osition o f a groundw ater body. In large aquifers, such
as U m m -E r-R adh u m a in th e A rabian Peninsula, th ere is also significant isotop ic
variability within the aquifer w hich m ay be o f great use in studying groundw ater
rech arge and flow conditions. T he m ost prom ising application is perhaps the study
o f the interrelations betw een aquifers, leakage problem s and w ater source in
springs. Such a stud y was m ade in Saudi A rabia in relation to th e Al-H asa springs
w ith an estim ated discharge o f 12 m 3/s. T he results have show n th a t the spring
IAEA-AG-158/2 2 9
20 30 40 50 60T E M P E R A T U R E OF THE SAN D °C
~ 0 6 0 0 /0 7 1 5 t im e „ '1 ) 8 5 0 „ - " 1 0 2 0 1400
V I , ,
4 ' "' ' < /-10
E
x f-
1-20 1 ,
I
-30 ---------------------------------------------- D R Y -M O IS T SAND INTERFACE
FIG.2. Diurnal temperature variation in the surface sand layers in July. (From R e f[4 \ )
w ater consists m ainly o f old w ater originating from the U m m -E r-R adh u m a form a
tion [2 ] . R ecen t studies have indicated th a t th ere is also a m in or com p on en t o f
old w ater [3 ].As th e iso top ic com p osition o f m o d em rech arge w aters, inferred from
samples co llected from alluvium , differs significantly from the isotop ic com p osition o f old w aters in som e arid cou ntries, this d ifference could be used in assessing
the presence o f m o d em recharge in the o u tcro p s o f the aquifer form ations which
essentially store old w ater. In p ractice such studies are u n fortu n ately handicapped
by the absence o f sam pling points a t desired location s. H ydrogeologists should th erefo re plan the drilling o f e x p lo rato ry wells by taking in to a cco u n t the benefits to be gained from th e iso top ic m easurem ents in evaluating th e recharge including
m easurem ents o f 14C and tritiu m .
R E C H A R G E O F A Q U IF E R S B Y S U R F A C E W A T E R
As m en tion ed earlier, large perennial rivers do flow in very arid lands. In
addition to th e Nile in E gy p t one can m en tion th e Niger in its middle reaches
and th e B o te ti R iver flow ing in the K alahari desert. Som e o f these rivers have
3 0 DINÇER
their w aters naturally labelled w ith stable isotop es as th ey originate from large
lakes and swamps (V icto ria L ake in the case o f the Nile, T im b u ctu swamps
in Niger and the O kavango sw am p fo r the B o te ti River). It is possible in such
situations to study th e relation o f these surface w aters with the groundw ater.
In th e study o f the O kavango sw am p for exam p le th e aquifers recharged from th e
swamp w ere very easily identified by com paring the isotop ic com p osition o f
swamp w aters w ith th at o f the grou nd w ater [6 ] . The n atural labelling o f th e Niger
River as a result o f the evap oration in th e T im b u ctu swamps was used to study
m ixing in the Kainji R eservoir in Nigeria [7 ] . F in ally , the degree o f iso top ic
en richm ent in three lakes in southern T u rk ey was used to evaluate th eir w ater balance and to determ ine the origin o f w ater in large karst springs on the
M editerranean coast o f T urkey [8 ] . A lthough these studies are n o t specific to
arid regions the isotop ic en richm ent o f surface w aters in arid zones is in general
m ore extensive than the hum id regions and allow b ette r com parisons to be m ade
betw een the iso top ic com p osition o f surface and groundw aters.
R E F E R E N C E S
[1 ] M OO RE, O.M. (United States Geol. Team ), Unpublished work, Ministry o f Agriculture and Water, Saudi Arabia, Riyadh (1 9 7 7 ).
[2] D IN ÇER, T ., N O O RY, M., JA V ED , A .R .K ., NUTI, S., TO N G IO RG I, E ., “ Study of groundwater recharge and movement in shallow and deep aquifers in Saudi Arabia with stable isotopes and salinity data” , Isotope Techniques in Groundwater Hydrology 1974 (Proc. Symp. Vienna, 1974), IA EA, Vienna (1 9 7 4 ).
[3] Quaternary Period in Saudi Arabia (SAAD, S ., A L-SA Y A RI, and ZÖ TL, J .G ., Eds), Springer-Verlag, Vienna-New Y ork (1 9 7 8 ).
[4 ] D IN ÇER, T ., Al-MUGRIN, A ., ZIMMERMANN, V ., Study o f the infiltration and recharge through the sand dunes in arid zones with special reference to the stable isotopes and thermonuclear tritium , J . Hydrol. 23 (1 9 7 4 ).
[5 ] SONNTAG, G ., THOMA, G ., MÜNNICH, K .O ., DIN ÇER, T ., KLITZSCH , E ., “ Environm ental isotopes in N orth African groundwaters — The Dahna sand-dune study, Saudi Arabia” , IA EA -A G -158/6, these Proceedings.
[6 ] D IN ÇER, T ., HUTTON, L .G ., KHUPE, B .B .J ., “ Study, using stable isotopes, o f flow distribution, surface-groundwater relations in the Okavango Swamp, Botsw ana” ,Isotope Hydrology 1978 (Proc. Symp. Neuherberg, 1978) I, IA EA , Vienna (1 9 7 9 ) 3 - 2 6 .
[7 ] Advisory Group Meeting on the Application o f Nuclear Techniques to the Study of Lake Dynamics, Vienna, 29 August - 2 Septem ber 1977.
[ 8] D IN ÇER, T ., PA YN E, В .R ., An environmental isotope study o f the south-western karst region o f Turkey” , J . Hydrol. 1 4 (1 9 7 0 ) .
IAEA-AG-158/3
I N T E R P R E T A T I O N O F E N V I R O N M E N T A L
IS O T O P IC G R O U N D W A T E R D A T A
A rid a n d s e m i-a rid z o n e s
M .A. G E Y H
N iedersächsisches L and esam t für
B odenforschung, H anover,
Fed eral R epublic o f G erm any
Abstract
IN TERPRETA TIO N O F ENVIRONM ENTAL ISO TO PE GROUN DW ATER DATA: ARID AND SEM I-ARID ZONES.
Various hydrodynamic aspects are discussed in order to show their implication for the hydrogeological interpretation of environmental isotope and hydrochemical groundwater data. Special attention is drawn to radiocarbon and tritium studies carried out in arid and semi-arid zones. An exponential model has been utilized to determine the mean residence time o f the long-term water from springs in karst and crystalline regions. Hydrogeological parameters such as the porosity can be checked by this result. In addition, the exponential model offers the possibility o f determining the initial 14C content of spring water, which is sensitively dependent on the soil o f the recharge area. A base-flow model has been introduced to interpret the 14C and 3H data o f groundwater samples from older karst regions. Differences between pumped and drawn samples exist with respect to the groundwater budget. Owing to pumping, the old base flow is accelerated and becom es enriched in pumped groundwater in comparison to the short-term water. Radiocarbon ages of groundwater in alluvium may be dubious because of isotope exchange with the C 0 2 in the root zone along the river bank. Under confined conditions I4C groundwater ages are diminished if the hydraulic head of the confined aquifer is lower than that o f the shallow one. This is due to the radiocarbon downwards transport by convection o f shallow groundwater. The same effect occurs, though much faster, if the groundwater table is depleted by groundwater withdrawal. The decrease of the radiocarbon groundwater ages in time can be used to determine the hydraulic transmissibility coefficient o f the aquitarde. According to the practical and theoretic results obtained the hydrodynamic aspects require at least the same attention for the interpretation of environmental isotope and hydrochemical data o f groundwater as do hydrochem ical and isotope fractionation processes.
IN TR O D U C TIO N
Environm ental isotop es have been increasingly used in hydrological studies.
Increasingly com p licated h yd roch em ical and isotop e m odels have been in trod u ced
to in terp ret the m easured d ata ( 1 - 3 ) . R e ce n tly , first a ttem p ts w ere m ade
to take in to a cco u n t h yd rod yn am ic aspects in the in terp retation o f environm ental
isotope data [4 , 5 ] and to reach quantitative conclusions.
3 1
3 2 GEYH
rrïH
F I G . l . R u n - o f f o f t h e B a r d a g u é R i v e r a t B a r d a i [ 6].
In arid zones, fossil groundw ater resources and grou nd w ater recharge are restricted . T hus, the d eterm in ation o f the grou nd w ater budget based on h y d ro chem ical o r environm ental isotop e d ata is o f in terest since the com m on hydraulic
m ethods are often to o expensive and tim e-consum ing. H ow ever, even simple m eth od s often can n ot be applied so far because hydraulic d ata have seldom been recorded .
In this p aper we discuss again the rad iocarb on and tritiu m data o f a case
stud y in the arid zone o f the Tibesti M ountains [6 ] . O bviously, the h yd rod yn am ic situ ation , rath er than the h yd roch em ical h istory o f the grou nd w ater, seem s to
have determ ined the environm ental iso top ic and h yd roch em ical com p osition .
1. H Y D R O G E O L O G IC A L C A S E S T U D Y : V A L L E Y O F T H E B A R D A G U E
R IV E R
T he largest tow n o f the Tibesti region in Chad is Bardai (elevation : 1 0 2 0 m
above sea level). The annual rainfall, w hich occu rs as occasional storm s, am ounts
to 1 5 —2 0 0 m m , increasing w ith altitude. T h e p oten tial evaporation is estim ated
to be 4 0 0 0 —6 0 0 0 m m [6 ] . G roun d w ater is only recharged by ru n -off in the
IAEA-AG-158/3 3 3
FIG.2. Sampling sites in the Bardagué Valley at Bardai [6].
alluvium o f the Bardagué R iver and its tributaries. A seepage o f 6 m 3/m o f river
bed was estim ated fo r a peak discharge o f 3 0 m 3/s , w hich m ay o c c u r at intervals
o f several years (F ig . 1).Sandstone overlain by volcan ic ro ck p redom inates in this region. T he high
m oun tain platform above 3 0 0 0 m above sea level serves as the recharge area o f
the tributaries o f the Bardagué River, w hich have eroded steep canyon s in to the
Bardagué V alley. A ccord in g to the isotop e results fo r sam ples draw n from wells
situated along the river bed (F ig . 2 ) , the grou nd w ater is several thousand years
old [6 ] . Present-day recharge o ccu rs a t rare intervals.
Since in terp retation m eth od s o f isotop e d ata have been im proved during the last decade, it is w orth while reconsidering the previously m easured isotope con cen tration s, although a q uan titative evaluation is still n o t possible owing to the lack o f supplem entary h yd roch em ical and hyd rau lic data.
2 . M ETH O D S F O R IN T E R P R E T IN G H Y D R O C H EM IC A L A N D R A D IO A C T IV EE N V IR O N M E N T A L ISO T O PE D A TA
2 .1 . H yd roch em ical m odel fo r 14C grou nd w ater dating
The only m eth od applied in p ractice fo r dating grou nd w ater is based on its
14C co n te n t. A ccord in g to the M ünnich m odel [7 ] , rain-w ater seeping through the top soil dissolves C 0 2 form ed in the ro o t zone and soil carb on ate , w hich is
usually assum ed to be fossil and o f m arine origin. T h e C 0 2 o f the ro o t zone
should have the 14C c o n te n t o f the atm osp h eric carb on dioxid e. H en ce, the 14C
34 GEYH
co n te n t o f recen tly recharged groundw ater, designated as initial 14C co n ten t, is determ ined by the p art o f adm ixed soil carb on ate and th at 14C con ten t.Changes in the 14C co n te n t in the grou nd w ater in a confined aquifer are due only
to rad ioactive d ecay.
T he m ost-discussed problem s in I4C grou nd w ater dating are still
(a ) H ow to determ ine the initial 14C co n ten t [ 8 ] ; and
(b ) W hether second ary processes o ccu r w hich change the 14C co n ten t in
the groundw ater besides the rad ioactive decay.
T here are various m ethods fo r th eoretically estim ating the initial 14C
co n te n t [ 1 —3 ]. T h eir applicability is questioned since the isotop ic c o n te n t o f soil
lim e and soil C 0 2 varies greatly [ 9 - 1 1 ]. A n em pirical m ethod based on the
application o f an exp on en tial m odel to rad iocarb on and tritium m easurem ents [1 2 ]
seem s to give b ette r estim ates [8 ]. A ccord in g to exp erien ce, the initial 14C co n te n t
seem s to be determ ined mainly by the geological and pedological situation in the
recharge area [1 3 ] rath er than the clim ate. Even barely distinguishable soil covers are reflected in this value [ 14].
S eco n d ary processes th at change the 14C co n ten t m ay be isotop e exchan ge or
hyd roch em ical reaction s. M ünnich [7 ] estim ated th at iso top ic exchan ge betw een the bicarbonate and the lim e in the ro ck is n o t likely to play a predom inant role
in the 14C age d eterm ination o f groundw ater. I f it did, the 14C data w ould be to o
large b y a small but con stan t factor. W igley considered open and closed system s
[2 ] and a steady-state process o f p recipitation and re-dissolution o f carb on ates [1 5 ] . A ccord in g to these studies,' large age shifts are to be exp ected and should som etim es be reflected by anom alous h yd roch em ical properties o f the groundw ater.
T o check w h ether second ary processes should be taken in to a cco u n t in p ractice , we re-evaluated the 14C data fo r a sandstone aquifer in the U nited K ingdom [1 6 ] . B ath et al. found th at co rrected and u n corrected grou nd w ater ages differed eith er by 3 7 0 0 years (= 63% -m od ern ) o r 5 6 0 0 years (= 50% -m o d em ) from each oth er independently o f the actu al groundw ater age. A h yd roch em ical
exp lan ation fo r these tw o values could n o t be given. T he initial 14C co n te n t
(referring to sample 9 ) , determ ined em pirically by applying the UK 3H input curve, yielded 65% -m o d em , in very good agreem ent w ith the above-m entioned low er
value. This shows us th a t, a t least during the last 3 0 0 0 0 years, second ary processes
could n o t have changed the environm ental isotop ic and h yd roch em ical com p osition
o f the grou nd w ater in this case. H ence, w e m ay sum m arize th a t the initial 14C c o n te n t o f groundw ater can be estim ated reliably, and second ary processes do
n o t need to be taken in to a cco u n t if a norm al h yd roch em ical situation exists.
2 .2 . E xp o n en tia l m odel in hard ro ck h yd rology
Spring w ater and shallow groundw ater from hard ro ck regions consist o f at
least tw o com p on en ts: short-term and long-term groundw ater. T he flow velocity
IAEA-AG-158/3 3 5
o f sh o rt-term grou nd w ater can be m easured by dye exp erim en ts. T h e am oun t o f this sh o rt-term grou nd w ater is alm ost alw ays greater than th at o f thé long-term
groundw ater.L on g-term grou nd w ater is a m ixtu re o f com p on en ts o f d ifferent ages.
T h e p rop ortion o f these old er com p on en ts in the long-term grou nd w ater decreases
exp on en tially w ith increasing age, because broad fissures are flow paths o f large
quantities o f youn g grou nd w ater and the sm aller ones o f sm aller am oun ts o f older
groundw ater. U nder stead y-state conditions betw een recharge and discharge, the
iso top e con cen tra tion B n o f long-term grou nd w ater o f m ean residence tim e (M R T )
is described by
The values fo r aj are the isotop e con cen tra tion s o f the rain-w ater o f the years j after A D 1 9 5 0 (in p u t curve). T h e initial 14C co n te n t o f the grou nd w ater
ranges from 1 0 0 to 50% -m o d em [7 , 13 ]. In th e case o f tritiu m , Aj is set at 100 .
T h e rad iocarb on in p u t curve can be taken from N ydal ( 1 7 ) . R egionally valid
tritiu m input values are obtainable from the IA E A , Vienna.
In ou r case stud y o f the B ard ai region, tw o grou nd w ater sam ples gave 14C
and 3H con ten ts w hich should be evaluated independently b y the exp on ential m odel (T ab le I) . Initial 14C con ten ts o f 9 0 % -m o d em and m ore have been found fo r grou nd w ater recharged in ca tch m e n t areas covered w ith sedim ents p o o r in
carb on ates [1 3 , 1 8 ]. This is the case in the T ibesti M ountains, w here the
conventional 14C ages e x ceed th e actual w ater ages by less than 8 5 0 years.T h e reliability o f the M R T estim ated by the exp on en tial m odel o f b oth the
rad iocarb on and tritiu m co n ten ts has been tested in various w ays:
(a ) The m ean residence tim e o f the long-term w ater o f perennial springs
has been found to be a fu n ction o f the recharge area F (k m 2), the p o ro sity o f the
aquifer P (% ), the thickness o f th e w ater-bearing form ation H (m ), and the m ean
annual discharge o f the long-term grou nd w ater MQ (m 3/s).
(j (n -1950)
A .1 0 0
BnM R T
1 -M R X
n
( 1 )
1951
P * H * F
R 3 X 15 * MQ( 2 )
T h e recharge rate R is given in % o f the annual rainfall.
3 6 GEYH
T A B L E I. 14C A N D 3H D A T A O F TW O S PR IN G -W A T ER SA M PLES FR O M
T H E B A R D A I R EG IO N A N D T H E C O R R ESPO N D IN G M R T V A L U E S
D E T E R M IN E D B Y T H E E X P O N E N T IA L M O D EL [6 ]
Sampling site Guelta (No. 9) Guelta (N o. 17)
Lab. number H v 4 3 3 0 Hv 432 7
14C content (%-modern) 101 89
3H content (TU ) 70 28
M RT (a) 35 5 0 - 1 0 0
Ai (%-modern) 90 85
1СГppm
юн
7 -
Z 6 ш >- Z
8 sh<a .ш 4 Z
3 -
2 -
M IG M A T IT E IN S E L B E R G
WEI
GN8SS4 у saitwater ¡ »rçüûûppm
10 20 30 40 50 60 70 80 90 yrs.
MEAN RESIDENCE TIME
FIG.3. Mineral contents (ppm) and MR T o f joint groundwater from the hard rock peneplain in NE Brazil [18].
IAEA-AG-158/3 37
T A B L E II. D IF F E R E N C E S IN T H E Y IE L D O F W E L L S IN N E B R A Z IL S IT E D
A CC O R D IN G TO TW O D IF F E R E N T C O N C EPT S [1 9 ]
Form er concept Wells in peneplain
New concept Wells at inselbergs
Number o f wells 67 32
Yield (m 3/h) 1.5 3
Depth (m) 78 55
Mineral content (ppm) 3 6 0 0 500
T h e validity o f E q . (2 ) has been tested m any tim es [1 4 ] and has been
utilized fo r estim ating p oro sity in u nexp lored , arid, karst regions from the 14C and 3H con ten ts o f som etim es only a single spring-w ater sample.
(b ) T he M RT o f the subsurface flow in the semi-arid peneplain o f N E Brazil
(annual p recipitation < 4 0 0 m m ) was found to be a function o f the m ineral
co n ten t o f the groundw ater [1 8 ] . This result (F ig . 3 ) revealed evidence for
present-day groundw ater recharge, forcing the geologists to m odify th eir form er
co n cep t for siting w ater wells [1 9 ] , w hich are now drilled at the fo o t o f the
inselbergs instead w ithin the peneplain. A s a result, the qualitative and quantitative yields o f the wells have n oticeably im proved (T able II). T he hydrogeological
estim ated groundw ater recharge rate , R , o f 3% corresponds to the observed M RT
o f 2 5 —4 0 years (E q . (2 ) ) .
2 .3 . Base-flow m odel
M R T values fo r h olok arst regions, i.e. old k arst regions, estim ated independently from the rad iocarb on and tritium con ten ts o f con du it spring w ater [2 0 ] ,
w ere frequently found to disagree w ith each oth er. T he reason is th a t such groundw ater is b e tte r described as a m ixtu re o f a young com p on en t, fo r w hich the
environm ental isotop e co n te n t can be estim ated by the exp on ential m odel, and
an old com p on en t (base flow ) o f an age o f several centuries o r millennia, and
free o f tritium .The 14C and 3H con ten ts o f the young com p on en ts ( 14C eXp and 3H exp)
w ere first estim ated using an assumed o r m easured M R T . This 3H exp was then
used to calculate the p rop ortion o f the base-flow com p on en t o f the sam ples q.
. 3HSpi^exp
(3)
3 8 GEYH
FIG.4. Content o f the base flow and its flow direction according to pumped samples from wells in the recharge area o f the mineral springs at Bad Canstatt, Fed. Rep. o f Germany [21 ].
t
The 14C co n te n t o f the base-flow com p on en t is given by
. ( 14Cspi — 14Cexp)14c bf = -------- --------------- C exp (4 )
A tte m p ts to apply th e base-flow m odel in ou r case study did n o t succeed because isotop e exchange w ith soil C 0 2 occu rred (see S ection 2 .4 .) . H ow ever, the application o f the base-flow m odel has given so m uch insight to the origin o f
the environm ental isotop e and h yd roch em ical com p osition o f w ell-w ater samples
th at the instructive case stud y o f the m ineral springs at Bad C a n sta tt, Fed eral
R epublic o f G erm any, should be discussed [21 ].T h e karst recharge area o f these m ineral springs is partly covered w ith a
lim ey and loam y sedim ent. T h e youngest w ell-w ater should have hydrogeologically
accep table M R T values betw een 5 and 4 0 years. A lthough isochrons could n o t be
delineated, w e have assumed an increase in the M R T o f 5 years in the SW to
4 0 years in the N E (F ig . 4 ) . This follow s from the hydrogeological con cep tion .T h e calcu lated flow velocity o f 3 5 0 to 4 0 0 m /a fo r this young com p on en t
fits the know n discharge o f the mineral springs. O n the o th er hand, accord ing
IAEA-AG-158/3 3 9
Q=1 (Us)
F I G . 5 . S c h e m a t i c p r e s e n t a t i o n o f t h e d i s t u r b a n c e o f t h e f l o w v e l o c i t i e s o f b o t h t h e y o u n g
c o m p o n e n t a n d t h e b a s e f l o w o w i n g t o p u m p i n g .
to the 14C d ata the base flow has a velocity betw een 0 . 7 - 4 . 7 m /a w hose direction
(derived from the h yd roisoch ron s) corresponds to the piezom etric lines.H ow ever, in apparent co n trast to the above-m entioned mass balance, the base-
flow com p on en t o f the w ater samples was determ ined to be 2 0 —60% instead o f
the 2 —5 %o th at was e x p e cte d from the ratio o f the flow velocity o f the base flow to th at o f the youn g com p on en t.
We reached the follow ing conclusion (F ig . 5 ) : G roun d w ater is pum ped o ff m ore o r less from the w hole aquifer (h ere ap p rox. 15 m ) w ith pum ping rates > 1 ltr/s. H en ce, the flow velocity at the surface o f a cylinder o f 5 m d iam eter and a rock p oro sity o f ap p roxim ately 3% already am ounts to vart > 2 2 0 m /a.As a result, the base flow becom es accelerated . In this case, the ratio o f the base flow to the young co m p o n en t is ap p roxim ated by the q u otien t o f Hbf and H exp
(F ig . 5 ).This result has im plications fo r the application o f environm ental isotopes
and h yd roch em ical d ata from pum ped w ater samples fo r determ ining the w ater
budget. A ccord in g to the vertical m ixtu re m odel [2 2 ] , the w idespread
im pression is th at the 14C grou nd w ater age reflects the young com p on en ts m ore
than the older ones. H ow ever, the reverse is true fo r w ater-budget considerations. A s the old w ater generally has a very small flow velo city , its p art in the mass
balance is over-em phasized by the 14C age o f pum ped w ater sam ples [2 0 ] .
40 GEYH
T A B L E III. 3H C O N T E N T O F W A T E R SA M PLES FR O M T H E W E L L O F T H E
R E S E A R C H STA TIO N B A R D A I B E F O R E AN D D U R IN G PUM PIN G [6 ]
Date State 3H content(TU )
2 April 1968 Unpumped [2 3 ] 322
7 Septem ber 1968 Start o f pumping [2 3 ] 2 - 3
5 January 1971 Continuous pumping [6 ] < 2
T A B L E IV . E X A G G E R A T E D 14C C O N T EN T IN W A T E R S A M PLES DRAW N
FR O M W E L L S S IT U A T E D IN T H E A L L U V IU M O F T H E B A R D A G U É R IV E R
U PS T R E A M AN D D O W N STREA M FR O M A N A T U R A L C A N Y O N B A R R IE R
IN T H E R IV E R BED
SampleNo. Hv
14C content (%-modern)
3H content (TU )
Location
8 4823 114 13 Upstream
1 0 4 8 2 4 1 1 1 35 Upstream
13 4825 96 1 2 Upstream
7 4 8 2 0 115 5 Downstream
15 48 2 7 1 2 0 < 2 Downstream
T A B L E V. V A R Y IN G 14C G R O U N D W A T ER A G ES W ITH IN C R E A S IN G D IS T A N C E FR O M B A R D A I IN T H E A L L U V IU M O F T H E B A R D A G U É R IV E R
Distance (km ) 0 2 23 63 80 87 113
14С age (b .p .)a 2 7 0 0 3 7 0 0 9 5 0 0 6 7 0 0 59 0 0 4 0 0 0 80 0 0
Well No. 6 18 - - - - -
a b.p. = before present.
IAEA-AG-158/3 41
D ifferences m ust be ex p e cte d betw een these and draw n samples. These differences
m ay be used to explain differing results b etw een radioactive and stable environm ental isotop e data.
It follow s th at the m easured, rapid disappearance o f tritiu m after the
com m en cem en t o f pum ping o f the well at the research station in Bardai
(T ab le III) m ay n o t be in terp reted as indicating a lack o f grou nd w ater recharge
as has been done in the past [6 ].
2 .4 . Isoto p e exch an ge betw een b icarbonate o f g rou nd w ater and soil C 0 2
W ater samples taken from wells situated in alluvium som etim es have a high
14C co n te n t and a low co n te n t, o r an absence, o f tritiu m [6 ] . T here are m any
exam ples o f this from the B ardai case stud y (T ab le IV ). In these cases we
alw ays found th a t grou nd w ater rises to the surface in the ro o t zone th rou gh ou t the year. T h e high 14C and C 0 2 con ten ts in the ro o t zone favour iso top ic
exchan ge w ith the b icarb on ate and the C 0 2 in the groundw ater. A t the same tim e, atm osp h eric o xy gen is dissolved [2 3 ] and salt crusts are form ed by evaporation o f grou nd w ater. A s a result o f this isotop e exch an ge, the 14C age
o f the grou nd w ater in the river valley does n o t increase steadily in direction
o f the river flow , as seen from ou r results o f the Bardai case study (T able V ).N o samples con tained tritiu m . A fte r tw o years w ith o u t rainfall, the 14C ground
w ater ages increased by 3 0 0 0 years.As the ru n -off recharge is the m ost im p o rtan t one in arid zones [2 4 ] , n either
radiocarbon n o r stable isotop e d ata fo r the grou nd w ater in alluvium allow
quantitative h ydrological in terp retation s.
2 .5 . H ydraulic (co n v e ctio n ) m odel
A t the end o f th e alluvium, the ru n -off grou nd w ater m ay b eco m e confined. W e w ould like to con sid er an aquifer o f thickness H 2 and p oro sity P 2 (F ig . 6 ).
It should be overlain by a p oorly perm eable layer (aq u itard e) o f a thickness H b
a p ore-w ater co n te n t P , and a coefficien t o f hydraulic perm eability K . Owing to assumed con tinu ou s recharge, the shallow grou nd w ater o f the u pper aquifer
should have a high and con stan t co n te n t o f radioactive environm ental isotopes in com parison w ith th at o f the confined groundw ater. A p art from radioactive
d ecay , tw o fu rth er physical processes m ay change the iso top ic com p osition o f
the confined groundw ater. These are:
(i) Diffusion: A ccord in g to F ic k ’s law , th ere m ust be a mass tran sport o f
radioactive isotopes from the shallow grou nd w ater w ith its high co n cen tration
to the confined one w ith its low co n cen tra tio n [2 5 ] . A s a result, the la tte r is
increased and the rad iom etric age o f the confined grou nd w ater appears to be to o
42 GEYH
OHWENCE UA-UPPERAQUFER
FIG. 6. Scheme o f an aquifer system consisting o f a shallow unconfined aquifer separated from a deeper confined aquifer by an aquitarde.
low . This case has to be considered if the shallow aquifer does n o t exist but
sporadic grou nd w ater recharge occurs in the region o f investigation. T h en , the pore w ater o f the upp erm ost p art o f the p oorly perm eable layer m ay have a high
rad ioactive isotop e co n te n t b u t there is n o h y d ro sta tic difference betw een this and the confined groundw ater.
(ii) Convection: T he radioactive isotop e tran sp o rt by diffusion becom es com p arab ly negligible if there is a h yd rosta tic pressure difference Д betw een the shallow and the confined grou nd w ater [5 ] . Provided th at the hydraulic head o f the confined groundw ater is higher than th at o f the shallow grou nd w ater, an
upw ards m ovem ent occu rs th rou gh the aquitarde. A hydraulic pressure difference
o f a few decim etres is sufficient to cancel com p letely the diffusion e ffe ct to the isotop e com p osition . In this case, the rad ioactive isotop e co n te n t o f the
confined grou nd w ater m ay be a fu n ction o f its actual age, second ary processes
exclud ed.
In the co n tra ry case, the dow nw ards m ovem ent o f the shallow grou nd w ater
w ith its high radioactive isotop e co n ten t increases the isotope co n ten t o f the
confined grou nd w ater by several orders o f m agnitude faster than diffusion
(F ig . 7 ) . T h e rad iom etric age approaches a m axim um value, w hich is a fu n ction
o f th e hydraulic pressure d ifference Л and the geom etric and hydraulic
properties o f the w hole aquifer system . E ven thick and very p oorly perm eable
layers do n o t p ro te c t the confined aquifer against a n oticeable input o f shallow
grou nd w ater over periods o f millennia. H en ce, the hyd roch em ical and stable
isotop e com p osition o f the con fin ed grou nd w ater also becom es n oticeably
influenced by the ad m ixtu re o f shallow groundw ater. Its co n te n t slowly
IAEA-AG-158/3 43
FIG. 7. Relationship between apparen t14 С ages and actual ages o f groundwater due to the convection fo r a special case study.
approaches 100% -m od em . O f course, exchan ge processes m ay also change the
h yd roch em ical and stable isotop e com p osition o f the shallow grou nd w ater while
passing through the aquitarde.T h e general observation th at the m ineral co n te n t o f confined grou nd w ater
increases w ith its age could be a result o f the con tinu ou s ad m ixtu re o f shallow groundw ater. A ccord in g to Bögli [ 2 6 ] a m ixing o f g rou nd w ater w ith different
tem p eratu res, C 0 2 , o r b icarbonate con ten ts , d istorts the h yd roch em ical equilibrium and results in a chem ical a tta ck on the ro ck in the aquifer.
T he hydraulic (co n v e ctio n ) m odel m ust be utilized if I4C d ata o f old groundw ater taken from wells situated in the S ahara desert is to be in terp reted paleo-
hydrogeologically. In m any cases, n o t the paleohydrogeology b u t the h yd ro-
d ynam ic situ ation during the past m ay be reflected .
2 .6 . M ining m od el
Confined grou n d w ater resources are being increasingly exp loited in arid
zones fo r irrigation w ater. A t the beginning o f the pum ping o f a well, a ground
w ater cone is form ed w hich quickly low ers the hyd rau lic head. L a te r , depletion rates o f 0 .5 m /a and m ore are com m on . A ccord in g to the co n vection m odel,
44 GEYH
-------------------->►GROUNDWATER WITHDRAWALS]
F I G . 8 . C h a n g e s o f t h e 14C w a t e r a g e s in a c o n f i n e d a q u i f e r o w i n g t o g r o u n d w a t e r m i n i n g a s
a f u n c t i o n o f t h e d u r a t i o n o f g r o u n d w a t e r w i t h d r a w a l a n d d i f f e r e n t d e p l e t i o n r a t e s o f t h e
h y d r o s t a t i c p r e s s u r e c o n s i d e r e d f o r a s p e c i a l s i t u a t i o n .
w ater from the shallow aquifer will com m ence to p enetrate the aq uitarde and eventually en ter the confined aquifer [5 ]. A t this m om en t, the radioactive
isotope co n ten t will rise and accordingly the grou nd w ater ages will decrease w ith increasing pum ping tim e (F ig . 8 ) . The fall o f the confined grou nd w ater age is a fu n ction o f the depletion rate o f the hyd rau lic head and the geo m etric and hydraulic properties o f the w hole aquifer system (F ig . 6 ) . T h erefo re , the change
in the confined grou nd w ater age w ith tim e m ay be used to check the assum ptions
on the hydraulic param eters o f the aquifer system .
3. CO N C LU SIO N
V arious h yd rod yn am ic aspects have been discussed in ord er to show their
im plications fo r the hydrogeological in terp retation o f the com p osition o f radioactive and stable environm ental isotopes in groundw ater. I t becam e obvious
th at these aspects have a sim ilar if n ot greater im p ortan ce than h yd roch em ical
p rocesses, isotope exchan ge and isotope fractio n atio n . A s the rad iom etric ages
IAEA-AG-158/3 45
o f old grou nd w ater are particu larly affected , the h yd rod ynam ic aspects require special a tten tio n to environm ental isotope and h yd roch em ical groundw ater studies in arid and semi-arid zones.
A C K N O W LED G EM EN T
I am grateful to Dr. A .H . B ath fo r providing the U nited K ingdom tritium
input data.
R E F E R E N C E S
WENDT, I., STAH L, W., G EY H , M., FAUTH , F ., “ Model experiments for 14C water-age determinations” , Isotopes in Hydrology, (Proc. Symp. Vienna, 1966), IA EA , Vienna(1 9 6 7 ) 3 2 1 - 3 7 .W IG LEY , T .M .L ., Carbon-14 dating of groundwater from closed and open systems, Water Resour. Res. 11 (1 9 7 5 ) 3 2 4 - 2 8 .
[ 2
[3
[4
[5
[ 6
V
[ 8
[9
[ 1 0
[ 1 1
[ 1 2
[13
[14
[15
MOOK, W.G., “The dissolution-exchange model for dating groundwater with 14C” , Interpretation o f Environmental Isotope and Hydrochemical Data in Groundwater Hydrology (Proc. Panel Vienna, 1975), IAEA, V ien n a (1 9 7 6 ) 2 1 3 —25.W A LLIK, E .I., TOTH, J . , “ Methods o f regional groundwater flow analyses with suggestions for the use o f environmental isotopes” , ibid., pp. 3 7 —64.G EYH , M.A., BACKHAUS, G ., “Hydrodynamic aspects o f 14C groundwater dating” , Isotope Hydrology 1978 (Proc. Symp. Neuherberg, 1978) 2, IA EA , Vienna (1 9 7 8 ) 6 3 1 —43. G EYH , M.A., O BEN A U F, K .-Р., Zur Frage der Neubildung von Grundwasser unter ariden Bedingungen, Pressedienst W issenschaft, Freie-Universität-Berlin 5 (1 9 7 4 ) 7 0 —91. MÜNNICH, K .O ., Isotopen-Datierung von Grundwasser, Naturwissenschaften 34(1 9 6 8 ) 3 2 - 3 3 .TA M ERS, M.A., Validity o f radiocarbon dates on groundwater, Geophys. Surv. 2 (1 9 7 5 ) 2 1 7 - 3 9 .G EYH , M.A., “ Carbon-14 concentration o f lime in soils and aspects o f the carbon-14 dating o f groundwater” , Isotope Hydrology 1970 (Proc. Symp. Vienna, 1970), IAEA, Vienna (1 9 7 0 ) 2 1 5 - 2 3 .SALOMONS, W., MOOK, W.G., Isotope geochemistry o f carbonate dissolution and reprecipitation in soils, Soil Sei. 122 (1 ) (1 9 7 6 ) 15—24.R IG H TM IRE, C .T ., Seasonal variations in P^o and 13C content o f soil atmosphere,Water Resour. Res. 14 (4 ) (1 9 7 8 ) 6 9 1 —92.G EYH , M.A., “ On the determination o f the initial I4C content in groundwater” , Proc.V III Int. Conf. on Radiocarbon Dating, Wellington ( 1972) D 5 8 - 6 9 .G EYH , M.A., Basic studies in hydrology and C-14 and H-3 measurements, Proc. X X IV Int. Geol. Congr., Montreal (1 9 7 2 ) Section 11, 2 2 7 —34.G EYH , M.A., GRO SCH O PF, P., Isotopenphysikalische Studie zur Karsthydrologie der Schwäbischen A lb, Jh . Geol. Landesamt Baden-Württemberg 8 (1 9 7 8 ) 7 —58.W IG LEY , T .M .L ., E ffect o f mineral precipitation on isotopic com position and 14C dating of groundwater, Nature (Lond .) 263 (1 9 7 6 ) 2 1 9 —21.
46 GEYH
[16] BATH, A.H., EDMUNDS, W.M., ANDREW S, J.N ., “Palaeoclimatic trends deduced from the hydrochemistry o f the Triassic sandstone aquifer, United Kingdom” , Isotope Hydrology 1978 (Proc. Symp. Neuherberg, 1978) 2 , IAEA, Vienna (1 9 7 8 ) 5 4 5 —68.
[1 7 ] N YDAL, R ., personal communication.. [1 8 ] G EY H , M.A., K R E Y SIN G , K ., Sobre a idade das aguas subterráneas no polígono des
secas do Nordeste Brasiliero, Rev. Bras. Geoscie. 3 (1 9 7 3 ) 5 3 —59.[19] K R E Y SIN G , K ., LENZ, R ., M Ü LLER, W., Problems of groundwater exploration in
m etamorphic rocks for domestic water supplies in northeastern Brazil, Proc. X X IV Int. Geol. Congr., Montreal (1 9 7 2 ) Section 1 1, 7 3 —79.
[2 0 ] SH U STER, E .T ., W HITE, W .B., Seasonal fluctuations in the chemistry o f limestone springs: A possible means for characterizing carbonate aquifers, J . Hydrol. 14 (1 9 7 1 ) 9 3 - 1 2 8 .
[2 1 ] G EY H , M.A., KÖ HLE, H., In preparation.[22] VO G EL, J .C ., “Carbon-14 dating of groundwater” , Isotope Hydrology 1970 (Proc.
Symp. Vienna, 1970), IA EA , Vienna ( 1970) 2 2 5 —37.[23] SIEG EN TH A LER, U., SC H O TTER ER , U., O ESCH G ER, H., M E SSER LI, B ., Tritium-
messungen and Wasserproben aus der Tibesti-Region, Hochgebirgsforsch. (High Mountain Res.) 2 (1 9 7 2 ) 1 5 3 -5 9 .
[24 ] D IN ÇER, T ., “ The use o f environmental isotopes in arid zone hydrology” , these Proceedings.
[25 ] K LITZ SC H , E ., SONNTAG, C., W E IST R O FF E R , K ., EL SH A ZLY, E.M ., Grundwasser der Zentralsahara: Fossile Vorräte, Geol. Rundsch. 65 (1 9 7 6 ) 264 .
[2 6 ] BÖ G LI, A ., Mischungskorrosion — ein Beitrag zum Verkarstungsproblem, Erdkunde 18 (1 9 6 4 ) 8 3 - 9 2 .
IAEA-AG-158/4
A G E O C H E M I C A L A N D IS O T O P IC A P P R O A C H
T O R E C H A R G E E V A L U A T I O N
IN S E M I -A R I D Z O N E S
P a s t a n d p re s e n t
W.M. ED M U N D S, N .R .G . W A LTO N
In stitu te o f G eological Sciences,
W allingford, O xford sh ire,
U nited K ingdom
Abstract
A GEOCHEM ICAL AND ISO TO PIC APPROACH TO REC H A RG E EV A LU A TIO N IN SEMI- ARID ZONES: PAST AND PRESEN T.
The magnitude o f any recharge to aquifers in semi-arid and arid zones is the principal uncertainty in estimating a water balance. Recent studies in Cyprus and Libyan Arab Jam ahiriya are currently being used to demonstrate the application o f geochem ical and isotopic techniques, to the determ ination o f both current and palaeo-recharge. In Cyprus, solute profiles o f the unsaturated zone have been interpreted to provide estimates o f the direct recharge component using a steady-state, mass-balance approach; results from the chloride profiles compare well with recharge estimates using tritium . In addition, it is found that some solute peaks, notably for specific electrical conductance, give a reasonably accurate record o f the rainfall history during the period 1 9 5 0 —1975 . The solute profile method is relatively unsophisticated and' could be more widely applied to recharge estim ation in other semi-arid areas o f the world. In Libya, a clear distinction can be made using the combined iso topic, hydrological and geochem ical results between regional groundwaters recharged to the upper, unconfined aquifer o f the Sirte Basin before 13 0 0 0 years BP and younger waters recharged locally during the period 5 0 0 0 —7 8 0 0 years BP. A well-defined fresh-water channel, superimposed upon the regional water quality pattern, can be traced within the aquifer for some 1 3 0 km and represents d i r e c t evidence o f recharge during the Holocene. Some shallow groundwaters o f similar com position to the fresh-water channel are also considered to represent recent, if interm ittent, recharge which took place during historical times. It is concluded that geochem ical and iso topic studies o f both the unsaturated zone and o f shallow groundwaters in semi-arid regions, can be used to determine not only the present-day direct recharge component, but also a recharge chronology o f immediate historic times, which may be im portant in the estimation o f long-term water resources.
I N T R O D U C T I O N
' N a t u r a l r e p l e n i s h m e n t o f g r o u n d w a t e r r e s e r v o i r s i n a r i d z o n e s c a n t a k e
p l a c e b y t w o m e c h a n i s m s - (i) d i r e c t i n f i l t r a t i o n o f r a i n w a t e r v i a t h e s o i l
a n d u n s a t u r a t e d z o n e a n d (ii) l o c a l r e c h a r g e o f s u r f a c e r u n o f f v i a p e r m e a b l e
w a d i b e d s o r d r a i n a g e s y s t e m s . T h e f l a s h y a n d u n p r e d i c t a b l e n a t u r e o f r a i n
f a l l e v e n t s i n s e m i - a r i d a n d a r i d z o n e s m a k e s t h e a c c u r a t e d e t e r m i n a t i o n o f
r a i n f a l l a m o u n t s a n d o f s u r f a c e r u n o f f a l m o s t i m p o s s i b l e . T h e p r o b l e m o f
4 7
48 EDMUNDS and WALTON
d e t e r m i n i n g a q u i f e r r e c h a r g e is f u r t h e r c o m p o u n d e d b y t h e d i f f i c u l t y in
m e a s u r i n g é v a p o t r a n s p i r a t i o n w i t h a n y a c c u r a c y . A d d t o t h i s t h e i n f i n i t e
v a r i e t y i n so i l s , t o p o g r a p h y , g e o l o g y a n d l a n d u s e a n d t h e p r o b l e m s of
d e t e r m i n i n g r e c h a r g e in s e m i - a r i d z o n e s a r e p l a c e d in p e r s p e c t i v e .
T h e m a g n i t u d e o f t h e n a t u r a l r e c h a r g e c o m p o n e n t is g e n e r a l l y th e
l a r g e s t u n c e r t a i n t y in w a t e r b a l a n c e c a l c u l a t i o n s in t h e a r i d z o n e s . M a n y
m a j o r g r o u n d w a t e r d e v e l o p m e n t s c h e m e s a r e p r o c e e d i n g w i t h t h e u n c e r t a i n t y
a s t o w h e t h e r n a t u r a l r e p l e n i s h m e n t is t a k i n g p l a c e a t all; o t h e r s c h e m e s
a r e b e i n g d e v e l o p e d o n t h e c o n c e p t o f ' g r o u n d w a t e r m i n i n g ' , w h i c h is p r o
b a b l y t h e o n l y s a f e b a s i s o f e x p l o i t a t i o n i n t h e a b s e n c e o f g o o d r e c h a r g e
f i g u r e s .
C o n v e n t i o n a l m e t h o d s o f c a l c u l a t i n g t h e d i r e c t r e c h a r g e c o m p o n e n t
(Penman, [l-2] , M o n t e i t h , [3] ) h a v e l i m i t a t i o n s i m p o s e d u p o n t h e m
w h e n a p p l i e d t o a r i d a n d s e m i - a r i d zo n e s , n o t o n l y b y t h e p r o b l e m o f t h e
a c c u r a c y o f t h e m e a s u r e d p a r a m e t e r s , b u t p o s s i b l y e v e n b y t h e m e t h o d
i t s e l f . L y s i m e t e r s h a v e b e e n u s e d s u c c e s s f u l l y t o m e a s u r e d i r e c t r e c h a r g e
( K i t c h i n g e t al., [4]). I n a d d i t i o n to t h e l o g i s t i c s a n d c o s t o f i n s t a l
l a t i o n i n r e m o t e a r e a s , t h e y p r e s e n t p r o b l e m s o f d i s t u r b i n g t h e t e r r a i n
d u r i n g c o n s t r u c t i o n a n d a l t h o u g h i n t e g r a t i o n o v e r a s i z e a b l e a r e a c a n b e
a c h i e v e d , t y p i c a l l y u p t o 9 m 2 , p r o b l e m s o f r e p r e s e n t a t i v e n e s s o f t h e r e
s u l t s o v e r a w i d e c a t c h m e n t a r e a s t i l l r e m a i n . I n d e e d , t h i s l a s t p o i n t is
p r o b a b l y t h e m o s t d i f f i c u l t f a c t o r i n a n y r e c h a r g e c a l c u l a t i o n .
T r i t i u m p r o f i l e m e t h o d s d e v e l o p e d b y Z i m m e r m a n e t a l .[5], Miinnich
e t al. [б] , S m i t h e t al. [7] , A n d e r s e n a n d S e v e l , ["g] , a m o n g o t h e r s ,
h a v e b e e n f o u n d a p p l i c a b l e to r e c h a r g e e s t i m a t i o n in v a r i o u s t e m p e r a t e
z o n e e n v i r o n m e n t s . T h e i r p r i n c i p a l a d v a n t a g e is t h e i n t e g r a t i o n o f i n
f o r m a t i o n f r o m a l a r g e t i m e s e q u e n c e , (as r e p r e s e n t e d b y t h e v e r t i c a l
i n t e r v a l ) a n d a r e a l e x t e n t , t h r o u g h r e p e a t e d p r o f i l e s m a d e o v e r a w i d e
g e o g r a p h i c a l area. S e v e r a l s t u d i e s h a v e n o w d e m o n s t r a t e d t h a t t r i t i u m
p r o f i l e s c a n b e u s e d i n s e m i - a r i d z o n e s i n c l u d i n g I n d i a ( S u k h i j a a n d
S h ah, [9 ])/ A u s t r a l i a ( A l l i s o n a n d H u g h e s , [ l o j ) an<3 S a u d i A r a b i a
( D i n ç e r e t al., [ i l ] ) .
W h i l s t t r i t i u m h a s b e e n s u c c e s s f u l i n c u r r e n t r e c h a r g e e v a l u a t i o n ,
o t h e r i s o t o p i c m e t h o d s , n o t a b l y th e c o n j u n c t i v e u s e o f c a r b o n , o x y g e n a n d
h y d r o g e n s t a b l e i s o t o p e r a t i o s , t o g e t h e r w i t h c a r b o n - 1 4 , h a v e p r o v e d
v a l u a b l e in d e m o n s t r a t i n g t h e e x i s t e n c e o f f o s s i l g r o u n d w a t e r and, in
f a v o u r a b l e c i r c u m s t a n c e s , i n p r o v i d i n g a n a p p r o x i m a t e g r o u n d w a t e r age.
S u c h s t u d i e s , u s u a l l y s h o w i n g t h a t s i g n i f i c a n t r e c h a r g e h a s n o t o c c u r r e d in
r e c e n t ti m e s , h a v e b e e n p a r t i c u l a r l y u s e f u l i n a r i d z o n e w a t e r r e s o u r c e
a p p l i c a t i o n s . H o w e v e r , m o s t i s o t o p e s t u d i e s i n a r i d z o n e s h a v e ' t e n d e d
t o d e a l w i t h r e g i o n a l r a t h e r t h a n l o c a l r e l a t i o n s h i p s . I n s u c h s t u d i e s ,
p u m p e d s a m p l e s a r e l i k e l y to r e p r e s e n t '.age m i x t u r e s ' a n d a t s h a l l o w
d e p t h s a m b i g u o u s r e s u l t s , o f t e n w i t h h i g h % m o d e r n c a r b o n , a r e p a r t i c u l a r l y
d i f f i c u l t t o i n t e r p r e t .
I t is l i k e l y h o w e v e r , t h a t r e c h a r g e i n p r e s e n t d a y a r i d z o n e s h a s b e e n
c o n t i n u i n g , a l t h o u g h i n t e r m i t t e n t , p r o c e s s d u r i n g t h e H o l o c e n e a n d t h a t
c a r e f u l f i e l d stu d y , c o m b i n e d w i t h i s o t o p i c a n d g e o c h e m i c a l a n a l y s i s , c a n
b e u s e d to i n v e s t i g a t e t h is. E v i d e n c e o f s i g n i f i c a n t , if i s o l a t e d , r e c h a r g
d u r i n g t h e p a s t 5 0 0 o r 1 , 0 0 0 y e a r s f r o m c a t a s t r o p h i c r a i n f a l l e v e n t s w o u l d
b e o f c o n s i d e r a b l e s i g n i f i c a n c e in e v a l u a t i n g t h e l o n g e r t e r m r e s o u r c e s o f
a r i d ar e a s .
IAEA-AG-158/4 49
T h e p u r p o s e o f t h i s p a p e r is f i r s t l y , t o i n v e s t i g a t e i s o t o p i c a n d
h y d r o g e o l o g i c a l e v i d e n c e t h a t r e c h a r g e h a s o c c u r r e d a t s u c c e s s i v e i n t e r v a l s '
d u r i n g t h e H o l o c e n e , u s i n g e x a m p l e s f r o m L i b y a ; a n d s e c o n d l y , t o d i s c u s s
t h e a p p l i c a t i o n o f g e o c h e m i c a l t e c h n i q u e s f o r t h e e s t i m a t i o n o f c u r r e n t
r e c h a r g e u s i n g r e s u l t s f r o m a c o n t i n u i n g s t u d y in C y p r u s . F u r t h e r m o r e , i t
is i n t e n d e d to e x a m i n e t h e p o s s i b i l i t y o f l i n k i n g t h e s e m e t h o d s o f i n v e s
t i g a t i n g c u r r e n t a n d p a l a e o - r e c h a r g e w i t h a v i e w t o d e t e r m i n i n g t h e h i s t o r i c
r e c h a r g e w h i c h is i m p o r t a n t f o r l o n g e r t e r m w a t e r r e s o u r c e e s t i m a t i o n s .
1. E V I D E N C E F R O M L I B Y A O F S U C C E S S I V E R E C H A R G E E V E N T S D U R I N G T H E L A T E
P L E I S T O C E N E A N D H O L O C E N E .
D e t a i l e d h y d r o g e o l o g i c a l s t u d i e s in t h e S i r t e a n d K u f r a b a s i n s in
L i b y a h a v e p r o v i d e d a n o p p o r t u n i t y t o c a r r y o u t i s o t o p i c a n d g e o c h e m i c a l
i n v e s t i g a t i o n s o f g r o u n d w a t e r s , b o t h o n a r e g i o n a l a n d o n a l o c a l s c a l e
(W right e t al., [l2] , E d m u n d s a n d W r i g h t , [l3] , B e n f i e l d a n d W r i g h t , [l4]).
G e o c h e m i c a l s t u d i e s h a v e b e e n u s e d t o d e f i n e l i m i t s o f p o t a b l e w a t e r ,
g r o u n d w a t e r s o u r c e a r e a s a n d t h e c o n n e c t i o n b e t w e e n i n d i v i d u a l a q u i f e r s
a n d b a s i n s as w e l l a s i n v e s t i g a t i n g t h e r e c h a r g e h i s t o r y . V a s t r e s e r v e s
o f f r e s h g r o u n d w a t e r h a v e b e e n p r o v e d in a q u i f e r s w i t h i n t h e t w o b a s i n s
w h i c h a r e s t o r e d i n c l a s t i c s e d i m e n t s o f C r e t a c e o u s a n d T e r t i a r y age.
T h e o v e r a l l g r o u n d w a t e r m i n e r a l i s a t i o n i n t h e P o s t M i d d l e M i o c e n e
(PMM) a q u i f e r o f t h e S i r t e B a s i n is r e p r e s e n t e d i n F i g u r e 1 in w h i c h
i s o c h l o r s , d r a w n f r o m o v e r 2 0 0 d a t a p o i n t s , d e f i n e t h e a v e r a g e w a t e r
q u a l i t y d i s t r i b u t i o n i n t h i s u p p e r , u n c o n f i n e d a q u i f e r s y s t e m . A d i s t i n c t
b o d y o f n o n - s a l i n e w a t e r e x t e n d s t h r o u g h o u t t h e a r e a , i n w h i c h t h e t o t a l
m i n e r a l i s a t i o n i n c r e a s e s , a l t h o u g h w i t h o u t a n y c h a n g e in i o n i c r a t i o s ,
f r o m 1 , 0 0 0 - 2 , 0 0 0 m g / 1 s o u t h t o n o r t h a l o n g t h e f l o w line. S u p e r i m p o s e d
u p o n t h i s g e n e r a l p a t t e r n t h e r e e x i s t s a s i g n i f i c a n t f r e s h w a t e r l e n s
(Cl < 50 m g /1) w h i c h is a t l e a s t 7 0 m t h i c k a n d w h i c h t r a n s e c t s t h e r e
g i o n a l w a t e r q u a l i t y c o n t o u r s S E o f J a l u o a s i s . T h i s f e a t u r e c a n be
t r a c e d as a b r o a d c h a n n e l a b o u t 1 0 k m w i d e f o r s o m e 1 3 0 k m a n d is c o n
s i d e r e d t o b e a s u p e r i m p o s e d f e a t u r e c r e a t e d b y r e c h a r g e f r o m a f o r m e r
w a d i , w h i c h is i l l u s t r a t e d i n c r o s s - s e c t i o n i n F i g u r e 2. It is a p p a r e n t
f r o m t h e r a t h e r l i m i t e d n u m b e r o f s h a l l o w w e l l s e x i s t i n g i n t h e r e g i o n ,
t h a t l o w s a l i n i t y w a t e r a l s o e x i s t s e x t e n s i v e l y as a t h i n l a y e r (< 1 0 m)
a t t h e w a t e r ta b l e ; t h i s l e s s s a l i n e w a t e r is a l s o c o n s i d e r e d t o r e p r e s e n t
r e c h a r g e f r o m o n e o r m o r e l a t e - s t a g e e v e n t s .
T o i n v e s t i g a t e t h e s e a n o m a l i e s i n t h e b a s i c w a t e r q u a l i t y p a t t e r n in
m o r e d e t a i l , i s o t o p i c a n a l y s e s w e r e m a d e i n c l u d i n g ^H, 6 2H, 6 13C a n d 6^**C
f o r w h i c h r e p r e s e n t a t i v e r e s u l t s a r e g i v e n in T a b l e 1 (the o t h e r s b e i n g
g i v e n in f u l l i n E d m u n d s a n d W r i g h t , [l3]). T r i t i u m r e s u l t s f o r t h e s h a l l o w
w e l l s a r e 0±2 T U a n d i n d i c a t e t h a t c u r r e n t r e c h a r g e is i m p r o b a b l e , a l t h o u g h ,
as s h o w n b y t h e r e s u l t s f r o m C y p r u s , d i s c u s s e d b e l o w , a n y r e c e n t r a i n f a l l
m a y w e l l b e s t o r e d w i t h i n t h e u n s a t u r a t e d zone.
T h e c a r b o n i s o t o p e r e s u l t s f a l l i n t o t w o g r o u p s :-
(a) t h o s e w i t h l o w ^ C a c t i v i t y r a t i o s ( b e l o w 4% m o d e r n ) a n d w i t h s l i g h t l y
e n r i c h e d 6 1 C c o n t e n t s S - 7 to - 1 1 °/„) , c o r r e s p o n d i n g t o t h e c h e m i c a l l y -
s i m i l a r r e g i o n a l , n o n - s a l i n e g r o u n d w a t e r s
(b) t h o s e w i t h m o d e r n li(c a c t i v i t y r a t i o s ( b e t w e e n 35 a n d 5 5 % m o d e r n ) a n d
w i t h c o n s i d e r a b l y e n r i c h e d 6 ^ C (-3 t o -5° /J w h i c h c o r r e s p o n d t o t h e .
l o w s a l i n i t y g r o u n d w a t e r s w i t h i n t h e f r e s h w a t e r c h a n n e l a n d a t s h a l l o w
d e p t h s , r e p r e s e n t e d b y t h e f i r s t t w o g r o u p s i n T a b l e 1.
50 EDMUNDS and WALTON
F I G . l . W a t e r q u a l i t y m a p o f t h e c e n t r a l S i r t e B a s i n a n d K u f r a , L i b y a n A r a b J a m a h i r i y a ,
s h o w i n g l o c a l i t i e s r e f e r r e d t o in t h e t e x t .
T w o s e p a r a t e m o d e l s w e r e n e c e s s a r i l y c o n s i d e r e d f o r t r e a t m e n t o f t h i s
b a s i c d i v i s i o n o f t h e d a t a t o a t t e m p t a d e r i v a t i o n o f age. T h e f i r s t is
b a s e d o n a c l a s s i c a l a p p r o a c h t o c a r b o n a t e d i s s o l u t i o n , w h i l s t t h e s e c o n d
t a k e s i n t o a c c o u n t s o m e o f t h e l i m i t a t i o n s i m p o s e d b y t h e s e m i - a r i d z o n e
e n v i r o n m e n t .
I s o t o p i c r e s u l t s f r o m t h e l o c a l e n v i r o n m e n t w e r e f i r s t u s e d w h e r e
p r a c t i c a b l e t o p r o v i d e a r e a l i s t i c c o n t r o l o n t h e i n t e r p r e t a t i o n o f t h e
c a r b o n a c q u i s i t i o n b y t h e g r o u n d w a t e r . T h e P M M a q u i f e r is p r e d o m i n a n t l y
f l u v i a t i l e , a n d m a r i n e c a r b o n a t e is c o n s i d e r e d t o b e v i r t u a l l y a b s e n t ;
IAEA-AG-158/4 51
F I G . 2 . H y d r o l o g i c a l m o d e l t o i l l u s t r a t e t h e p r o b a b l e d e v e l o p m e n t o f t h e f r e s h w a t e r c h a n n e l ,
( i ) M a j o r r e c h a r g e p e r i o d - c o r r e s p o n d s t o r e g i o n a l , l a t e P l e i s t o c e n e a c c u m u l a t i o n o f g r o u n d
w a t e r ; ( i i ) A r i d p h a s e - c h a r a c t e r i z e d b y l o w r a i n f a l l , l i t t l e o r n o r e c h a r g e a n d c a l c r e t e
a c c u m u l a t i o n ; ( i i i ) L a t e - s t a g e w e t p h a s e - s i g n i f i c a n t r e c h a r g e t a k e s p l a c e r e p r e s e n t e d b y
g r o u n d w a t e r h a v i n g C l~ < 2 5 0 m g / l ; ( i v ) P r e s e n t d a y - a l i t t l e o r n o r e c h a r g e ; s i t u a t i o n
c o m p a r a b l e t o ( i i ) .
Lf\to
TABLE 1 . STABLE ISOTOPE AND RADIOCARBON RESULTS FOR GROUNDWATERS AROUND JA L U , L IB Y A , THE LOCATIONS OF WHICH ARE INDICATED IN FIGURE 1 .
S IT ENO.
(MAP)
APPROX SAMPLE DEPTH BELOW WATER TABLE (XoSMCW)
F IE L DPH HCO3
(m g/ I)
S I C CALCITE SATURATION 61 3C
INDEX
С UNCORRECTED-(% MODERN) AGE
CORRECTED AGE I
GROUNDWATER IN FRESHWATER CHANNEL
1 E l - 1 0 5 1 / 7 3
SHALLOW GROUNDWATER FROM JALU AREA
2 JALU ( 0 . FADEL) 1 2 / 7 3
3 JALU (A . RE JA B ) 1 2 / 7 3
4 GATMIR 1 2 / 7 3
2 - 4
2 - 4
2 - 4
- 8.6 - 6 7 7 . 6 8 2 8 4 + 0 .0 8 - 3 . 6 3 7 . 6 7 8 5 0 NO F I T
- 6.8 - 6 0 7 . 9 9 8 + 0 .6 1 - 3 . 2 5 1 . 2 5 3 8 0 NO F I T
- 7 . 1 - 7 0 7 . 9 9 8 + 0 .2 5 - 5 . 2 3 9 . 1 7 5 4 3 NO F I T
- 0 . 5 - 3 4 8.1 3 9 5 +0. 7 5 - 4 . 9 5 3 . 6 5 0 1 0 NO F I T
REGIONAL GROUNDWATERS MAINLY FROM INTERMEDIATE DEPTH
5 D -1 0 2 1 / 7 3 1 7 - 2 1 - 8.1 - 7 2 7 . 5 3 1 2 5 + 0 .3 3 - 6.2 5 . 4 2 3 4 0 0 9 3 5 0
6 JA -P 3 / 7 3 2 5 - 7 3 - 6.8 - 7 6 7 . 6 5 2 5 4 + 0 .3 9 - 7 . 2 1.2 3 5 7 6 0 2 4 1 0 0
7 W 81-103A 1 / 7 3 6 3 - 1 0 3 - 8.6 - 7 4 7 . 3 3 1 9 5 +0.10 - 7 . 1 - 1.6 3 3 3 0 0 2 1 5 0 0
8 W 52-103D 3 / 7 2 6 6 - 1 0 3 - 9 . 0 - 7 4 7 . 2 9 1 3 5 - 0 . 0 9 - 6.0 1 . 6 3 3 2 2 0 1 8 5 0 0
9 J D -P 1 2 / 7 2 3 2 - 8 2 - 8 . 3 - 7 3 7 . 4 1 1 8 6 + 0 .2 2 - 6 . 4 3 . 4 2 7 2 1 0 1 3 8 5 0
1 0 J E - P 1 / 7 3 2 3 - 7 3 - 8 . 5 - 7 0 7 . 6 7 1 9 2 + 0 .4 0 - 5 . 9 4 . 8 2 4 3 5 0 9 1 0 0
1 1 W 1 5 3 -5 9 E 8 / 7 1 5 0 - 9 4 - 8 . 7 - 7 4 7 . 6 8 1 1 7 - 0 . 3 3 - 7 . 0 2 . 8 2 8 6 3 0 1 6 4 0 0
1 4 T / F F I - 6 5 P 1 0 / 7 3 7 8 - 1 2 9 - 9 . 5 - 7 8 7 . 4 3 ’ 2 8 9 + 0 .2 8 - 6 . 8 0 . 7 3 9 8 5 0 2 7 7 0 0
1 5 T / U I-6 5 P 2 9 / 7 3 5 9 - 1 1 9 - 9 . 6 - 7 9 7 . 6 3 3 5 1 + 0 .3 9 - 1 0 . 7 0 . 7 > 4 0 3 6 0 3 3 8 0 0
1 6 T / T 2 -6 5 P 1 1 / 7 3 6 3 - 1 1 3 - 1 0 . 5 - 8 0 7 . 3 1 2 7 -Ю .1 1 - 1 1 . 7 0 . 7 > 4 0 3 6 0 3 4 3 6 0
EDM
UN
DS
and W
ALT
ON
IAEA-AG-158/4 5 3
t h e c a r b o n a t e c a r b o n s o u r c e is l i k e l y t o b e s o i l c a r b o n a t e o r c a l c r e t e
w h i c h h a d 6 I3C = - 2 . 6 t o - 4 . 1 % 0. A m e a n i n g f u l v a l u e f o r b i o g e n i c o r s o i l
C O 2 w a s m u c h m o r e d i f f i c u l t to o b t a i n i n v i e w o f t h e p r o b l e m o f e x t r a p o l a
t i o n t o f o r m e r v e g e t a t i o n c o n d i t i o n s ; i n t h e c a l c u l a t i o n , a v a l u e o f
6 = - 2 1 . 5 % 0 w a s u s e d - t h i s w a s a n a v e r a g e v a l u e , m e a s u r e d d u r i n g t h e study,
f o r m o d e r n v e g e t a t i o n a n d w o o d p r e s e r v e d i n s o i l h o r i z o n s ( r a n g e <S13C =
- 2 0 . 3 to - 2 7 . 3 %o) . U s i n g t h e s e d a t a , t h e o v e r a l l i s o t o p i c e v o l u t i o n w a s
m o d e l l e d u s i n g t h e W A T E Q F - I S O T O P p r o g r a m d e v e l o p e d b y R e a r d o n a n d F r i t z
! 1 5 1 f r o m t h e o p e n a n d c l o s e d s y s t e m d i s s o l u t i o n m o d e l o f D e i n e s e t al.,
J 1 6 J . In t h i s p r o g r a m , a v a r i e t y o f p o s s i b l e o p e n - s y s t e m p H a n d рСОг
v a l u e s a r e c a l c u l a t e d f r o m t h e o b s e r v e d d a t a . T h e 6 13C o f t h e e q u i l i b r i u m
C O 2 g a s p h a s e is c a l c u l a t e d f r o m t h e m o d e l l e d w a t e r a n d t h e n c o m p a r e d w i t h
t h e w a t e r s a m p l e s ' C - i s o t o p e v a l u e . T h e d e t a i l e d m o d e l c h o s e n a s b e i n g
m o s t a p p l i c a b l e i n t h e p r e s e n t i n s t a n c e , a s s u m e s t h a t t h e i n i t i a l p H w a s
d e t e r m i n e d s o l e l y b y e q u i l i b r i u m w i t h РСО2 ; t h e g r o u n d w a t e r t h u s r a p i d l y
e n t e r s c l o s e d s y s t e m c o n d i t i o n s i n t h e u n s a t u r a t e d zone. T h i s m o d e l g i v e s
a c c e p t a b l e c o r r e c t e d g r o u n d w a t e r a g e s f o r t h e f i r s t g r o u p o f w a t e r s i d e n t i f i e d
a b o v e , b u t c a n n o t m a t c h w i t h t h e s e c o n d g r o u p , f o r w h i c h n o f i t w a s
o b t a i n e d .
A s e p a r a t e m o d e l h a s t h e r e f o r e b e e n p r o p o s e d ( E d m u n d s a n d W r i g h t ,
[1 3 ]) t o e x p l a i n t h e i s o t o p i c c h a r a c t e r i s t i c s o f t h i s s e c o n d s e t o f g r o u n d
w a t e r s . I t is c o n s i d e r e d t h a t t h e <513C e n r i c h m e n t h a s b e e n b r o u g h t a b o u t
b y t h e r e l a t i v e p r e d o m i n a n c e o f i n o r g a n i c c a r b o n a t e d i s s o l u t i o n a s c o m
p a r e d w i t h b i o g e n i c p r o d u c t i o n . F o r t h e d e r i v a t i o n o f a g e s o f t h e s e c o n d
g r o u p o f w a t e r s t h e r e f o r e , t h e i m p o r t a n t c o n c l u s i o n is t h a t t h e p e r c o l a t i n g
w a t e r o f s h o r t d u r a t i o n r e c h a r g e e p i s o d e s w o u l d , i n a d d i t i o n t o t h e <513
e n r i c h m e n t , a l s o d i s s o l v e a l a r g e a m o u n t o f a c t i v e c a r b o n - 1 4 d e r i v e d f r o m
s o i l c a r b o n a t e o r c a l c r e t e s w h i c h h a d a c c r e t e d d u r i n g t h e p r e c e d i n g 'arid'
p h a s e ; t h e e v o l u t i o n o f t h e f r e s h w a t e r c h a n n e l i n t h i s w a y is s c h e m a t i c a l l y
i l l u s t r a t e d in F i g u r e 2. T h e g r o u n d w a t e r a g e i s t h e r e f o r e r o u g h l y e q u i v a
l e n t t o th e m e a s u r e d ' r a d i o c a r b o n age' a n d a n y c o r r e c t i o n is u n l i k e l y to
g i v e a m o r e a c c u r a t e e s t i m a t e .
S t a b l e o x y g e n a n d h y d r o g e n i s o t o p e r e s u l t s ( q u o t e d i n d e t a i l in
S d m u n d s a n d W r i g h t , , [1 3 ] ) s h o w a c o n s i s t e n t r e g i o n a l t r e n d w i t h i n t h e
3ir t e a n d K u f r a b a s i n s , b e c o m i n g g e n e r a l l y e n r i c h e d i n *® 0 a n d 2H f r o m
n o r t h t o s o u t h . A l l t h e r e s u l t s a r e r e l a t e d b y a n e v a p o r a t i v e l i n e
( F i g u r e 3) w i t h a s l o p e 6 2H = 4 . 5 <5lsO - 3 5 w h i c h i n t e r c e p t s t h e m e t e o r i c
l i n e a t <S180 = - 1 2 . b % o <52H = - 8 9 % o s u g g e s t i n g t h a t a l l t h e w a t e r s a r e r e
l a t e d t o t h e s a m e p a r e n t r a i n . T h e r e is n o c o r r e s p o n d e n c e o f a n y o f t h e
g r o u n d w a t e r c o m p o s i t i o n s t o p r e s e n t d a y ' M e d i t e r r a n e a n ' r a i n f a l l a n d it
is c o n c l u d e d t h a t a l l a r e p a l a e o g r o u n d w a t e r s w i t h a p a r e n t r a i n d e r i v e d
f r o m t r o p i c a l a i r m a s s e s s o u t h o f t h e S a h a r a , c o r r e s p o n d i n g t o a m o d e r n
a n a l o g u e s u c h as B a m a k o , M a l i . T h e s t r o n g l y e v a p o r a t i v e c h a r a c t e r i s t i c
r e l a t i n g t h e g r o u n d w a t e r s , i m p l i e s t h a t c o n s i d e r a b l e n o n - e q u i l i b r i u m
e v a p o r a t i v e c o n c e n t r a t i o n m u s t h a v e t a k e n p l a c e w i t h i n t h e a i r m a s s e s
d u r i n g t h e i r n o r t h w a r d t r a c k i n g ; t h e g e n e r a l t e n d e n c y f o r y o u n g e r a n d
s h a l l o w e r w a t e r s t o h a v e th e h e a v i e r i s o t o p e c o m p o s i t i o n s , s u g g e s t s a
c h a n g e i n t h e n a t u r e o f r e c h a r g e e v e n t s w i t h t i m e d u r i n g t h e l a t e P l e i s t o
c e n e a n d H o l o c e n e . T h e e x t r e m e c a s e is s e e n a t t h e o a s i s o f G a t m i r -
w h e r e a c o m p o s i t i o n o f 6 180 = 0 ° /o o is f o u n d i n a l o w s a l i n i t y g r o u n d w a t e r .
T h e s e r e s u l t s t i e in w i t h t h e b r o a d e r p i c t u r e o f t h e S a h a r a n g r o u n d w a t e r s
p r e s e n t e d b y S o n n t a g e t al. [l7-18] w h o f i n d a ' c o n t i n e n t a l e f f e c t ' o f
i n c r e a s i n g l y m o r e n e g a t i v e 6 H v a l u e s m o v i n g f r o m w e s t t o e a s t f r o m t h e
A t l a n t i c c o a s t . I n L i b y a h o w e v e r , t h e p r e s e n t r e s u l t s s u g g e s t t h e l o c a l l y
d o m i n a n t i n f l u e n c e o f t r o p i c a l r a i n f r o m s o u t h o f t h e S a h a r a d u r i n g th e
H o l o c e n e , w h i c h s u p p o r t s t h e p a l y n o l o g i c a l e v i d e n c e o f M a l e y [”l9] a n d th e
p a l a e o c l i m a t i c r e c o n s t r u c t i o n o f R o g n o n a n d W i l l i a m s [20] .
Ln
Ь'в0
F I G . 3 . O x y g e n a n d h y d r o g e n s t a b l e i s o t o p e r a t i o s f o r S i r t e a n d K u f r a b a s i n g r o u n d w a t e r s , l i n k e d b y a n ‘e v a p o r a t i v e l i n e - 8 D = 4 . 5 8 l s O - 3 5 .
C u r r e n t r e c h a r g e ( l i g h t i s o t o p i c l i m i t s o n l y ) a r e s h o w n f o r s t a t i o n s a t T u n i s ( 1 ) ; F a y a L a r g e a u , F o r t L a m y a n d K a n o ( 2 ) a n d B a m a k o ( 3 ) . A l l
r e s u l t s a r e r e l a t i v e t o S M O W a n d b a s e d o n d a t a in I A E A ( V i e n n a ) T e c h n i c a l R e p o r t s S e r i e s N o s 9 6 , 1 1 7 , 1 2 9 , 1 4 7 , 1 6 5 .
EDMUNDS
and W
ALTON
F I G . 4 . G r o u n d w a t e r ‘a g e s ’ in t h e S i r t e a n d K u f r a b a s i n s c o m p a r e d a g a i n s t i n d i r e c t p a l a e o c l i m a t i c e v i d e n c e f o r t h e S a h a r a r e g i o n f o r t h e l a t e
P l e i s t o c e n e a n d H o l o c e n e , s o u r c e s o f w h i c h a r e g i v e n in E d m u n d s a n d W r ig h t [ 13].
5 6 EDMUNDS and WALTON
T h e r e s u l t s h a v e a s i g n i f i c a n c e i n r e l a t i o n to p o s s i b l e h i s t o r i c a l
r e c h a r g e e v e n t s i n t h e S a h a r a a n d it s m a r g i n s . T h e g r o u n d w a t e r r a d i o
c a r b o n r e s u l t s a r e s u m m a r i s e d i n F i g u r e 4 in r e l a t i o n to o t h e r p a l a e o
c l i m a t i c d a t a f o r t h e S a h a r a n r e g i o n d u r i n g t h e l a t e P l e i s t o c e n e a n d
H o l o c e n e . F r o m t h e v a r i o u s s t u d i e s c i t e d i t h a s b e e n e s t a b l i s h e d t h a t
s i g n i f i c a n t h u m i d e p i s o d e s p r o b a b l y o c c u r r e d a t l e a s t t w i c e d u r i n g t h e
H o l o c e n e . T h e i s o t o p i c r e s u l t s f r o m L i b y a , s u b s t a n t i a t e d b y t h e h y d r o -
g e o l o g i c a l e v i d e n c e , p r o v i d e d i r e c t e v i d e n c e t h a t g r o u n d w a t e r r e c h a r g e
o c c u r r e d d u r i n g t h i s p e r i o d , w h i c h c a n b e c l e a r l y d i s t i n g u i s h e d f r o m
w a t e r r e c h a r g e d d u r i n g e a r l i e r e p i s o d e (s). T h e r e p l e n i s h m e n t is c o n
s i d e r e d to h a v e t a k e n p l a c e b o t h b y d i r e c t r e c h a r g e a n d b y s u r f a c e r u n
off. I t is l i k e l y t h a t a q u i t e m a j o r r i v e r s y s t e m w a s s t i l l a c t i v e in
e a s t e r n L i b y a u p u n t i l c a . 5 0 0 0 B P a n d p o s s i b l y lat e r . P a c h u r [2l] r e c o r d s
e l e p h a n t b o n e s w i t h a n a g e o f c a . 3 5 0 0 B P in f l u v i a t i l e s e d i m e n t s in t h e
W a d i B e h a r B e l a m a ( F i g u r e 1) - a s h o r t d i s t a n c e s o u t h - w e s t o f t h e k n o w n
g r o u n d w a t e r c h a n n e l - w h i c h is a l i k e l y e x t e n s i o n o f t h e s a m e h y d r o l o g i c a l
s y s t e m . T h e s h a l l o w , l o w s a l i n i t y g r o u n d w a t e r s in t h e a r e a w i t h c o n s i d e r
a b l e c o n t e n t s g i v e c o r r e c t e d a g e s a r o u n d 5 , 0 0 0 - 7 , 0 0 0 B P a n d a r e
c o n s i d e r e d to r e p r e s e n t H o l o c e n e r e c h a r g e . W h i l s t t h e s e s a m p l e s m a y
c o r r e s p o n d s o l e l y t o t h e s a m e p l u v i a l e v e n t s t h a t c a u s e d t h e f r e s h w a t e r
c h a n n e l , t h e r e is t h e p o s s i b i l i t y t h a t t h e y r e p r e s e n t a c c u m u l a t i o n s o f
d i r e c t r e c h a r g e m u c h y o u n g e r t h a n 5 , 0 0 0 - 7 , 0 0 0 y e a r s m i x e d w i t h m u c h o l d e r
w a t e r d u r i n g p u m p i n g .
F u r t h e r s t u d y is c l e a r l y r e q u i r e d i n L i b y a a n d e l s e w h e r e o f t h e w a t e r
a t t h e w a t e r - t a b l e a n d i m m e d i a t e l y b e l o w , s i n c e t h i s z o n e is o f t e n i g n o r e d
d u r i n g w a t e r w e l l d r i l l i n g . I t is t h e r e f o r e l i k e l y t h a t s o m e r e c h a r g e
m a y h a v e o c c u r r e d d u r i n g t h e p a s t 5 0 0 - 2 , 0 0 0 y e a r s , s i g n i g i c a n t f r o m a
w a t e r r e s o u r c e s v i e w p o i n t , y e t w h i c h is n o t d e t e c t a b l e w i t h o u t a c a r e f u l
e v a l u a t i o n t e c h n i q u e .
2. G E O C H E M I C A L M E T H O D S F O R E S T I M A T I N G T H E C U R R E N T R E C H A R G E C O M P O N E N T -
C Y P R U S .
E n v i r o n m e n t a l t r i t i u m p r o f i l e s o f u n s a t u r a t e d z o n e m o i s t u r e a r e n o w
f a i r l y w i d e l y e s t a b l i s h e d a s a t e c h n i q u e f o r i n v e s t i g a t i n g r e c h a r g e r a t e s
a n d t h e m e c h a n i s m o f w a t e r m o v e m e n t to t e m p e r a t e z o n e a q u i f e r s ( Z i m m e r m a n
et al. , [5], M ü n n i c h e t al., '[б]» S m i t h e t a l . £7] A n d e r s e n , [s] ) • T h e r e
h a v e a l s o b e e n s e v e r a l s u c c e s s f u l a t t e m p t s a t t h e u s e o f t r i t i u m p r o f i l e s
in s e m i - a r i d a n d a r i d z o n e s , e.g. I n d i a ( S u k h i j a a n d S h ah, [э]), A u s t r a l i a
( A l l i s o n a n d H u g h e s , [lo]), a n d S a u d i A r a b i a ( D i n ç e r et al., [il] ) ■ W h i l s t
i t is r e c o g n i s e d t h a t t r i t i u m o f f e r s a v a l u a b l e a n d s u c c e s s f u l m e t h o d o f
r e c h a r g e e s t i m a t i o n , e s p e c i a l l y w h e n c o m p a r e d w i t h t h e o t h e r a v a i l a b l e t e c h
n i q u e s , it m u s t a l s o b e r e c o g n i s e d t h a t t h e m e t h o d h a s s e v e r a l l i m i t a t i o n s .
(a) t h e r e l a t i v e l y s h o r t t r i t i u m h a l f l i f e o f 1 2 . 3 y e a r s l i m i t s t h e l o n g
t e r m u s e f u l n e s s o f t h e m e t h o d
(b) t h e m e t h o d is v u l n e r a b l e t o c o n t a m i n a t i o n u u r i n g s a m p l i n g , a n d s u b
s e q u e n t p r o c e s s i n g , a f a c t o r t h a t is e n h a n c e d i n r e m o t e a r e a s a n d a t
l o w t o t a l m o i s t u r e l e v e l s
(c) a n a l y s i s is h i g h l y s p e c i a l i s e d a n d r e l a t i v e l y c o s t l y , m a k i n g d e t a i l e d
p r o f i l e s r a t h e r e x p e n s i v e
(d) a l t h o u g h t h e 1 9 6 3 / 4 t r i t i u m p e a k c a n b e e x p e c t e d t o b e r e s o l v e d , a t
l e a s t i n t h e n o r t h e r n h e m i s p h e r e , a n n u a l i n c r e m e n t s o f m o i s t u r e c a n n o t
b e i d e n t i f i e d .
IAEA-AG-158/4 5 7
T r i t i u m s t u d i e s h a v e u s u a l l y b e e n c a r r i e d o u t w i t h o u t a n y s u p p o r t i n g
d a t a o n t h e g e o c h e m i s t r y o f t h e d i s s o l v e d s o l u t e s . T h e a i m o f t h e c u r r e n t
r e s e a r c h p r o g r a m m e w a s t h e r e f o r e t o i n v e s t i g a t e a w i d e r p o s s i b l e r a n g e of
g e o c h e m i c a l m e t h o d s t h a t m i g h t b e a p p l i c a b l e t o t h e e s t i m a t i o n o f r e c h a r g e
i n s e m i - a r i d a n d a r i d z o n e e n v i r o n m e n t s . S i n c e e s t i m a t e s o f r e c h a r g e a r e
u r g e n t l y r e q u i r e d i n r e m o t e a r e a s w h e r e s o p h i s t i c a t e d m e t h o d s a r e i n
a p p r o p r i a t e , t h e t a r g e t w a s s e t t h a t a n y t e c h n i q u e h a d t o b e l o g i s t i c a l l y
s i m p l e a n d i n e x p e n s i v e , a s w e l l a s r e l i a b l e .
It w a s c o n s i d e r e d t h a t c e r t a i n c o n s t i t u e n t s , n o t a b l y Cl, S O ^ a n d p o s
s i b l y NO3 , m i g h t b e c o n s e r v a t i v e w i t h i n t h e u n s a t u r a t e d z o n e e n v i r o n m e n t ,
t h e r e b y r e t a i n i n g a c h r o n o l o g y o f r e c h a r g e e v e n t s w i t h i n t h e p r o f i l e r e c o r d
t h a t c o u l d b e u s e d a t l e a s t t o s u p p l e m e n t t h e i n f o r m a t i o n o b t a i n e d f r o m
t r i t i u m , if n o t t o p r o v i d e a n a l t e r n a t i v e m e a n s o f a s s e s s m e n t . T h e i d e a
o f u s i n g c h l o r i d e f o r r e c h a r g e e s t i m a t i o n is n o t n e w - it h a s b e e n u s e d
f o r e x a m p l e b y E r i k s s o n [22] i n I n d i a , a n d A l l i s o n a n d H u g h e s [23]
in A u s t r a l i a . In b o t h t h e s e i n v e s t i g a t i o n s t h e o b j e c t i v e w a s t o c o m p a r e
t h e s a t u r a t e d z o n e c h l o r i d e v a l u e s w i t h a t m o s p h e r i c i n p u t s t o d e r i v e a
m a s s b a l a n c e . H o w e v e r , i n u s i n g w a t e r - t a b l e s a m p l e s , i t is c l e a r l y d i f
f i c u l t t o d i s c r i m i n a t e b e t w e e n t h e c o n t r i b u t i o n f r o m t h e a q u i f e r l i t h o l o g y
a n d t h a t f r o m a t m o s p h e r i c in p u t . T h e v a l u e o f u s i n g t h e u n s a t u r a t e d z o n e
p r o f i l e s d e s c r i b e d i n t h e p r e s e n t s t u d y , is t h a t i t is t h e n p o s s i b l e to
r e a d i l y d i s t i n g u i s h b e t w e e n these' t w o c o m p o n e n t s a n d t o h a v e a f a r g r e a t e r
c o n t r o l o v e r t h e r e c h a r g e i n t e r p r e t a t i o n . T w o m a i n a p p r o a c h e s , a n a l a g o u s
t o t r i t i u m p r o f i l e i n t e r p r e t a t i o n , a r e t h e r e f o r e b e i n g a d o p t e d :
(a) M a s s B a l a n c e : T h e u n s a t u r a t e d z o n e p r o f i l e s s h o u l d c o n t a i n a s o l u t e
l o a d i n g w h i c h i s c o m p o s e d o f t w o i n p u t s (i) r a i n f a l l , d r y d e p o s i t i o n
a n d a e r o s o l s , (ii) a q u i f e r l i t h o l o g y . T h e s e t w o c o m p o n e n t s s h o u l d
b e c l e a r l y d i s t i n g u i s h a b l e w i t h i n t h e p r o f i l e r e c o r d . T h e v a l u e o f
(i) c a n t h e n b e u s e d to c a l c u l a t e t h e r e c h a r g e , a s s u m i n g a l s o t h a t
s o i l m o i s t u r e , r a i n f a l l a m o u n t a n d r a i n f a l l c o m p o s i t i o n o v e r a n u m b e r
o f y e a r s a r e k n o w n .
(b) S o l u t e P e a k s : T h e w o r k i n g h y p o t h e s i s is c o m p a r a b l e t o t r i t i u m i n t e r
p r e t a t i o n , t h a t i n a r e a s w h e r e a f i n i t e a m o u n t o f a n n u a l r e c h a r g e o c c u r s ,
s o l u t e i n c r e m e n t s a r e t r a n s m i t t e d a n n u a l l y b y p i s t o n d i s p l a c e m e n t
t h r o u g h t h e u n s a t u r a t e d zone. T h e a m o u n t o f s o l u t e t r a n s m i t t e d w i l l
b e d e p e n d e n t u p o n a n u m b e r o f f a c t o r s , p a r t i c u l a r l y t h e d e g r e e o f
e v a p o r a t i v e c o n c e n t r a t i o n a n d t h e i n t e n s i t y a n d a m o u n t o f t h e f o l l o w i n g
s e a s o n s ' r a i n f a l l . I n a h o m o g e n e o u s m e d i u m t h e r e f o r e , a s e r i e s of
p e a k s s h o u l d b e g e n e r a t e d c o r r e s p o n d i n g to a n n u a l r e c h a r g e e v e n t s ,
w h i l s t v a r i a t i o n s i n t h e s o l u t e p r o f i l e m a y a l s o b e a p p a r e n t o n a w i d e r
s e c u l a r b a s i s f o l l o w i n g g r o u p s o f d r y o r w e t y e a r s . A n i n v e r s e c o r r e l a
t i o n b e t w e e n m e a n a n n u a l r a i n f a l l a n d p r o f i l e r e c o r d o v e r a n u m b e r of
y e a r s c o u l d t h e n b e u s e d t o c a l i b r a t e t h e p r o f i l e o n a t i m e b a s i s a n d
e n a b l e a r e c h a r g e c a l c u l a t i o n a n a l o g o u s t o t h e u s e o f t r i t i u m p e a k s .
It is s t r e s s e d t h a t t h i s is p r i m a r i l y a n e m p i r i c a l a p p r o a c h d e s i g n e d to
b y - p a s s , a t l e a s t i n i t i a l l y , t h e c o m p l e x i t y o f s o i l p h y s i c s t h e o r y ,
w h i c h , i n t h e c a s e o f w a t e r i t s e l f , i s m a d e m o r e p r o b l e m a t i c a l b y th e
l i k e l i h o o d o f 2 - p h a s e m o v e m e n t . T h i s i d e a l i s e d m o d e l m i g h t b e c o m
p l i c a t e d w h e r e r e c h a r g e w a s n o t a n a n n u a l e v e n t , o r w h e r e p e r c o l a t i o n
w a s n o n - h o m o g e n e o u s d u e t o f r a c t u r e o r p i p e f l o w fo r e x a m p l e , o r to
v a r i a b l e l i t h o l o g y o r s e d i m e n t o l o g y . S o m e o f t h e l i m i t a t i o n s t h a t
a p p l y t o t r i t i u m p r o f i l e i n t e r p r e t a t i o n w o u l d a l s o t h e r e f o r e a p p l y to
s o l u t e p r o f i l e s . A l t h o u g h t r i t i u m l e v e l s v a r y c o n s i d e r a b l y b e t w e e n
s u m m e r a n d w i n t e r r a i n i n m o s t p a r t s o f t h e w o r l d , t h e r e is n o c o r r e
s p o n d i n g c o n t r a s t i n t h e p e r c o l a t i n g w a t e r , s i n c e o n l y t h e h e a v i e s t
5 8 EDMUNDS and WALTON
r a i n s (with s e a s o n a l l y c o n s t a n t 3H) a r e s i g n i f i c a n t a s r e c h a r g e ; it
m a y b e t h a t s o l u t e p r o f i l e s c a n t h e r e f o r e r e c o r d a n n u a l e v e n t s w h e r e
t r i t i u m c a nnot.
T h e p r o j e c t w a s i n i t i a t e d i n C y p r u s f o r l o g i s t i c a s w e l l a s s c i e n t i f i c
r e a s o n s . T h i s w a s p a r t l y b e c a u s e t h e g e o c h e m i c a l r e s u l t s c o u l d t h e n e v e n t u
a l l y b e c o m p a r e d b o t h w i t h s i m u l t a n e o u s l y s i m e t e r m e a s u r e m e n t s o f r e c h a r g e
a n d a l s o w i t h r e c h a r g e e s t i m a t e s f r o m l o c a l w a t e r r e s o u r c e s t u d i e s . T h e
A k r o t i r i p e n i n s u l a i n t h e e x t r e m e s o u t h o f t h e i s l a n d w a s c h o s e n i n i t i a l l y
fo r g e o c h e m i c a l p r o f i l e w o r k o n a c c o u n t o f i t s l i t h o l o g y , w h i c h c o m p r i s e s
r e l a t i v e l y h o m o g e n e o u s u n c o n s o l i d a t e d H o l o c e n e / Q u a t e r n a r y d u n e s a n d s a n d b e
c a u s e a w a t e r b a l a n c e s t u d y o f t h e m i n o r a q u i f e r o f t h e p e n i n s u l a h a d
a l r e a d y b e e n m a d e (Fink, [24]). A n u n s a t u r a t e d z o n e u p to 50 m t h i c k e x i s t s ,
a n d l a n d u s e v a r i e s f r o m a g r i c u l t u r a l t o u n d e v e l o p e d a r e a s o f s t a t e forest.
I n a d d i t i o n , a p r i n c i p a l m e t e o r o l o g i c a l s t a t i o n w a s l o c a t e d w i t h i n t h e
r e s e a r c h a r e a a t A k r o t i r i , w h i c h c o u l d b e u s e d a s a c o n t r o l t o g e t h e r w i t h
a l o c a l r a i n f a l l s t a t i o n o n site. T h e m e a n a n n u a l r a i n f a l l a t A k r o t i r i
(1965-76) w a s 4 2 0 m m a n d t h e s e r e c o r d s c o u l d b e m a t c h e d o v e r a 6 0 y e a r
p e r i o d w i t h t h o s e a t L i m a s s o l , 5 k m t o t h e north.
E x p e r i m e n t a l M e t h o d s
T h e p r o b l e m o f o b t a i n i n g u n c o n t a m i n a t e d s a m p l e s f o r c h e m i c a l a n d
i s o t o p i c i n v e s t i g a t i o n t o a d e p t h o f 30 m p r e s e n t s s e v e r e p r a c t i c a l d i f
f i c u l t i e s , n o t l e a s t in s e m i - a r i d o r a r i d z o n e s w h e r e l o w s o i l m o i s t u r e
c o n t e n t s a r e found. D r i l l i n g d u r i n g t h e c u r r e n t p r o j e c t w a s c a r r i e d o u t
u s i n g a P i l c o n W a y f a r e r 1 5 0 0 p o r t a b l e rig. V a r i o u s a t t e m p t s a t d r y
d r i l l i n g b y p e r c u s s i o n a n d a i r f l u s h r o t a r y w e r e m a d e a t t h e s t a r t o f t h e
p r o j e c t , i n c l u d i n g U4 c o r i n g , b u t w i t h o u t s u c c e s s . H o w e v e r , a s i m p l e p e r
c u s s i o n t e c h n i q u e w a s e v e n t u a l l y d e v e l o p e d w h i c h w i l l b e d e s c r i b e d in f u l l
e l s e w h e r e .. ( E d m u n d s e t al., Г 25j ). T h i s m e t h o d a v o i d e d c o n t a m i n a
t i o n a n d o v e r h e a t i n g in u n c o n s o l i d a t e d m a t e r i a l d i d n o t o c c u r ; a c o n t i n u o u s
s a m p l i n g r e c o r d e v e r y 20 c m w a s o b t a i n e d a t a r a t e o f 4-7 m / w o r k i n g day.
T h e m e t h o d w a s f o u n d to b e s i m p l e , c h e a p a n d e f f e c t i v e a n d p r o b a b l y w i d e l y
a p p l i c a b l e i n s e m i - a r i d a n d a r i d t e r r a i n s .
It w a s i n i t i a l l y h o p e d t h a t d i r e c t e x t r a c t i o n o f s o i l m o i s t u r e b y
c e n t r i f u g e ( E d m u n d s a n d B a t h , [2б]) m i g h t b e p o s s i b l e , b u t t h i s w a s t o t a l l y
i m p r a c t i c a b l e a t t h e l o w (4 m l / 1 0 0 g) w a t e r c o n t e n t s found. I m m i s c i b l e
f l u i d d i s p l a c e m e n t ( P a t t e r s o n e t a l . , [27]) w a s a l s o t r i e d , b u t w a s no t
s u c c e s s f u l f o r th e g r a i n s i z e / m o i s t u r e c o n t e n t o f t h e A k r o t i r i f o r m a t i o n .
A n é l u t r i a t i o n t e c h n i q u e w a s s u b s e q u e n t l y a d o p t e d , w h e r e b y 30 m l d i s t i l l e d /
d e i o n i s e d w a t e r w a s a d d e d t o e a c h 5 0 g s a n d s a m p l e , w h i c h w a s s t i r r e d
o c c a s i o n a l l y b e f o r e r e m o v i n g t h e s u p e r n a t a n t s o l u t i o n a f t e r 1 h o u r . T h i s
w a s t h e n f i l t e r e d t h r o u g h 0.45 цт f i l t e r s a n d t h e s p e c i f i c e l e c t r i c a l c o n
d u c t a n c e m e a s u r e d i n t h e field. A p p r o x . 15 m l o f c l e a r e l u t r i a t e w a s
o b t a i n e d f r o m e a c h 20 c m p r o f i l e s a mple. T h i s w a s s e a l e d i n p o l y c a r b o n a t e
t u b e s a n d t r a n s p o r t e d t o t h e O K fo r m i c r o c h e m i c a l a n a l y s i s . S o i l m o i s t u r e
w a s d e t e r m i n e d w i t h i n 1 h o u r o f s a m p l i n g a n d 5 k g d o u b l e - w r a p p e d s a n d
s a m p l e s w e r e a i r f r e i g h t e d t o U K fo r a n a l y s i s b y A E R E H a r w e l l .
T h i s m e t h o d h a s o b v i o u s l i m i t a t i o n s w i t h r e g a r d t o a f u l l g e o c h e m i c a l
i n t e r p r e t a t i o n o f th e s o l u t e , n o t a b l y t h e c a r b o n a t e s y s t e m , b u t p r o b a b l y
r e p r e s e n t s t h e o n l y m e t h o d o f o b t a i n i n g s o l u t e d a t a i n s e m i - a r i d o r a r i d
z o n e s . H o w e v e r , in t h e p r e s e n t a p p l i c a t i o n , i t is a r g u a b l y t h e m o s t
e f f e c t i v e t e c h n i q u e s i n c e i t is u n s o p h i s t i c a t e d a n d r e q u i r e s o n l y t h e m i n i
m u m o f f i e l d e q u i p m e n t . F u r t h e r m o r e , a n o n - t h e - s p o t a s s e s s m e n t o f t h e
IAEA-AG-158/4 5 9
SPECIFIC ELECTRICAL CONDUCTANCE Cl (mg/100 g) log Cl (mg/ litre)u Scm -1 at 25‘C -E LU TR IATE SOIL MOISTURE
N03 (mg/100g)ELUTRIATE
SOIL MOISTURE (mg/100g)
F I G . 5 . S o l u t e , s o i l m o i s t u r e a n d p a r t i a l t r i t i u m p r o f i l e s f o r b o r e h o l e A K 2 , A k r o t i r i , C y p r u s .
S p e c i f i c e l e c t r i c a l c o n d u c t a n c e p e a k s a r e n u m b e r e d f o r r e f e r e n c e p u r p o s e s ; c o r r e s p o n d i n g
p e a k s in C l a n d N O 3 a r e a l s o i n d i c a t e d . T h e m e a n v a l u e f o r C l u s e d in r e c h a r g e c a l c u l a t i o n s
i s i n d i c a t e d in t h e l o g C l ( s o i l m o i s t u r e ) p r o f i l e .
6 0 EDMUNDS and WALTON
s a l i n i t y o f t h e p r o f i l e c a n b e m a d e f r o m t h e c o n d u c t a n c e logs, w h i c h g i v e s
a n i m m e d i a t e f e e d - b a c k f o r j u d g i n g t h e s u c c e s s o f t h e d r i l l i n g a n d in
p l a n n i n g t h e s u b s e q u e n t p r o g r a m m e .
P R E L I M I N A R Y R E S U L T S
P a r t i a l , p r e l i m i n a r y a n d o n l y s e m i - q u a n t i t a t i v e r e s u l t s f r o m t h e f i r s t
t w o o f t h r e e f i e l d s e a s o n s (1977-8) a r e p r e s e n t e d h e r e a s a m e a n s o f i l l u s
t r a t i n g t h e t y p e o f s o l u t e p r o f i l e s o b t a i n e d a n d o f d e r i v i n g a n i n i t i a l
c o m p a r i s o n w i t h t r i t i u m p r o f i l e s . S o f a r e i g h t b o r e h o l e s h a v e b e e n d r i l l e d ,
b u t c o m p l e t e r e s u l t s a r e o n l y a v a i l a b l e f o r t h e f i r s t two. T h e f i r s t
d r i l l e d b o r e h o l e , A K 2 w i l l b e t a k e n as a m o d e l f o r t h e st u d y . T h i s w a s
d r i l l e d o n h o r i z o n t a l g r o u n d i n s a n d y s o i l w h i c h w a s a t th e b o u n d a r y o f t h e
a g r i c u l t u r a l l a n d o f th e v i l l a g e ; v a r i a b l e c a l c r e t e (Kafkalla) w a s f o u n d
e v e r y w h e r e b e l o w t h e s u r f a c e . T h e p r o f i l e s f o r s o i l m o i s t u r e , t r i t i u m ,
s p e c i f i c e l e c t r i c a l c o n d u c t a n c e c o r r e c t e d t o 25 С ( S E C ) , c h l o r i d e a n d
n i t r a t e a r e g i v e n i n F i g u r e 5 f r o m w h i c h i t c a n b e s e e n that:
1. D i s t i n c t p e a k s a r e e v i d e n t for SEC, C l a n d N O 3, w h i c h i n d i c a t e t h a t
h o m o g e n i s a t i o n b y d i s p e r s i o n h a s n o t t a k e n p l a c e . M o s t o f t h e p e a k s
c o r r e l a t e b e t w e e n a l l t h r e e p a r a m e t e r s a n d t h e r e is an a p p a r e n t
p e r i o d i c i t y o f r a t h e r l e s s t h a n 1 m.
2. S E C a n d C l o s c i l l a t e a b o u t m e a n v a l u e s , i m p l y i n g a s t e a d y s t a t e p e r
c o l a t i o n t h r o u g h t h e p r o f i l e w i t h o u t a n y u p t a k e f r o m t h e l i t h o l o g y .
N i t r a t e , o n t h e o t h e r h a n d , i n c r e a s e s d o w n t h e p r o f i l e a n d t h i s is
c o n s i d e r e d to r e p r e s e n t a f o r m e r , d i f f e r e n t l a n d use.
3. C l p r o f i l e s , p l o t t e d a s w e i g h t / u n i t v o l u m e a n d as e q u i v a l e n t p o r e
w a t e r c o n c e n t r a t i o n ( c o r r e c t e d u s i n g s o i l m o i s t u r e ) , b o t h r e t a i n
p e a k s . T h i s d e m o n s t r a t e s t h a t t h e v a r i a t i o n s in t h e e l u t r i a t e сощ-
p o s i t i o n s a r e n o t a f u n c t i o n o f v a r i a t i o n s i n t h e s o i l m o i s t u r e a n d
i m p l y t h a t t h e w a t e r a n d s o l u t e s a r e m o v i n g d o w n w a r d s a t a p p r o x i m a t e l y
t h e s a m e rate.
4. A f a i r l y w e l l d e f i n e d t r i t i u m p e a k o c c u r s a r o u n d 10 m a n d t h e r e is
a p p a r e n t l y l i t t l e d i s p e r s i o n o r l e a k a g e to t h e d e e p e r p r o f i l e , a l t h o u g h
t r i t i u m m a s s b a l a n c e c a l c u l a t i o n s h a v e y e t to b e m a d e .
T h e e q u i v a l e n t p r o f i l e s f o r t h e s e c o n d b o r e h o l e d r i l l e d , A K 3 , ( F i g u r e
6 ) r e p r e s e n t a c o m p l e t e s e c t i o n o f t h e u n s a t u r a t e d zone. T h i s b o r e h o l e w a s
d r i l l e d w i t h i n 5 m e t r e s o f A K 2 o n h o r i z o n t a l g r o u n d w h e r e t h e o n l y e s s e n t i a l
d i f f e r e n c e w a s t h e e x t e n t o f c a l c r e t e d e v e l o p m e n t a t 0 . 8 0 - 2 . 0 0 m b e l o w th e
s u r f a c e . T h e d i f f e r e n c e s o r s i m i l a r i t i e s w i t h A K 2 m a y b e s u m m a r i s e d as
f o l l o w s :
1. S o l u t e p e a k s a r e n o t w e l l d e v e l o p e d i n t h e u p p e r 16 m a n d i t is c o n
s i d e r e d t h a t a d e g r e e o f h o m o g e n i s a t i o n h a s o c c u r r e d . A n e x c e p t i o n
w o u l d b e a t t h e S E C p e a k b e t w e e n 11 m a n d 15 m w h i c h b r o a d l y r e f l e c t s
t h e f o r m o f t h e A K 2 p r o f i l e .
2. T h e C l p r o f i l e c a n b e a s c r i b e d a m e a n v a l u e o f lo g C l = 2 . 0 8 ( 1 2 0 mg/l)
o v e r t h e i n t e r v a l 3 . 0 - 1 6 . 5 m . A t t h i s p o i n t a m i x e d s a n d a n d g r a v e l
h o r i z o n w a s p e n e t r a t e d a n d t h e i n c r e a s e i n C l i l l u s t r a t e d o n t h e
p l o t o f s o i l m o i s t u r e c o m p o s i t i o n r e f l e c t s a l i t h o l o g i c a l i n p u t w h i c h
a f f e c t s t h e p r o f i l e b e l o w t h i s p o i n t . T h e p r o f i l e a b o v e 3 . 0 m , w h i c h
is s t i l l s u b j e c t t o s o i l m o i s t u r e flux, is i g n o r e d .
DE
PT
H
IAEA-AG-158/4 61
SPECIFIC ELECTRICAL CONDUCTANCE Cl (mg/100g)4iS cm'1 at 25'C-ELUTRIATE
log Cl (mg/ litre)SOIL MOISTURE
200 300 400 500 0.5 1.0
SOIL MOISTURE (mg 100g)
TRITIUM (TU)
0 2 4 6 0 25 50 75 100
F I G . 6 . S o l u t e , s o i l m o i s t u r e a n d t r i t i u m p r o f i l e s f o r b o r e h o l e A K 3 , A k r o t i r i , s o u t h e r n
C y p r u s . T h e m e a n v a l u e f o r C l u s e d in r e c h a r g e c a l c u l a t i o n s i s i n d i c a t e d in t h e l o g C l ( s o i l
m o i s t u r e ) p r o f i l e . S W L = s t a t i c w a t e r l e v e l .
6 2 EDMUNDS and WALTON
T A B L E 2. V A L U E S O F M E A N A N N U A L R A I N F A L L , R A I N F A L L C H L O R I D E A N D S O I L
M O I S T U R E C O M P O S I T I O N U S E D T O C A L C U L A T E R E C H A R G E A T A K R O T I R I SITE.
M e a n a n n u a l r a i n f a l l A k r o t i r i M e t . S t a t i o n (1963-1976) = 4 2 0 m m
A n n u a l r a i n f a l l A k r o t i r i Met. S t a t i o n (1977-8) = 4 5 6 . 2
A n n u a l r a i n f a l l A k r o t i r i V i l l a g e s i t e (1977-8) = 4 7 0 . 8
M e a n r a i n f a l l c h l o r i d e - A k r o t i r i Met. S t a t i o n 1 9 7 7 - 8 = 1 2 . 3 m g / 1 Cl_
C p M e a n r a i n f a l l c h l o r i d e - A k r o t i r i V i l l a g e s i t e 1 9 7 7 - 8 1 6 . 3 m g / 1 Cl
A v e r a g e v a l u e u s e d i n c a l c u l a t i o n s = 1 4 . 3 m g / 1 Cl
M e a n s o i l m o i s t u r e c o m p o s i t i o n A K 2 = 1 2 0 m g / 1
M e a n s o i l m o i s t u r e c o m p o s i t i o n A K 3 = 1 2 0 m g / 1
RdR e c h a r g e a t A K 2 = 5 0 m m
R e c h a r g e a t A K 3 = 5 0 m m
3. T h e t r i t i u m p r o f i l e c o n t a i n s a p e a k c o r r e s p o n d i n g t o th e 1 9 6 3 / 4
a t m o s p h e r i c m a x i m u m , a n d t h e v e r y l o w v a l u e s i n t h e l o w e r h a l f o f
t h e p r o f i l e a n d 0 . 0 T U a t t h e water' t a b l e , s u g g e s t t h a t b y - p a s s
f l o w h a s n o t b e e n s i g n i f i c a n t .
T h i s r a t h e r s t r i k i n g d i f f e r e n c e b e t w e e n t w o a d j a c e n t p r o f i l e s w a s u n - .
e x p e c t e d b u t m a y p o s s i b l y b e a g o o d i n d i c a t i o n o f t h e p r o b l e m s t o b e e x p e c t e d
i n s e m i - a r i d t e r r a i n s , t h e e x p l a n a t i o n is t h o u g h t to l i e e i t h e r i n t h e
v a r i a b i l i t y o f c a l c r e t e d e v e l o p m e n t , o r i n l o c a l f l u x d u e to r o o t t r a n s
p i r a t i o n . N e v e r t h e l e s s , i t is c o n s i d e r e d t h a t b o t h C l p r o f i l e s c a n b e u s e d
to d e r i v e r e c h a r g e e s t i m a t e s a s s u m i n g s t e a d y s t a t e c o n d i t i o n s a n d t h a t t h e
A K 2 p r o f i l e m i g h t a l s o b e u s e d to d e r i v e r e c h a r g e e s t i m a t e s u s i n g t h e
s o l u t e p e a k s .
(i) C h l o r i d e M a s s B a l a n c e
T h e v o l u m e o f t h e d i r e c t r e c h a r g e c o m p o n e n t (R^) c a n b e e x p r e s s e d as
f o l l o w s :
R d = P - Ë - S
w h e r e P = m e a n a n n u a l r a i n f a l l ( m m ) , E = m e a n é v a p o t r a n s p i r a t i o n a n d S =
m e a n s u r f a c e r u n o f f . If o n l y t h o s e s i t e s w h e r e s u r f a c e r u n o f f is n e g l i g i b l e
a r e s e l e c t e d fo r p r o f i l e d r i l l i n g , t h e n S = 0 . S i n c e é v a p o t r a n s p i r a t i o n
r e p r e s e n t s t h e g r e a t e s t u n c e r t a i n t y i n t h e c a l c u l a t i o n , t h i s t e r m c a n be
r e p l a c e d b y a t e r m f o r t h e s o l u t e m a s s b a l a n c e if it c a n b e d e m o n s t r a t e d
(or a s s u m e d ) t h a t t h e t o t a l a t m o s p h e r i c s o l u t e l o a d i n g f o r a n y c o n s e r v a t i v e
e l e m e n t f o r a g i v e n s i t e is e v e n t u a l l y t r a n s m i t t e d t h r o u g h t h e s o i l in a
s t e a d y s t a t e p r o c e s s . T h e n :
IAEA-AG-158/4 6 3
w h e r e C p is t h e s o l u t e . c o n c e n t r a t i o n i n t h e r a i n f a l l (P) a n d C s is t h e s o i l -
m o i s t u r e c o m p o s i t i o n - in t h i s c a s e t h a t of C l -. T h u s i t is p o s s i b l e t o s o l v e
t h i s e q u a t i o n b y s i m p l y k n o w i n g t h e s e t h r e e p a r a m e t e r s .
T h e m e a n a n n u a l r a i n f a l l a t t h e A k r o t i r i s i t e is k n o w n f o r t h e 14
y e a r p e r i o d 1 9 6 3 - 7 6 (T a b l e 2), w h i c h c o r r e s p o n d s r o u g h l y to t h e t i m e s c a l e
o f t h e p r o f i l e . T h e r a i n f a l l c h l o r i d e l e v e l s m e a s u r e d a t t h e t w o s t a t i o n s
fo r th e f i r s t y e a r d i f f e r b y 4 m g / l ; f o r p r e l i m i n a r y c a l c u l a t i o n , t h e
a v e r a g e v a l u e h a s b e e n u s e d . T h e s o i l m o i s t u r e c h l o r i d e c o n c e n t r a t i o n s (Cs )
h a v e b e e n d e r i v e d u s i n g a m e a n v a l u e f o r s o i l m o i s t u r e f o r b o t h p r o f i l e s of
4. 8 m g / 1 0 0 g, w i t h m e a n c h l o r i d e l e v e l s p e r 1 0 0 g d e r i v e d f r o m F i g u r e s 5 a n d
6 , a n d a f o r m a t i o n b u l k d e n s i t y v a l u e o f 1.50. F o r b o t h p r o f i l e s , i d e n t i c a l
r e c h a r g e v a l u e s o f 50 m m / a n n u m a r e t h u s c a l c u l a t e d , d e s p i t e t h e a p p a r e n t
d i f f e r e n c e i n t h e f o r m o f t h e p r o f i l e s .
(ii) S o l u t e P e a k s
T h e A K 2 p r o f i l e c o n t a i n s s e v e r a l r e s o l v e d p e a k s w i t h a n a p p a r e n t
p e r i o d i c i t y o f r a t h e r l e s s t h a n 1 m f o r SEC, C l a n d N O 3 . In o r d e r to c h e c k
w h e t h e r s o m e o r a l l o f t h e s e c o r r e s p o n d t o a n n u a l i n p u t s t o t h e p r o f i l e ,
t r i t i u m w a s u s e d t o c a l i b r a t e t h e l i k e l y r e c h a r g e rat e . U s i n g t h e A K 3
p r o f i l e a d e f i n i t e m a x i m u m o f 9 9 T U o c c u r s a t c a . 9 . 5 m w h i c h c o r r e s p o n d s
w i t h t h e A K 2 m a x i m u m o f 10 5 T U a t a c o m p a r a b l e d e p t h . I d e n t i f y i n g t h i s
a s t h e 1 9 6 3 / 4 r e c h a r g e p e a k t h e n p e r m i t s a d o w n w a r d p e r c o l a t i o n v e l o c i t y
o f 0 . 6 6 m / y r to b e d e t e r m i n e d . A n i n d e p e n d e n t r e c h a r g e e s t i m a t e o f 48
m m / y r u s i n g t r i t i u m c a n t h e n b e o b t a i n e d u s i n g a s o i l m o i s t u r e v a l u e o f
4 . 8 m g / 1 0 0 g a n d a b u l k d e n s i t y o f 1 . 5 0 , a s s u m i n g n o b y - p a s s flow. T h i s
is r e m a r k a b l y c l o s e t o t h e c h l o r i d e m a s s b a l a n c e r e s u l t g i v e n ab o v e ;
h o w e v e r a t t h i s sta g e , it h a s n o t b e e n p o s s i b l e t o f u l l y i n t e r p r e t th e
t r i t i u m p r o f i l e w i t h o u t k n o w i n g t h e u p - t o - d a t e r a i n f a l l t r i t i u m i n p u t s
f o r s t a t i o n s i n C y p r u s .
U s i n g t h i s r e c h a r g e r a t e t h e m e a n a n n u a l r a i n f a l l d a t a f o r A k r o t i r i
h a s b e e n p l o t t e d a t a n e q u i v a l e n t s c a l e a g a i n s t t h e S E C a n d C l p r o f i l e s
o f A K 2 ( F i g u r e 7). M e t e o r o l o g i c a l d a t a f o r L i m a s s o l h a v e b e e n u s e d to
p r o v i d e a n e x t e n s i o n o f t h e c a l i b r a t i o n b e f o r e 1965. It is f o u n d t h a t an
i n v e r s e c o r r e l a t i o n e x i s t s b e t w e e n r a i n f a l l a m o u n t a n d t o t a l s o l u t e c o n
c e n t r a t i o n a s e x p r e s s e d b y SEC. T h e t w o d r y p e r i o d s o f 197 2-4 a n d 1 9 5 5 - 6 0
b o t h c o r r e l a t e w i t h a b r o a d e r z o n e o f h i g h e r S E C a n d c e r t a i n i n d i v i d u a l
p e a k s , e.g. 5 a n d 8 c o u l d l i b e r a l l y b e i n t e r p r e t e d t o c o i n c i d e w i t h t h e
d r y y e a r s o f 1 9 6 7 a n d 1 9 6 3 r e s p e c t i v e l y . T h e c h l o r i d e p r o f i l e d o e s n o t
r e f l e c t t h e p e r i o d i c c l i m a t i c c h a r a c t e r i s t i c s i n th e s a m e w a y as SEC. T h e
r e a s o n f o r t h i s is n o t e n t i r e l y c l e a r w i t h o u t a d d i t i o n a l c h e m i c a l r e s u l t s
f o r t h e o t h e r m a j o r ions. O n l y 8 o r 9 r e c o g n i s a b l e p e a k s a r e f o u n d i n th e
1 9 6 4 - 1 9 7 7 i n t e r v a l , a n d a n e x a c t a n n u a l c o r r e l a t i o n d o e s n o t s e e m p o s s i b l e
w i t h o u t m o r e d e t a i l e d a n a l y s i s o f t h e d a ta. H o w e v e r , i t is l i k e l y t h a t s o m e
p e a k s w o u l d b e l e s s d i s t i n c t i n y e a r s o f s i m i l a r a n n u a l r a i n f a l l a n d c o n
v e r s e l y , t h a t t h e m o s t s i g n i f i c a n t p e a k s w o u l d a p p e a r i n a d r y y e a r ( s )
f o l l o w e d b y a w e t y e a r (e.g. 1 9 6 0 / 1 ) . I n a d d i t i o n , e m b r y o p e a k s w i t h i n
t h e t o p f e w m e t r e s w i l l n o t b e f u l l y d e v e l o p e d u n t i l t h e y h a v e p a s s e d b e l o w
t h e z o n e o f e v a p o r a t i v e flu x ; t h u s a t l e a s t t h r e e p e a k s c a n p r o b a b l y b e
a c c o u n t e d f o r w i t h i n t h i s u p p e r zone. O n b a l a n c e ,t h e r e f o r e , i t w o u l d s e e m
u n l i k e l y t h a t t h e f u l l q u o t a o f p e a k s w o u l d e v e r b e d e v e l o p e d w i t h i n a n y
p r o f i l e .
In t h e c a s e o f t h e A K 2 p r o f i l e t h e r e f o r e it is c o n c l u d e d t h a t a n i n
v e r s e c o r r e l a t i o n b e t w e e n m e a n a n n u a l r a i n f a l l a n d t h e S E C p r o f i l e c a n b e
m a d e a n d t h a t t h i s c a n b e u s e d t o c a l i b r a t e t h e p r o f i l e r e c o r d o v e r a t
ANNUAL RAINFALL mm
6 4 EDMUNDS and WALTON
0 2 0 0 4 0 0 6 0 0
F I G . 7. A n n u a l r a i n f a l l f o r A k r o t i r i a r e a f o r p e r i o d 1 9 3 8 - 1 9 7 6 w i t h S E C ( s p e c i f i c e l e c t r i c a l
c o n d u c t a n c e ) a n d C l p r o f i l e s ( A K 2 ) d r a w n a t a v e r t i c a l ( t i m e ) s c a l e , d e r i v e d f r o m t h e p o s i t i o n
o f t h e t r i t i u m p e a k in F i g s 5 a n d 6 . C o n c e n t r a t i o n s c a l e s a r e g i v e n in F ig . 5 .
l e a s t a 25 y e a r p e r i o d . A t t h i s s t a g e i t is p o i n t e d o u t t h a t t h e c o r r e l a
t i o n is n o t i n d e p e n d e n t o f t r i t i u m a n d f u r t h e r r e s u l t s w o u l d b e r e q u i r e d to
se e w h e t h e r t h e s o l u t e p r o f i l e s a l o n e , c a l i b r a t e d a p p r o x i m a t e l y
u s i n g t h e C l m a s s b a l a n c e r e s u l t s , r e f l e c t t h e p r e c e d i n g r a i n f a l l r e c o r d .
H o w e v e r , t h e S E C a n d t r i t i u m p e a k s t a k e n t o g e t h e r s t r e n g t h e n t h e c o n c l u s i o n
t h a t t h e d o w n w a r d p e r c o l a t i o n is 0 . 6 6 m / y r , r e p r e s e n t i n g a m e a n r e c h a r g e o f
48 m m / y r .
T h e f o r e g o i n g r e s u l t s p r o v i d e e n c o u r a g i n g e v i d e n c e t h a t s o l u t e p r o f i l e s m a y
b e a s u s e f u l , if n o t m o r e u s e f u l , i n r e c h a r g e e s t i m a t i o n t h a n t r i t i u m p r o
f i l e s a l o n e , a n d t h a t b o t h t e c h n i q u e s u s e d t o g e t h e r c o u l d p r o v i d e t h e m o s t
e f f e c t i v e m e t h o d f o r d e t e r m i n i n g d i r e c t r e c h a r g e . In c o m p a r i s o n w i t h c o n
v e n t i o n a l m e t h o d s o f r e c h a r g e e s t i m a t i o n , t h e s o l u t e p r o f i l e a p p r o a c h is
t o t a l l y i n d e p e n d e n t o f s o i l p h y s i c s t h e o r y . O n e o f i t s m a i n a d v a n t a g e s
IAEA-AG-158/4 6 5
is t h e a b i l i t y t o i n v e s t i g a t e t h e v a r i a b i l i t y o f r e c h a r g e o v e r a w i d e
g e o g r a p h i c a r ea; p r o f i l e s o f o n l y 1 0 - 1 5 m a r e r e q u i r e d f o r i n t e r p r e t a t i o n ,
r e p r e s e n t i n g l i t t l e m o r e t h a n 3 d a y s d r i l l i n g p e r sit e , a n d a p r e l i m i n a r y
i n t e r p r e t a t i o n o f t h e r e s u l t s c a n b e m a d e in t h e field.
T h e r e a r e l i m i t a t i o n s t o t h e u s e o f s o l u t e p r o f i l e s , a s i n d e e d t h e r e
a r e f o r o t h e r m e t h o d s o f r e c h a r g e e s t i m a t i o n . T h e m e t h o d is l i m i t e d b y t h e
d e p t h o f t h e u n s a t u r a t e d z o n e a n d is f a v o u r e d i n u n c o n s o l i d a t e d l i t h o l o g i e s ;
r o t a r y a i r f l u s h d r i l l i n g , o n c e s u c c e s s f u l l y d e v e l o p e d , m i g h t o f f e r a
s a t i s f a c t o r y i f l o g i s t i c a l l y m o r e d i f f i c u l t a n d e x p e n s i v e m e t h o d fo r c o n
s o l i d a t e d l i t h o l o g i e s . T h e g r e a t e s t s o u r c e o f e r r o r is l i k e l y t o b e in
t h e r a i n f a l l c h l o r i d e r e s u l t s a n d i d e a l l y , s e v e r a l y e a r s c o n s e c u t i v e c h e m i c a l
a n a l y s i s s h o u l d b e c a r e f u l l y o b t a i n e d f r o m s t a t i o n s w i t h i n t h e a r e a of
r e c h a r g e i n v e s t i g a t i o n .
F u r t h e r s t u d i e s a r e b e i n g c a r r i e d o u t i n C y p r u s a n d e l s e w h e r e o n th e
a p p l i c a t i o n o f t h e s o l u t e p r o f i l i n g t e c h n i q u e a n d i t is i n t e n d e d t o p u b l i s h
a f u l l a c c o u n t o f t h e w o r k a t th e e n d o f t h e t h r e e y e a r s t u d y . It is p l a n n e d
t o c o m p a r e d a t a f r o m s o m e 15 b o r e h o l e s a t t h e s a m e s i t e d u r i n g t h e p r o j e c t
f o r w h i c h t h e r e l a t i v e b e h a v i o u r o f o t h e r s o l u t e s is a l s o b e i n g s t u d i e d .
T h e r e s u l t s g i v e n i n t h i s p a p e r t h e r e f o r e s h o u l d b e r e g a r d e d a s p r o v i s i o n a l
a n d m u s t a w a i t a f u l l e r c o m p a r i s o n w i t h o t h e r r e c h a r g e e s t i m a t e s , p a r t i c u
l a r l y f r c m a l y s i m e t e r w h i c h is b e i n g c o n s t r u c t e d o n t h e s a m e s i t e a t
A k r o t i r i .
C O N C L U D I N G D I S C U S S I O N
T h e r e s u l t s f r o m C y p r u s d e m o n s t r a t e t h a t c u r r e n t , d i r e c t r e c h a r g e
e s t i m a t i o n i n s e m i - a r i d z o n e t e r r a i n s , m a y b e p o s s i b l e u s i n g t h e i n f o r
m a t i o n s t o r e d i n t h e s o l u t e p r o f i l e s o f t h e u n s a t u r a t e d zone. It is
a r g u e d t h a t t h e u s e o f s o l u t e s s u c h a s c h l o r i d e o r t h e s p e c i f i c e l e c t r i c a l
c o n d u c t a n c e o f t h e s o i l m o i s t u r e , o f f e r a m o r e s t r a i g h t - f o r w a r d m e a n s o f
r e c h a r g e a s s e s s m e n t t h a n t h e s t u d y o f t h e s o i l m o i s t u r e i t s e l f , s i n c e th e
m a s s t r a n s p o r t o f t h e s o l u t e s is l e s s c o m p l e x t h a n t h a t o f t h e s o i l
m o i s t u r e . T h e s o l u t e p r o f i l e m e t h o d is a l s o , in p r i n c i p l e , w e l l s u i t e d f o r
a p p l i c a t i o n to r e m o t e a r e a s d u e t o it s r e l a t i v e l a c k o f s o p h i s t i c a t i o n .
In a d d i t i o n , th e m e t h o d is p r o b a b l y a p p l i c a b l e t o w i d e g e o g r a p h i c a l a r e a s
o f t h e s e m i - a r i d a n d a r i d r e g i o n s o f t h e w o r l d , w h e r e t h i c k u n s a t u r a t e d
z o n e s w i t h i n a r e n a c e o u s l i t h o l o g i e s a r e f a i r l y w i d e s p r e a d .
F u r t h e r f i e l d t r i a l s a r e n e e d e d to d e m o n s t r a t e th e w i d e r a p p l i c a b i l i t y
o f t h e m e t h o d h o w e v e r , and, a t t h i s s t a g e , s e v e r a l p o s s i b l e l i m i t a t i o n s
s h o u l d b e m e n t i o n e d . It is c o n s i d e r e d t h a t r e p r o d u c i b i l i t y o f t h e s o l u t e
p r o f i l e s is l i k e l y t o b e t h e e x c e p t i o n r a t h e r t h a n t h e r u l e ; v a r i a t i o n in
t h e s o l u t e d i s t r i b u t i o n m a y b e d u e t o t h e f o l l o w i n g ! ( a ) l a n d u s e (b) t h e
i n f l u e n c e o f p h r e a t o p h y t i c v e g e t a t i o n (c) t h e e x t e n t a n d n a t u r e o f c a l c r e t e s
o r s i m i l a r a c c r e t i o n a r y d e p o s i t s s (d) g e o l o g i c a l f a c t o r s i n c l u d i n g s e d i
m e n t a r y f e a t u r e s , p i p e s a n d f i s s u r e s . I t m a y b e d i f f i c u l t t o f i n d s i t e s
w h e r e i t is k n o w n t h a t l a n d u s e h a s r e m a i n e d c o n s t a n t f o r 1 0 - 5 0 y e a r s ,
a l t h o u g h it i s c o n s i d e r e d l i k e l y t h a t a c h a n g e in l a n d u s e d u r i n g t h i s t i m e
m a y b e i d e n t i f i a b l e b y u s e o f n i t r a t e , as m a y b e t h e e x p l a n a t i o n in th e
c a s e o f t h e A K 2 p r o f i l e f r o m C y p r u s ( F i g u r e 5). F u r t h e r s t u d i e s a r e b e i n g
c a r r i e d o u t i n C y p r u s t o e x a m i n e t h e e f f e c t s o f d e e p r o o t e d v e g e t a t i o n o n
t h e m e t h o d , s i n c e c l e a r l y , i n a n y r e c h a r g e s t u d y , t h e r e l a t i v e i m p o r t a n c e
o f v e g e t a t i o n t y p e s m u s t b e t a k e n i n t o a c c o u n t . T h e e f f e c t s o f c a l c r e t e
a p p e a r t o h a v e a p r o f o u n d i n f l u e n c e o n p e r c o l a t i o n a n d i t is c o n s i d e r e d
t h a t c a l c r e t e d e v e l o p m e n t m a y b e o n e o f t h e m o s t s i g n i f i c a n t f a c t o r s
6 6 EDMUNDS and WALTON
affecting both the development of the solute p ro files, and the amount of d irect recharge. This conclusion has also been reached by Foster e t a l . [28] • in recharge estimation studies in Botswana, during which,solute p rofiles were also determined; chloride p ro files, taken together with other evidence indicate very low recharge rates through the Kalahari Beds.
Results from the Cyprus profiles show that there is the p ossib ility of a method to determine not only the recharge amount, but also the recharge history. I t has been noted in the case of Libya, that i t is possible to distinguish isotopically and geochemically between major recharge events with a time difference of the order of 2000-10000 years. There is , therefore, a likelihood that, by careful examination of the solute, as well as the isotopic record of the unsaturated zone and the immediate w ater-table, that a chronology of recharge events throughout the h isto rica l period could be established. During periods of clim atic o scillatio n in semi-arid zones, there is likely to be a change from steady-state percolation, to a period of solute accumulation when recharge has ceased. However, on a longer time scale , of several decades or several hundred years for example, the accumulated solutes will eventually be flushed towards the w ater-table. In the thick ('vlOOm) unsaturated zones that e x ist in many parts of the semi-arid regions of the world, therefore, there may be a record of medium term clim atic changes, and corresponding changes in recharge h istory, that can be investigated by geochemical profile studies, by methods analogous to those developed in Cyprus.
From both the preceding examples - Cyprus and Libya - two general re quirements for further research emerge. F irs tly , i t is considered that greater attention must be paid to the geochemical processes taking place within the unsaturated zone. This is not only because of the immediate need for understanding the development of solute p rofiles for recharge estimation, but also because the accumulation of ca lcretes and similar deposits d irectly a ffe c t recharge to aquifers; conversely, the dissolution of accreted carbonates and soluble sa lts a ffe c ts , amongst other problems, the interpretation of radiocarbon ages. Furthermore, i t is likely that the investigation of stable isotope profiles of carbon, as well as hydrogen and oxygen, w ill also increasingly a ss is t the understanding of the record contained in the water and solutes of the unsaturated zone.
Secondly, i t is considered worthwhile to stress the almost complete lack of ra in fa ll chemical data currently available for most parts of the world, and for semi-arid and arid zones in p articu lar. If recharge studies using solute mass balance calculations are to continue, then i t is impprtant that chloride a t least of the input constituents, be measured on a global basis, in addition to tritium and stable isotopes, possibly by the IAEA or a sim ilar agency - using the established network, which could then be augmented by detailed ra in fa ll studies in the area of in terest to recharge assessment.
ACKNOWLEDGEMENTS
The work in Libya was carried out during a hydrogeological study of the S irte Basin and the cooperation of s ta ff of the Kufra-Sarir Authority is gratefully acknowledged. The radiocarbon analyses were performed by Dr D Harkness of the Scottish Research Reactor Centre, East Kilbride.
The results from Cyprus have been obtained with the cooperation of the Water Development Department; Mr J Jacovides and the s ta ff of the
IA EA -A G -158/4 6 7
Limassol office are thanked for their assistance. The boreholes were drilled by Mr M Howard assisted by Mr G H adjifilactou. Analyses for major ions were carried out in the UK by Mr D L Miles, Ms J M Cook and Ms J Trafford. Tritium determinations were made by AERE Harwell. John Barker, IGS is thanked for his help in the computer production of the solute p ro files. Our colleagues in the Hydrogeology Department, In stitu te of Geological Sciences, in particu lar Drs A H Bath, R Herbert, R Kitching and E P Wright have helped considerably in the discussion of the resu lts . The Cyprus studies fora part of the Semi Arid Zone Recharge project which is being funded by the Ministry of Overseas Development, London. This paper is published by permission of the D irector, In stitu te of Geological Sciences, London.
R E FER E N C ES
[lj PENMAN, H .L ., Proc. Roy. Soc. A 193 (1948) 120
[2] PENMAN, H .L ., Neth. J . Agrie. S c i . ,£ (1956) 9
[3] MONTE I TH, J . L . , Quart. J . Roy. Met. Soc., 87_ (1961) 171
[4] KITCHING, R ., e t a l . , J . Hydrol., 33 (1977) 217
[5] ZIMMERMAN, U., e t a l . , Isotopes in Hydrology (Proc. Symp. Vienna1966) IAEA, Vienna (1967) 584
[6] MUNNICH, K.O., e t a l . . Isotopes in Hydrology (Proc. Symp. Vienna1966) IAEA, Vienna (1967) 305
[7] SMITH, D.B., e t a l . , Isotope Hydrology (Proc. Symp. Vienna 1970)IAEA, Vienna (1970) 73
[8] ANDERSEN, L .J . , SEVEL, T ., Isotope Techniques in GroundwaterHydrology (Proc. Symp. Vienna 1974) IAEA, Vienna, 1 (1974) 3
[9] SUKHIJA, B .S ., SHAH, C.R,, J . Hydrol., 30 (1975) 167
[10] ALLISON, G.B., HUGHES, M.W., Isotope Techniques in GroundwaterHydrology (Proc. Symp. Vienna 1974) IAEA, Vienna, 1 (1974) 57
[11] DINCER, T ., et a l . , J . Hydrol., 23 (1974) 79
[1 2 ] WRIGHT, E .P ., e t a l . , Jalu-Tazerbo P roject: Phase I . Final Reportto Kufra Sarir Authority, Libya. (1974) unpublished.
[13 ] EDMUNDS, W.M., WRIGHT, E .P ., J . Hydrol., 40 (1979) 215
[14 ] BENFIELD, A.C., WRIGHT, E .P ., Proc. Second Symposium on Geologyof Libya, Tripoli (1978) in press
[1 5 ] REARDON, E .J . , FRITZ, P ., J . Hydrol., 36_ (1978) 201
[16] DEINES, P ., e t a l . , Geochim. Cosmochim. Acta, 3£ (1974) 1147
[1 7 ] SONNTAG, C ., et a l . , Geol. Rundsch-, 6 7 (1978) 413
SONNTAG, C ., e t a l . , Isotope Hydrology 1978 (Proc. Symp. Neurerberg) IAEA, Vienna, 2(1979) 569
MALEY, J . , Nature, 269 (1977) 573
ROGNON, P ., WILLIAMS, M.A.J., Palaeogeogr., Palaeoclim atol. , Palaeoecol. , 2l_ (1977) 285
PACHUR, H -J., Erde, 106 (1975) 21
ERIKSSON, E ., Interpretation of Environmental Isotope and Hydro- chemical Data in Groundwater Hydrology, IAEA, Vienna (1976) 171
ALLISON, G.В ., HUGHES, M.W., Aust. J . Soil Res., 16 (1978) 181
FINK, M., Preliminary Report on the Hydrogeology of the Akrotiri Peninsula. Tahal Ltd. Isreal (1965) unpublished.
EDMUNDS, W.M., e t a l . , in preparation
EDMUNDS, W.M., BATH, A.H., Environ. Sei. Techol., 10 (1976) 467
PATTERSON, R .J . , e t a l . , Canad. J . Earth Sei., (197 )
FOSTER, S .S.D., in preparation
EDMUNDS and WALTON
IA EA -A G -158/5
A N E X A M I N A T I O N O F R E C H A R G E M O U N D
D E C A Y A N D F O S S I L G R A D I E N T S I N
A R I D R E G I O N A L S E D I M E N T A R Y B A S IN S
J.W. LLOYD Hydrogeological Section,Department of Geological Sciences,University of Birmingham,United Kingdom
Abstract
A N E X A M IN A T IO N OF REC HARG E M O UND D EC AY AN D FOSSIL G R AD IE N TS IN A R ID R E G IO N A L S E D IM E N TA R Y BASINS.
In many o f the vast arid sedimentary basins o f the w orld, groundwater gradients exist
that appear to be anomalous in the con text o f the probable modem recharge potentia l. The possib ility that such gradients are in fact rem nant fossil conditions representing the decay o f
ancient recharge mounds is examined. An example o f decay cond ition is represented using a
resistor-network analogue m odel in w hich the tim e con tro l is based on 14C ages. The decay
hypothesis is found to be plausible w ith realistic aquifer characteristics bu t a non-homogeneous
f lo w is indicated from the 14C data.
INTRODUCTION
In spite of the relatively isolated nature of many of the groundwater studies carried out in the vast sedimentary basins of North Africa and Arabia a general interpretation is emerging of groundwater equi-potential surfaces. These surfaces appear normally to slope away from the aquifer outcrop areas in the direction of the predominant geological dip. As such they represent classical groundwater flow patterns consistent with those found in temperate and other areas. A major difference exists, however, in that it is difficult in some cases to account for the gradients and the apparently large volumes of flow moving through the systems on the basis of the present-day recharge.
HYDROGEOLOGICAL CONDITIONS
The average annual rainfall over aquifer outcrop areas in the basins is low (usually < 50 mm), and frequently no rainfall occurs for long periods. Although limited comprehensive precipitation and recharge control is available, it is obvious
6 9
7 0 LLOYD
that only minor direct recharge can occur. In certain areas, such as southern Jordan and parts of Saudi Arabia.it is probable that important indirect recharge occurs to the basin system through run-off, from for example the Precambrian Basement mountains adjacent to the sedimentary outcrop areas. Undoubtedly due to the arid nature of the climate, major storm and run-off events produce intermittent indirect recharge in all the basins. Unfortunately such instances are not well documented, but they must be few in number and the resultant recharge can only equate to a small proportion of the throughput volume in many areas.
Environmental isotope data support the presence of some modern recharge at outcrop. Away from outcrop, however, radiocarbon data throughout most of Arabia and North Africa indicate that the bulk of the groundwater is old, up to and exceeding 35 000 years (the reasonable limit of radiocarbon dating).
Even in the outcrop areas local gradient control solely by modern recharge may be questionable as, for example, in Jordan where the well hydrographs in the confined areas close to outcrop do not always show evidence of seasonal water-level fluctuation as would be expected with such recharge (Lloyd, 1970) [ l ] ,1 a similar lack of fluctuations is reported from central Saudi Arabia (Clarke, 1977) [2].
Burdon (1977) [3] in an excellent discussion has postulated a variety of mechanisms other than modem recharge to account for the groundwater gradients found in these arid basins. Some of the suggestions, though possible, are unlikely to be significant. However, the hypothesis that the existing gradients can be attributed to the creation of recharge mounds in the pluvial Pleistocene periods and subsequent long-term head decay under distant gradual discharge appears plausible and merits further consideration.
INDICATIVE MODEL
Unfortunately, to study the hypothesis only broad physical dimensions can be considered at present owing to limited regional data. The hypothesis, however, can be tested by flow model techniques using reasonable parameter assumptions as detailed by Burdon in combination with control data from a particular basin.For this purpose, in the present study Burdon’s data have been combined'with data from the Western Desert of Egypt and certain groundwater control based on the Bahiriya Oasis (Farag, 1978) [4] to provide a representative basin.
To model the basin a resistance network analogue has been used representing a section of the Lower Bahariya Formation aquifer extending north-eastwards from the Sudan, Libya and Egyptian border intersection through Bahariya Oasis to the Mediterranean. The aquifer is a Cenomanian sandstone with an average thickness of 500 m. The data for the existing groundwater gradient is based on work by Farag (Fig. 1) and it is assumed that leakage occurs from the system north
IA EA -A G -158/5 7 1
FIG .l. Regional groundwater piezometric surface fo r the lower Bahariya Formation aquifer.
Pie
zom
etr
ic
Hea
d (m
]
7 2 LLOYD
200О
£ E 150 <p —É "OS 2 100
T im e -h e a d re la tio n s h ip a t B a h a r iy a O a s is
• • ♦В A B
U C dates * -j-0p 0f a q u ife rTop I Bottom ~ .. . . .
a q u ife r I Ja q u ife r ♦ Bottom of a q u ife r----1—*-----•—r------------ 1------------ 1-------------1------------2 0 ¿ 0 6 0 8 0 10 0
T i m e 1 1 0 0 0 y e a r s )
К = m /day h = m
Time (1000 ye a rs )
FIG.2. Details o f gradient decay in an unconfined area.
IA EA -A G -158/5 7 3
900-
800 -
- 700 - S£эQ 6 0 0 -
* 500 -x» a>Za»О Д 0 0 Ч£3a*ooa».c
0 3 0 0 -
01EoNa<¿ 200-
100-
A ssu m e d p ie z o m e tr ic
leve l 10.000 y e a rs b e fo re p re s e n t
P ie zo m e tric leve ls
a f te r O iab a n d Farag
P ie z o m e tric leve l o b ta ined fro m m odel
u s in g a q u ife r c h a ra c te r is t ic s
a s show n on F ig u re 3 * \
100 200 300 400 '5 0 0 600
Distance (km)
700 800 900
FIG.3. Simulated piezometric surface through a southwest to northeast section o f the Western Desert o f Egypt.
of the Mediterranean coast. It will be noted that the head in the aquifer, although it is confined, is at Mediterranean datum at the coast. Pumping test data from the Bahariya Oasis give a permeability range o f 6 to 0 .4 m/d while Burdon uses a permeability of ! .7 and 0.8 m/d for his conceptual basin. In the present study these values have been taken as a guide. In the absence of reliable measured specific yield data reasonable estimates have been made.
The control of the Bahariya Oasis is important to the model since, apart from head control, hydrochemical data distinguish a calcium-sodium chloride water at depth. These data suggest that the groundwater at depth has traversed
7 4 LLOYD
longer flow paths in the aquifer than the shallower water. This supposition is supported by radiocarbon data indicating an age of 25 000 years for the upper water and approximately 35 000 or a little older for the deep water.
The model has been constructed assuming steady-state Laplace conditions with the time variance of the groundwater flow being approximated to a series of steady-state situations (Herbert and Rushton, 1966) [5]. For initial conditions in the model a head of 9 0 0 m has been assumed in the outcrop area with 0 m at the discharge point. To simulate head decay in the recharge mound the release of groundwater from storage with decreasing head has been determined by the following equation:
Ah,sz = (A t/S y)K — * (1)
Az
where sz is the decline in head in the vertical (z) direction At is the time interval Sy is the specific yield for the outcrop area К is the permeability
is the head difference between two vertical nodesAhLAz is the vertical distance between two nodes
To represent realistic conditions an analogue was constructed using a decreasing permeability distribution with depth, as this parameter is considered to be the most likely control of the differential flow considered to be present at Bahariya. On the basis of past climatic records (McBurney, 1967) [6] it was assumed that maximum head (9 0 0 m) was operative in the outcrop until 10 000 years ago. From 10 000 years to the present time, head decay was assumed to operate (Eq. ( 1 )) and was measured over a series of decline steps.The rate of decline in the outcrop area is shown in Fig. 2 for various permeability and specific yield values. For control the flow velocities from the model were calculated at Bahariya and converted to ages. It will be seen that the В combination provides a reasonable correlation with the Bahariya control data and that the aquifer characteristics are of a good acceptable order. Further, as can be seen in Fig. 3 the simulated decayed gradient using the В parameters shows a good approximation to that believed to extend through the Egyptian Western Desert today.
CONCLUSIONS
It is considered that the study supports the view that present-day gradients in the regional arid basins of Saudi Arabia and North Africa can in part be attributed to head decay of recharge mounds created in the Pleistocene.
IAEA-AG-15 8 /5 7 5
The model study presented is not claimed to be definitive for any particular basin as similar results may be possible from a variety of parameter permutations. However, the simulation obtained using reasonable basin dimensions, aquifer parameters and climatic assumptions, clearly shows that a gradient can persist for a long period and that fossil gradients and their implications should be sensibly considered in proposals for long-term groundwater developments in the basins under discussion.
REFERENCES
[1] LLOYD, J.W., Hydrogeological procedures in consolidated sediments of underdeveloped arid areas, Ph. D. Thesis, University of Bristol, England (1970).
[2] CLARKE, L., (In discussion of BURDON, D.J., 1977), Flow of fossil groundwater,Q J. Eng. Geol. 10(1977) 97.
[3] BURDON, D.J., ibid.[4] FARAG, M.H., The geology and hydrogeology of the Bahariya Oasis Western Desert,
Egypt, Ph. D. Thesis, University of Birmingham, England (1978).[5] HERBERT, R., RUSHTON, K.R., Groundwater flow studies by resistance networks,
Geotechnique 16(1966) 53.[6] McBURNEY, C.B.M., The Haua Fteah, Cyrenaice and the Stone Age of the South-East
Mediterranean, Cambridge University Press (1967) 387pp.
IAEA-AG-15 8 /6
E N V I R O N M E N T A L IS O T O P E S IN
N O R T H A F R I C A N G R O U N D W A T E R S ;
A N D T H E D A H N A S A N D - D U N E
S T U D Y , S A U D I A R A B I A
C. SONNTAG, G. THOMA, K.O. MÜNNICH Institut für Umweltphysik der
Universität Heidelberg,Heidelberg, Federal Republic of Germany
T. DINÇERFood and Agriculture Organization
of the United Nations, Rome
E. KLITZSCH Technical University,D-1000 Berlin
Abstract
E N V IR O N M E N T A L ISOTOPES IN NO RTH A F R IC A N G ROUNDW ATERS; AN D THE
D A H N A SAND-DUNE STUDY, SAU D I A R A B IA .
I. N orth Saharian palaeowaters were m ainly form ed during a long hum id period between
50 000 and 20 000 years BP., which was fo llow ed by a cool dry period from 20 000 to
14 000 years BP. These palaeowaters show a significant west-east decrease in deuterium and
180 because o f past groundwater fo rm ation by local ra in fa ll from the western d r ift . Sahel zone groundwaters seem to show m eridional variation o f deuterium and ls O due to a tropical
convective influence.II. A com puter model estimate o f the alternate play between rainwater in f iltra tio n and
evaporation in the Dahna sand-dune (near R iyadh, Saudi Arabia) yields a mean annual groundwater recharge o f 20 mm annually w h ich agrees w ith that obtained from bomb tr it iu m vertical
profiles o f the sand m oisture. The m odel also describes the deuterium and 180 profiles.
I. ISOTOPE DATING OF SAHARIAN GROUNDWATERS
The west-east decrease of the heavy stable isotopes (D and 180 ) in the groundwaters from the central and eastern Sahara suggests that these groundwaters were mainly formed by local rainfall during humid phases of the past [1 ,2 ] . Carbon-14 data give no indication of long-range movement, with increasing 14C age in the direction of flow as suggested by Ambroggi [3]. This means that the inner Saharian groundwater storage is not recharged under present climatic
7 7
7 8 SONNTAG et al.
conditions. There may exist groundwater recharge in some places on a regional scale, particularly near mountainous terrain with sufficient rainfall.
An extension of our isotope dating to the western and the southern Sahara, including the Sahel Zone, is under way. Ulf Thorweihe (geologist, Berlin) and Jochen Rudolph (physicist, Heidelberg) are now on their way from Algier to Lake Tchad, collecting groundwater samples for isotope and noble gas analyses. It is hoped that the noble gas data will provide information on the ground-level temperature at the time and place of groundwater formation [4]. Up to now we have no available data.
Our data collection now contains environmental isotope data, hydrological and hydrochemical data, of several hundred groundwaters from northern Africa.The data are of various origins and cover Algeria, Tunisia, Libya, Egypt, Sudan, Chad, Nigeria, and Senegal. The coverage of the western Sahara, and the region south of the Sahara, however, has been relatively poor up to now. A tendency of meridional variation of D and 180 is to be seen in the southern area groundwaters indicating a tropical convective origin. Part of the deep groundwater ( 14C age > 20 000 years) [5] in the Bara Basin (Kordofan/Sudan) is considerably lower in D and 180 than modern groundwater at the same place. This may be due to a different water vapour origin in the past. The groundwater in northern Senegal, an area which is surrounded by the rivers Senegal and Gambia, shows uniform isotopic composition in both aquifer systems, the Maestrichtian Sands, and the Continental Terminal [6, 7]. The D and 180 appear to be low (5D s - 4 0 %o,50-18 = - 6 .2%c) if compared with local rainfall; presumably it is derived from the rivers mentioned. The 14C and tritium data indeed show groundwater flow towards the centre of the basin indicating recharge by river-water infiltration.This conclusion holds for the deep groundwater showing high 14C ages as well as for the modern groundwater containing tritium. Hydrochemical data support this by an increasing salt content towards the basin centre as well as by a typical development, from a bicarbonate type at the periphery to sulphate and, eventually, chloride type in the centre.
II. ENVIRONMENTAL ISOTOPES IN THE DAHNA SAND-DUNE
A t the beginning of 1978 T. Dinçer resumed his 1972 /73 isotope study at the Dahna sand-dune, Saudi Arabia [8]. The vertical profiles of soil moisture, tritium, D and 180 as observed in 1972 and in 1978, are presented in Fig. 1.The D and 180 profiles are fairly similar. This is more apparent in the 5D vs.5 180 diagram (Fig. 2) where all data points fall'on nearly the same evaporation Une. The low slope is due to kinetic fractionation by molecular diffusion of the water vapour through the stagnant air in the sand. The evaporation line cuts the meteoric water line at a point which approximately represents local groundwater. We intend to check this concept experimentally with sand samples to be taken
IAEA-AG-15 8 /6 7 9
FIG .l. Vertical distribution o f soil moisture and environmental isotopes in the Dahna sand- dune, Saudi Arabia. The 1972 data are taken from Dinçer et al. [8].
6 D [• /..]
FIG.2. SD versus 180 diagram o f the Dahna sand-dune moisture. The 1972 data com e from Ref. [8]. Low er part shows the laboratory column sand evaporation experiment.
8 0 SONNTAG et al.
[ w e ig h t °U]
0 2 4 6 0 2 í, 6
MARCH '70
FIG.3. Model estimate of the moisture development in the Dahna sand-dune, starting with a uniform moisture distribution below field capacity.
on our current North Africa expedition mentioned earlier. The data points on the other evaporation line in the lower part of Fig. 2 come from an evaporation experiment with a sand column in the laboratory. This line, naturally, cuts the meteoric water line at the point representing Heidelberg groundwater since the column had originally been loaded with tap water.
The 1972 tritium profile shows (Fig. 1) a broad maximum in 4 to 6 m depth, and the 1978 profile is at constant tritium level all the way down to 6 m. With Dinçer’s previous [8] recharge estimate (23 mm/a) and a 5 vol.% field capacity, the 1972 tritium peak is expected to have moved down by about two metres. The infiltration/evaporation model described in the following section yields about the same groundwater recharge rate as estimated by Dinçer [8]. This model further predicts that the sand should essentially be at field capacity below about one metre deep (Fig. 3). This, however, is not confirmed by observation. The moisture content of the 1978 profile is considerably lower throughout. This finding presents a problem.
EVAPORATION MODEL
Our model (Fig. 4) describes the alternate play of rain-water infiltration and evaporation from the sand with its effect on the moisture profile and its isotopic
IA EA -A G -158/6 81
box 1 BOX 2SPHERE 1
h 1 2 H 2 1. . N N N-1
VAPOR PRESSURE = ELECTR. POT. MOISTURE = ELECTR LOAD
R1 r 2 u2 «3 r n
=02_
/TRANSPORT RESISTANCEVAPOR
LIQUID
FIG.4. Desk computer evaporation model and electric analogue. Moisture transfer between boxes is by molecular diffusion of vapour, depending on saturation pressure difference, and by capillary flow. The vapour pressure decreases at very low moisture content (lower part of the Figure).
composition. We take the unsaturated zone as a series of N boxes with moisture content Qi, temperature Ti, and vapour pressure p¡. Starting with the highest box number the vapour pressure in each box is compared with that of the two adjacent boxes. If a pressure difference exists, vapour is transported to the lower pressure box by molecular diffusion through the soil air (with different diffusion constants for the different isotopic species of water) during the time interval selected, and the programme loop starts anew. The model includes liquid water flow by capillary suction. At high moisture content the saturation pressure depends exclusively on temperature; below the permanent wilting point, however, the pressure decreases with moisture content. This causes the boundary between dry and moist sand to cease being steplike, but rather to be of the error-function type.
The moisture development of the Dahna sand-dune as given by the model for the period 1964 to 1972 is shown in Fig. 3. To illustrate the downward movement of moisture we started with an arbitrary uniform moisture content below field capacity (which is estimated to be about 6 wt.%). It is evident in this case that infiltration exceeds evaporation loss. Calculations further show that a single heavy annual rainfall each 2 0 years or each 100 years must exceed 35, 77, and 196 mm, respectively, to compensate for evaporation under these
8 2 SONNTAG et al.
DAHNA SAND DÜNE
О 200 [TU]
2
3-
k
5-
6
7D E P T H [m l
■ ♦ ■ — D IN C E R E T A L . ■ 1972.. » 1 9 7 2 ------ * . - - . » — .-.*.1978
♦ ♦♦♦♦\ E VA P O R AT IO N M O D EL , R A IN : RIYADH 1972
1978 T R IT IU M : B A H R A IN
FIG.5. Observed bomb tritium profiles (1972, Ref. [8 ], and 1978) together with model estimates of the vertical tritium distribution in the Dahna sand-dune. Time period 1964-1972: rainfall from Riyadh, and tritium rain data from Bahrain (taken from Ref. [8 ]j. Time period 1972-1978: rainfall extrapolated from the previous data; tritium estimated from European rain data.
conditions. These lower limits then correspond to mean annual evaporation rates of 35, 3.8 , and 2 .0 mm, respectively.
The tritium profiles of the Dahna dune as estimated by the model are presented in Fig. 5, together with the observed profiles of 1972 and 1978. Since the rain data after 1972 were not available to us when the calculation was done, we assumed constant annual rainfall as extrapolated from the 1964—1972 data.
CONCLUSION AND RECOMMENDATIONS
Isotope studies in dune-sand areas should be intensified. With our new suction technique [9] samples can be taken from depths down to 30 m. Such studies can yield information on infiltration, evaporation and ablation in sand sea areas. The technique is relatively simple and cheap. A specific point in groundwater balance is the question of evaporation loss through the unsaturated zone. As an example Fig. 6 shows an estimate of the drawdown of the groundwater surface under a sand sea. It is assumed that at the end of the last humid phase the water table was close to the ground surface. During the following hyper-arid phase (no rainfall at all) the water table then decreases with the square root of time (age), the evaporation rate decreases correspondingly with the increasing thickness of the diffusion diaphragm. In this simplified model the
IA EA -A G -158/6 8 3
d e p th g roundw ate r s u rfa c e z Q pe te r? |
FIG . 6. Time (age) dependence o f drawdown o f the groundwater surface owing to evaporation through a completely dry sand cover. A t T = 0 (end o f the last hum id phase) the water-table is assumed to have been close to the ground surface.
unsaturated zone is assumed to be completely dry. After a 4000-year dry period, for instance, the drawdown is about 10 m, the remaining annual evaporation rate only about 1 mm. As in the case of the diffusion/advection experiment of Zimmermann [10] one must expect isotope fractionation effects in this case which, however, do not reach a steady state. A stepwise calculation is now under way.
REFERENCES
[1 ] SONNTAG, C., K LITZ SC H , E ., EL SH A ZLY, E.M ., K A LIN K E, Chr., MÜNNICH, K .O ., “Paläoklimatische Inform ation im Isotopengehalt C-14 datierter Saharawässer: Kontinentaleffekt in D u n d O -1 8 ” , Geol. Rdsch. 67 Heft 2 (1 9 7 8 ) 4 1 3 - 2 4 .
[2 ] SONNTAG, C., et al., “Paleoclim atic inform ation from deuterium in and oxygen-18 in carbon- 14-dated North Saharian groundwaters: Groundwater form ation in the past” , Isotope Hydrology 1978 (Proc. Symp. Neuherberg, 1978), IA EA, Vienna (1 9 7 8 ) 5 6 9 —81.
[3] AM BROGGI, R .P., Water under the Sahara, Sei. Am. 214 (1 9 6 6 ) 2 1 - 2 9 .[4 ] MAZOR, E ., “Paleotemperatures and other hydrological parameters deduced from noble
gases in groundwaters; Jordan R ift Valley, Israel” , Geochim. Cosmochim. A cta 36 (1 9 7 2 ) 1 3 2 1 -3 6 .
[5 ] M ABROOK, B ., A BD EL SH A FI, M .Sh., “Hydrological and environmental isotope studies o f Bara Basin, central Sudan” , Symp. on Trace Elem ents in Drinking Water, Agriculture
SONNTAG et al.
and Human L ife , M iddle Eastern Radioisotope Centre and Goethe In s titu t, Cairo,Jan. 1 0 -1 3 , 1977.
C ASTAN Y, G., et al., “ Etude par les isotopes du m ilieu du régime des eaux souterraines
dans les aquifers de grandes dimensions” , Isotope Techniques in Groundwater H ydrology
1974 (Proc. Sym p.Vienna, 1974) 1, IA E A , Vienna (1974) 243.
K L U B M A N N , P., SONNTAG, C., (unpublished w ork).
D IN Ç ER , T., A L-M U G R IN , A ., Z IM M E R M A N N , U., “ Study o f the in filtra tio n and
recharge through the sand dunes in arid zones w ith special reference to the stable isotopes
and thermonuclear tr it iu m ” , J. H ydrol. 23 (1974) 79— 109.
TH O M A , G., et al., “ New technique o f in-situ soil-moisture sampling fo r environmental
isotope analysis applied at Pilat Sand Dune near Bordeaux: HETP m odelling o f bomb
tr it iu m propagation in the unsaturated and saturated zones” , Isotope H ydrology 1978 (Proc.
Symp. Neuherberg, 1978) 2, IA E A , Vienna (1978) 753— 68.
Z IM M E R M A N N , U., E H H A L T , D., M Ü N N IC H , К .О ., “ Soil-water movement and
évapotranspiration: changes in the isotopic com position o f the water” . Isotopes in H ydrology (Proc. Sym p.Vienna, 1966), IA E A , Vienna (1967) 567.
IAEA-AG-15 8 /7
A N IN J E C T E D G A M M A - T R A C E R M E T H O D
F O R S O IL - M O IS T U R E M O V E M E N T
I N V E S T IG A T IO N S I N A R I D Z O N E S
A.R. NAIR, S.V. NAVADA, S.M. RAO Isotope Division,Bhabha Atomic Research Centre,Bombay, India
Abstract
A N INJECTED G AM M A-TR A C ER M ETHOD FOR SO IL-M O ISTURE M O VEM EN T
IN VE STIG A TIO N S IN A R ID ZONES.
A method fo r the in-situ determ ination o f soil-moisture transport rates using K 360Co(CN)6
is discussed. The tracer compares w ell w ith tr itia te d water in laboratory investigations and the
results obtained in lim ited fie ld studies are very encouraging. The method promises to be o f
specific interest in arid-zone investigations where the soil-moisture fluxes in liq u id and vapour
phases could cause complications fo r tr it iu m tracer data in terpre ta tion.
INTRODUCTION
In regions where the rainfall is low only a small fraction of the precipitation contributes to groundwater recharge, a major portion being lost by evaporation from the surface layers. Estimation of the direct recharge to the groundwater body is of special importance in such regions where often the only source of water supply is sub-surface.
Tracer methods either involving the study of natural tritium profiles in the soil [1] or the injection of artificial tritium [2] are now well established and extensively applied by many investigators.
The main disadvantage of the tritium method is that it is not amenable to in-situ and non-disturbing detection. The procedure involves collection of soil samples by augering, extraction of moisture by vacuum distillation in the laboratory and finally liquid scintillation counting for tritium activity.
The advantage that the tritiated water is an “ideal” tracer for water transport could sometimes “paradoxically” be a disadvantage. For example, in low moisture areas such as arid zones, the tracer could get into vapour phase during the day and may condense at points above the injection point during the night and thus cause complications in tracer profile interpretation. The authors had this experience while using the tritium tracer in soils with 1% moisture content in an arid zone in Rajasthan.
8 5
NAIR et al.
COBALT-60 METHOD
TRACER - Kj^CoiCNle COMPLEX OR 6°Co-EDTA COMPLEX
• Labelling of water layer around a borehole with tracer at desired depth
• In-situ detection of tracer position withscintillation detector
FIG .l. Soil-moisture movement study with radiotracers.
2. Experimental set-up for soil-moisture tracing with K3Co(CN)6 and НТО.
IAEA-AG-15 8 /7 8 7
o-£ Ю-U
*20- I 30 iI—CL
2000 6000 6000 8000 Ю000 12000
INJECTION iréórНТО, Kj[6ôCo(CN) ]
— *■ C o u n ts /m in
S c in t illa t io n C o u n tin g
WATER SPRAYED'71 mm
10000r 0-
Ю- 20* 30-
40-
Ю2030'40-
50-
20-30-
4 0 -
50-
6 0 -
5 0 0
20Ó 00 — -» • C o u n ts /m in
S c in t i l la t io n C ounting
12 days after injection WATER 5PRAYED‘86mm
L iqu id S c in t i l la t io n C o u n tin g
1000 ------- -- C o u n ts /m in
500 0 Ю 000 » C o u n ts /m in
S c in tilla tio n C o u n tin g
26 days after injection
WATER SPRAY ED : 1A6 m m
loo’o 20'00 3000 4000 5000
1000
L iq u id S c in tilla tio n C o un ting 1500 — C o u n ts /m in
C o u n ts /m in
S c in t i l la t io n C o u n tin g
56 days after injection WATER 5PRAVE0-. 157mm
L iq u id S c in t i l la t io n C o u n tin g
- ■ Counts / min
FIG.3. Comparison between К 3 [^CofCNjf,] and НТО.
Also Bath [3], while discussing the possibility of assessing modern recharge using natural tritium profiles, reported on the absence of peaks in the unsaturated zone in the Kalahari desert. Sonntag et al. also reported [4] on the disappearance of tritium peak which was observed in 1972 in the unsaturated zone of the Dahna sand-dune (Saudi Arabia).
GAMMA-TRACER METHOD
The gamma-tracer method (Fig. 1) being used by us both in laboratory models as well as in the field studies is as follows:
The selected radiotracer is injected around an access hole at several points on a circle at a suitable depth;
DEPT
H IN
8 8 NAIR et al.
FIG.4. Laboratory comparison o f K 3 [ ’CoiCN)^] and НТО movement in clayey soil.
The tracer is allowed to diffuse and form a ring-like layer around the access hole; and
The position of the tracer peak is determined at regular intervals by simply logging the hole with a scintillation detector.
The advantages of the gamma tracer method over the tritium method are:
No repeated augering to extract soil samples and hence no disturbance to soil conditions;
No elaborate laboratory support is needed;
The same borehole or access hole can be used for soil moisture and bulk density measurement using a neutron/gamma gauge; and
Higher accuracy in positioning the tracer patch.
IA EA -A G -158/7 8 9
FIG.5. Comparison between 6aCo-EDTA and K 3 [^Co/CN)^] in clayey soil.
« НТО
X * « Injections ite
FIG. 6. K 3 \ °Co(CN)6] and НТО injections at Chikli.
OEPTH
IN
DEPT
H IN
cm
SITE: CHIKL1 VILLAGE . BHUSAVAL DIST: JALGAON. MAHARASHTRA
\oо
SOIL SURFACE
Z
FIG. 7. Soil-moisture movement study with НТО and K* [Co(CN)6\
DEPT
H IN
cm
IAEA-AG-15 8 /7 91
Initial column studies carried out with K 3Co(CN)6 labelled with shorter- lived S8Co and reported elsewhere [5] showed that the tracer maintained its position integrity in a dry soil column for over four months and it was possible to follow its movement with intermittent spraying at the surface of the column.
To compare the transport characteristics of K 3 Co(CN)6 with those of tritiated water, a special laboratory model has been built (Fig. 2), consisting of a large drum filled with soil and having a bottom outlet for water. Rigid plastic tubes with nearly 30 to 40% perforations are placed around a central access hole. The cobalt-cyanide tracer labelled with 60Co and tritiated water are injected together at points between the perforated tubes at a specified depth. The position of the gamma-emitting tracer is located by logging the access hole with a scintillation probe. The position of the tritium tracer is detected by extracting one of the plastic tubes, cutting it into sections, extracting the moisture and assaying for tritium by liquid scintillation counting. Figure 3 compares the two tracers in sandy soil. Similar studies were carried out on clayey soils. Figure 4 gives an example of such a comparison.
' Laboratory studies carried out using 60Co-EDTA indicate that this tracer was retarded with respect to the cyanide tracer (Fig. 5).
L A B O R A T O R Y E V A L U A T IO N O F T H E M E T H O D
FIELD STUDY
Tracer studies are being carried out to estimate direct recharge to the groundwater reservoir in the Tapi alluvium between the Satpura hills and the Tapi river, in western India. The alluvial deposits consisting of clays, silt, sand, gravels and pebbles vary in thickness from 0 to 3000 m and have a number of water-bearing sand formations with interspersed clay lenses. The main problem in the region is the rapid depletion of the water-table, particularly in the case of the dug wells nearer to the Satpuras.
Besides the usual artificial tritium method, the cobalt method is also being tried to examine its feasibility under field conditions. The cobalt-cyanide tracer was injected around an unused 20-cm bore-well in Chikli village in the region in September 1976. Figure 6 gives details of cobalt and tritium injections at this site.
The position of the gamma tracer is determined by logging the bore-well with a gamma-scintillation detector. The tritium profile is obtained by the usual core sampling and tritium analysis procedures.
Figure 7 shows the results obtained at Chikli with the cobalt and tritium tracers for the period December 1976 to April 1978. Below is the tabulated recharge data obtained by the two methods:
9 2 NAIR et al.
Tracer Tracer Moisture Rainfall Recharge % ofdisplacement content rainfall(cm) (cm) (cm)
60Co 20(C .G .) 6.1 74 8
НТО 35 (peak) 10.6 74 14
DISCUSSION AND CONCLUSION
The reason for the difference between the transport rates of the cobalt complex and the tritiated water is not readily discernible in view of their similar behaviour in laboratory experiments carried out on the same soil. It should, however, be noted that the error in peak positioning in the НТО method is at least ± 5 cm. Considering the extremely slow movement of the soil moisture and the possibility of differences in transport rates from one point to another in the same site, the results obtained in the field study are quite encouraging. However, the study has not been undertaken on a larger scale with cobalt and tritium injections in five different sites in the region.
The field and laboratory experience with the cobalt complex method indicates the feasibility of its use in arid regions in understanding the mechanisms by which moisture is transported downwards through the unsaturated zone. By using a double-tracer method involving the cobalt complex and tritiated water it should be possible to differentiate between the liquid and vapour phase movements of soil moisture in the arid zones.
REFERENCES
[1 ] M Ü N N IC H , К.О ., ROETHER, W., T H ILO , L., “ Dating o f groundwater w ith tr it iu m
and 14C” , Isotopes in H ydro logy (Proc. Sym p.Vienna, 1966), IA E A , Vienna (1968)
3 0 5 -2 0 .[2 ] Z IM M E R M A N N , U., M Ü N N IC H , K.O ., ROETHER, W., “ Downward movement o f soil
moisture traced by means o f hydrogen isotopes” , Isotope Techniques in Hydrological
Cycle, Proc. Symp. University o f Illino is , 1 0 -1 2 November ( 1965) 28— 36.
[3 ] BA TH , A .H ., Private com m unication.[4 ] SONNTAG, C., TH O M A, G., M Ü N N IC H , K .O ., D INÇER, T., K L ITZS C H , E., “ Environ-
mental isotopes in N orth A frican groundwaters, and the Dahna sand-dune study, Saudi
Arabia” , IA E A -A G -158/6 , these Proceedings.
[5 ] RAO, S.M., Use o f injected radioactive tracers in groundwater hydro logy, Indo-German
Workshop on Approaches and Methodologies fo r Development o f G roundwater Resources,
H yderabad,India ,26— 30 May, 1975.
IA EA -A G -158/8
A S P E C T S O F T H E IS O T O P E H Y D R O L O G Y
O F T W O S A N D S T O N E A Q U IF E R S
I N A R I D A U S T R A L I A
P.L. A IREY, G.E. CALF, P.E. HARTLEY,D. ROMAN Isotope Division,Australian Atomic Energy Commission,Research Establishment,Lucas Heights, NSW, Australia
Abstract
ASPECTS OF THE ISOTOPE H Y D R O LO G Y OF TWO SANDSTONE AQ U IFER S IN A R ID
A U S T R A LIA .
Much o f Austra lia ’s arid zone lies in a high-temperature region where po ten tia l evaporation
rates can exceed the mean annual ra in fa ll by more than an order o f magnitude. Underground
water therefore tends to be a biased sample o f precip ita tion. The e ffect o f this biasing on the
stable isotopic com position and the deuterium excess o f water from the Mereenie Sandstone
aquifer and the western extrem ities o f the Great Artesian Basin is discussed. A n explanation
is offered fo r an observed systematic decrease in the standard deviation o f stable isotope ratios
w ith increasing residence time. An analytical solution o f a sim plified version o f the ground
water transport equation is presented w hich allows a clear mathematical d is tinc tion to be made
between a ‘flow ing ’ and a ‘non-flow ing ’ aquifer system.' This d is tinction can be useful in arid
zones where the isotopic data may no t be inherently predictable from the modern conditions
at inpu t because o f substantial differences in recharge rates over the accessible tim e scales.
The mathematical expressions also provide a basis fo r assessing when 14C data are more logically
presented as absolute activities than as the conventional 14C (per cent modern carbon) ratios.
IN TR O D U C TIO N
A u s t r a l i a n a r i d z o n e s h a v e h i g h p o t e n t i a l e v a p o r a t i o n r a t e s w h ic h c a n e x c e e d m e a n a n n u a l r a i n f a l l b y m o r e t h a n a n o r d e r o f m a g n i t u d e . T h u s , i f p r e c i p i t a t i o n r a t e s w e r e c o n s t a n t t h e r e w o u ld b e n o a c c u m u l a t i o n o f g r o u n d w a t e r d u e t o l o c a l r e c h a r g e ; h o w e v e r g r o u n d w a t e r r e s e r v e s e x i s t b e c a u s e t h e r a i n f a l l d i s t r i b u t i o n i s f a r f r o m u n i f o r m . N e t r e c h a r g e o c c u r s d u r i n g p r o l o n g e d p e r i o d s o f h i g h e r t h a n a v e r a g e i n t e n s i t i e s . T h e u n d e r g r o u n d w a t e r i s , t h e r e f o r e , a v e r y b i a s e d s a m p le o f p r e c i p i t a t i o n a n d t h i s i s r e f l e c t e d i n i t s i s o t o p i c c o m p o s i t i o n .
T h e b o u n d a r i e s o f a r i d A u s t r a l i a , a s d e f i n e d f o r a m a jo r s y m p o s iu m h e l d a t C a n b e r r a i n 1 9 6 9 [ 1 ] a r e sh o w n i n F i g u r e 1 .T h e t o t a l a r e a o f t h e z o n e i s a p p r o x i m a t e l y 5 . 0 x 1 0 6 km2 . " B y
9 3
VO4
TABLE I
STABLE ISOTOPE RATIOS
Area Sample age (a) grouping
<5d(per mille)
fi^O(per mille)
d (b)(per mille)
SD (ÔD) (с) (per mille)
Newer Basalts aquifer [4]
upper aquifer groundwater 1 -32.8 -5.64 12.3 4.3lower aquifer и " 1 -30.8 -5.08 9.8 5.6
Melbourne rain -31.2 -5.31 11.3 18
Mereenie Sandstone aquifer [3]
groundwater 2 -61.5 -8.9 9.8 1.87
Alice Springs rain -27.3 -3.7 22(d) 30(d)
Great Artesian Basin- western extremiti es groundwater 3 -48.3 -7.03 8.0 1.33- Jurassic aquifer
Queensland [10] .. 4 -42.4 -6.78 11.8 1.10
(a) Residence times increase in the order 1 < 2 < 3 < 4(b) Deuterium excess d = ÔD - 8 6180(c) Standard deviation of the population(d) Standard deviation of values exceeding the 90 percentile level
\
AIREY et al.
IAEA-AG-158/8 9 5
© LA K E EYRE
© COOPERS CREEK
© D IAM ANTIN A RIVER
B R ISB AN E
PERTHSYDNEY
URNE
FIG .l. Location map. The dotted line delineates arid Australia as defined in Ref. [1]. The Great Artesian Basin is shown; the region o f the basin discussed in this paper is cross- hatched.
f a r t h e g r e a t e s t p a r t o f t h i s , n e a r l y 3 . 8 x 10 ® km^ i s a n a r e a o f i n t e r n a l d r a i n a g e , w h e r e s t r e a m s a r e e p h e m e r a l o r n o n - e x i s t e n t , m o s t h a v e n o d e f i n e d d e s t i n a t i o n , a n d t h o s e t h a t d o , s u c h a s t h e C o o p e r a n d D i a m a n t i n a s y s t e m s t e r m i n a t i n g i n L a k e E y r e , r e a c h i t o n l y o n r a r e o c c a s i o n s o f t h e l a r g e s t f l o o d s . . T h e u n r e l i a b i l i t y
o f r u n o f f a n d t h e h i g h r a t e o f e v a p o r a t i o n p r e v a l e n t t h r o u g h o u t t h e a r e a e n f o r c e a w i d e s p r e a d d e p e n d e n c e o n g r o u n d w a t e r f o r p a s t o r a l a n d d o m e s t i c p u r p o s e s " [ 2 ] .
I n t h e p r e s e n t p a p e r , t h e i n t e r p r e t a t i o n o f h y d r o g e n , o x y g e n a n d c a r b o n i s o t o p e d a t a i s d i s c u s s e d . R e f e r e n c e i s m ad e t o p u b l i s h e d d a t a f r o m t h e M e r e e n i e S a n d s t o n e a q u i f e r s o u t h o f A l i c e S p r i n g s , N o r t h e r n T e r r i t o r y [ 3 ] a n d t o u n p u b l i s h e d i n f o r m a t i o n . f r o m t h e w e s t e r n e x t r e m i t i e s o f t h e G r e a t A r t e s i a n b a s i n ( F i g u r e 1 ) . T h e r e s u l t s f r o m t h e a r i d z o n e a r e c o m p a r e d w i t h t h o s e f r o m a s t u d y o f t h e N e w e r B a s a l t s a q u i f e r n e a r M e l b o u r n e , V i c t o r i a [ 4 ] .
ST A B L E IS O T O P E DATA
T h e s t a b l e i s o t o p e d a t a f r o m t h e a q u i f e r s o f i n t e r e s t , a n d f r o m m o n t h ly s a m p l e s o f p r e c i p i t a t i o n i n t h e r e c h a r g e a r e a a r e s u m m a r is e d i n T a b l e 1 .
9 6 A IR EY et al.
"I Q( i ) t h e ÔD a n d 6 0 v a l u e s p r o v i d e e x c e l l e n t e v i d e n c e
t h a t g r o u n d w a t e r i n a r i d z o n e s i s a b i a s e d s a m p leo f t h e p r e c i p i t a t i o n i n t h e r e c h a r g e a r e a ;
( i i ) i n lo w r a i n f a l l a r e a s , g r e a t c a r e n e e d s t o b e e x e r c i s e d i n t e r p r e t i n g d i f f e r e n c e s i n t h e m a g n i t u d e o f d e u t e r i u m e x c e s s d ( = ÔD - 8 ô ^ ® 0 ) f r o m r e g i o n t o r e g i o n ; a n d
( i i i ) t h e r e i s so m e e v i d e n c e t h a t t h e s t a n d a r d d e v i a t i o n s SD (6 D ) d e c r e a s e w i t h i n c r e a s i n g a g e .
V a r i a b i l i t y o f ¿D a n d б -^ О
A h i s t o g r a m c o m p a r in g t h e f r e q u e n c y d i s t r i b u t i o n o f 6Dv a l u e s i n m o n t h ly r a i n f a l l s a m p l e s f r o m M e lb o u r n e a n d A l i c eS p r i n g s i s sh o w n i n F i g u r e 2 [ 5 ] . T h e sa m e s c a l e h a s b e e n u s e dt o e m p h a s i s e t h e g r e a t e r v a r i a b i l i t y a t t h e a r i d z o n e s t a t i o n .T h e d a t a f o r m o n th s w i t h r a i n f a l l e x c e e d i n g t h e 7 0 a n d 9 0 p e r c e n t i l e v a l u e s a r e d i s t i n g u i s h e d [ 6 ] . A t A l i c e S p r i n g s , e x t e n d e d r a i n f a l l e v e n t s t e n d t o b e a s s o c i a t e d w i t h d e p l e t e d r a i n f a l l .T h i s h a s b e e n sh o w n q u a n t i t a t i v e l y b y H a r t l e y [ 7 ] w ho h a s r e c e n t l y a n a l y s e d i n d e t a i l t h e <5D l e v e l s i n m o n t h ly r a i n f a l l s a m p l e s f r o m 1 4 s t a t i o n s t h r o u g h o u t A u s t r a l i a f o r t h e y e a r s 1 9 7 3 - 1 9 7 5 .He h a s c a l c u l a t e d t h e c o r r e l a t i o n c o e f f i c i e n t s w i t h t h e p a r a m e t e r s t e m p e r a t u r e , r a i n f a l l a n d d e w p o i n t w h i c h h a v e a l l b e e n w e i g h t e d b y r a i n f a l l . T h e r e s u l t s f o r A l i c e S p r i n g s a n d M e lb o u r n e a r e r e s p e c t i v e l y : t e m p e r a t u r e ( - 0 . 0 8 , + 0 . 4 0 ) ; r a i n f a l l ( - 0 . 6 2 , - 0 . 3 5 )d e w p o i n t ( + 0 . 4 4 , + 0 . 1 6 ) ; r a i n f a l l a n d d e w p o in t ( 0 . 6 6 , 0 . 5 1 ) , . I t i s r e l e v a n t t h a t f o r A l i c e S p r i n g s t h e r e i s a s i g n i f i c a n t c o r r e l a t i o n w i t h d e w p o i n t a n d r a i n f a l l , b u t n o t w i t h t e m p e r a t u r e .
T h e m e a n 6D v a l u e s f o r t h e g r o u n d w a t e r a n d t h e w e i g h t e d m e a n
v a l u e s f o r t h e p r e c i p i t a t i o n a r e l i s t e d i n T a b l e 1 a n d sh o w n i n t h e h i s t o g r a m ( F i g u r e 2 ) . I t i s s i g n i f i c a n t t h a t a l t h o u g h t h e s t a b l e i s o t o p e r a t i o s f r o m t h e N e w e r B a s a l t s a q u i f e r [ 4 ] w a t e r
a r e c l o s e t o t h e p r e c i p i t a t i o n m e a n , t h o s e f o r t h e M e r e e n i e S a n d s t o n e a q u i f e r s a m p l e s a r e h i g h l y d e p l e t e d ( F i g u r e 2 , l o w e r i n s e r t s ) . I n t h e l a t t e r c a s e , t h e s t a b l e i s o t o p e d a t a i m p ly t h a t s i g n i f i c a n t r e c h a r g e o c c u r s o n l y i f t h e m o n t h ly r a i n f a l l e x c e e d s t h e 9 0 p e r c e n t i l e v a l u e .
T h e a v e r a g e <$D a n d 6 ^ ® 0 v a l u e s f o r 1 9 w e l l s s a m p le d i n t h e r e g i o n o f t h e G r e a t A r t e s i a n B a s i n ( c r o s s - h a t c h e d i n F i g u r e 1 ) a r e - 4 8 . 1 3 ± 1 . 3 3 p e r m i l l e a n d 7 . 0 3 ± 0 . 6 p e r m i l l e . T h e s t a n d a r d d e v i a t i o n s a r e o f t h e p o p u l a t i o n s . T h e r a t i o s a r e s i g n i f i c a n t
I n t h e d i s c u s s i o n s i t i s sh ow n t h a t
IAEA-AG-158 /8 9 7
15
10
GROUNDWATER
J
J FMAMJ J ASONO MONTH
S-20
0 30 50 70 90P
№
I A )
MELBOURNE
MONTHLY RAINFALL EXCEEDING
I 90 PERCENTILE
70 PERCENTILE
( В
ALICE SPRINGS
MONTHLY RAINFALL EXCEEDING
I 90 PERCENTILE
70 PERCENTILE
-1 5 0 -1 0 0 - 5 0
6 d per m ille
FIG.2. The frequency distribution o fb D values fo r monthly samples o f Melbourne (Part A ) and Alice Springs (Part B) rainfall. The mean ÔD fo r groundwater sampled in the Newer Basalts aquifer (A) and the Mereenie Sandstone aquifer (B) are shown. The insert (upper) are the mean monthly rainfall fo r the periods 1856 to 1965 (Melbourne) and 1874-1965 (Alice Springs) [6]. The inserts (lower) are the mean 5D values fo r monthly rainfall exceeding the percentile value P.
9 8 A IR EY et al.
l y d i f f e r e n t f r o m s a m p l e s t a k e n f r o m w e l l s t a p p i n g t h e p r i n c i p a l J u r a s s i c a q u i f e r i n Q u e e n s l a n d (6 D = - 4 1 . 7 9 ± 1 . 1 0 , ô ^ O = - 6 . 6 ± 0 . 9 w h ic h i s g o o d s u p p o r t i n g e v i d e n c e f o r r e c h a r g e n e a r t h e w e s t e r n e x t r e m i t i e s o f t h e b a s i n [ 8 , 9 ] . T h e s t a b l e i s o t o p e r a t i o s[ 1 0 ] a r e s u b s t a n t i a l l y l e s s d e p l e t e d t h a n t h o s e f r o m t h e M e r e e n i e
a q u i f e r . T h i s c o u l d b e d u e t o a n u m b e r o f f a c t o r s r e s u l t i n g f r o m t h e g e o g r a p h i c a l s e p a r a t i o n i n c l u d i n g t h e f o l l o w i n g :
( i ) T h e r e l a t i v e c o n t r i b u t i o n s o f d i f f e r e n t v a p o u r s o u r c e s t o t h e r a i n b e a r i n g c l o u d s c o u l d b e s i g n i f i c a n t l y d i f f e r e n t , a n d t h i s c o u l d a f f e c t t h e i s o t o p i c c o m p o s i t i o n o f t h e p r e c i p i t a t i o n .
( i i ) T h e t h r e s h o l d r a i n f a l l l e v e l f o r s i g n i f i c a n t r e c h a r g e m i g h t v a r y a p p r e c i a b l y i n t h e tw o r e c h a r g e a r e a s . I n g e n e r a l l o w e r t h r e s h o l d s w o u ld b e a s s o c i a t e d w i t h g r o u n d w a t e r t h a t i s l e s s d e p l e t e d i n ÔD a n d 6 1 8 0 t h a n t h e w e i g h t e d m e a n o f p r e c i p i t a t i o n i n t h e r e c h a r g e a r e a .
D e u t e r i u m e x c e s s
T h e ÔD a n d 6 ^ ® 0 v a l u e s f o r m e t e o r i c w a t e r s f r e q u e n t l y s a t i s f y t h e l i n e a r r e l a t i o n s h i p
<SD = a <S180 + d (1)
w h e r e t h e s l o p e , a , h a s t h e v a l u e o f 8 a n d t h e d e u t e r i u m e x c e s s d i s c l o s e t o + 1 0 p e r m i l l e . S o n n t a g a n d c o w o r k e r s [ 1 1 ] h a v e s u r v e y e d a w id e r a n g e o f d a t a f r o m S a h a r a n g r o u n d w a t e r ; t h e y h a v e f o u n d t h a t e q u a t i o n ( 1 ) i s s a t i s f i e d , h a v i n g a s l o p e c l o s e t o 8 b u t w i t h a r e d u c e d d e u t e r i u m e x c e s s . T h i s i s i n c o n t r a s t w i t h t h e r e s u l t s f r o m C e n t r a l E u r o p e a n g r o u n d w a t e r w h e r e t h e e x c e s s i s + 1 0 p e r m i l l e . T h e o b s e r v e d v a l u e o f d i n a r i d c o n d i t i o n s i s i n g e n e r a l . a c c o r d w i t h t h e a c c e p t e d t h e o r y t h a t r e d u c e d d e u t e r i u m e x c e s s e s a r e a s s o c i a t e d w i t h e x t e n s i v e e v a p o r a t i o n b e f o r e i n f i l t r a t i o n . I n a r i d z o n e s , c o m p l i c a t i o n s m i g h t a r i s e d u e t o t r a n s p o r t p r o c e s s e s w i t h i n t h e u n s a t u r a t e d z o n e .On a p i s t o n f l o w m o d e l , e x t e n s i v e e v a p o r a t i o n a n d t r a n s p i r a t i o n c o u l d l e a d t o m e a s u r a b l e e f f e c t s , i f t h e d e u t e r i u m e x c e s s v a r i e d s y s t e m a t i c a l l y t h r o u g h o u t t h e d u r a t i o n o f t h e e x t e n d e d r a i n f a l l e v e n t s w h i c h a r e t h o u g h t t o l e a d t o s i g n i f i c a n t r e c h a r g e .
T h e o b s e r v e d v a l u e s o f d e u t e r i u m e x c e s s a r e l i s t e d i n T a b l e 1 . I t i s s e e n t h a t g r o u n d w a t e r f r o m t h e N e w e r B a s a l t s a q u i f e r h a s a n a v e r a g e v a l u e ( + 1 1 p e r m i l l e ) w h i c h i s e x p e c t e d , w h e r e a s t h a t s a m p le d f r o m t h e w e s t e r n e x t r e m i t i e s o f t h e G r e a t A r t e s i a n B a s i n i s a p p r o x i m a t e l y 8 . 0 m i l l e , w h i c h i s t y p i c a l o f S a h a r a n g r o u n d w a t e r . I t i s s u r p r i s i n g t h a t t h e d e u t e r i u m e x c e s s
f o r M e r e e n i e S a n d s t o n e g r o u n d w a t e r i s c l o s e t o 1 0 p e r m i l l e ; h o w e v e r p a r a d o x i c a l l y t h i s i s a f u r t h e r c o n s e q u e n c e o f g r o u n d w a t e r b e i n g a b i a s e d s a m p le o f p r e c i p i t a t i o n .
IAEA-AG-15 8 /8 9 9
GROUNDWATER ( A )
18161412
CLLd 10æ1»z 6
i
LÜCD5
MELBOURNE
JU L
DEUTERIUM EXCESS d ( - 6 D -8 6 1a0)
FIG.3. The frequency distribution o f the values o f the deuterium excess d (= ÔD - 8 5 lsO) fo r monthly samples o f Melbourne (Part A) and Alice Springs (Part B) rainfall. Those months exceeding the 70 percentile and 90 percentile levels are distinguished by cross-hatching and full shade respectively. The mean values fo r groundwater samples from the Newer Basalts aquifer (A) and the Mereenie Sandstone aquifer (BJ are shown.
A f r e q u e n c y d i s t r i b u t i o n o f d e u t e r i u m e x c e s s v a l u e s f o r r a i n f a l l a t A l i c e S p r i n g s a n d M e lb o u r n e i s sh o w n i n F i g u r e 3 . T h e d a t a f o r r a i n f a l l a b o v e t h e 7 0 a n d 9 0 p e r c e n t i l e v a l u e a r e d i s t i n g u i s h e d . C l e a r l y , i n t h e A l i c e S p r i n g s r e g i o n , r e l a t i v e l y h i g h m o n t h ly r a i n f a l l s a r e a s s o c i a t e d w i t h l a r g e d e u t e r i u m e x c e s s e s .T h e w e i g h t e d m e a n v a l u e f o r m o n th s e x c e e d i n g t h e n o m in a t e d r e c h a r g e t h r e s h o l d ( 9 0 p e r c e n t i l e ) i s + 2 2 p e r m i l l e . T h i s i s c l e a r l y c o n s i s t e n t w i t h a g r o u n d w a t e r v a l u e o f + 1 0 p e r m i l l e f o r t h e d e u t e r i u m e x c e s s w h en a l l o w a n c e i s m ad e f o r s u r f a c e e v a p o r a t i v e e f f e c t s .
T h e a b s o l u t e m a g n i t u d e o f t h e d e u t e r i u m e x c e s s s o m e t i m e s o b s e r v e d i n p r e c i p i t a t i o n a t A l i c e S p r i n g s r e q u i r e s co m m e n t a s i t c a n e x c e e d a l l v a l u e s o b s e r v e d a t c o a s t a l s t a t i o n s f o r w h i c h b o t h ÔD a n d 6 1 8 0 d a t a a r e a v a i l a b l e . T h e n o r m a l d e u t e r i u m e x c e s s i s a t t r i b u t e d t o k i n e t i c i s o t o p e e f f e c t s a s s o c i a t e d w i t h m a r i n e e v a p o r a t i o n u n d e r n o n - e q u i l i b r i u m c o n d i t i o n s . A s c o n d e n s a t i o n u s u a l l y o c c u r s b y a n e q u i l i b r i u m p r o c e s s , i t d o e s n o t l e a d t o a c h a n g e
1 0 0 A IR EY et al.
i n t h e v a l u e o f t h e p a r a m e t e r . I f d u r i n g t h e p a s s a g e o f v a p o u r a c r o s s t h e c o n t i n e n t , w a t e r r e m o v e d b y c o n d e n s a t i o n i s i n p a r t r e t u r n e d b y n o n - e q u i l i b r i u m e v a p o r a t i o n o f e i t h e r t h e r a i n d r o p s , o r t h e s u r f a c e w a t e r , t h e d e u t e r i u m . e x c e s s a s s o c i a t e d w i t h t h e w a t e r i n t h e c l o u d w i l l b e i n c r e a s e d . S u b s e q u e n t c o n d e n s a t i o n w i l l r e f l e c t t h e e n h a n c e d d e u t e r i u m e x c e s s p r o v i d e d t h a t t h e e f f e c t i s n o t m a s k e d b y e v a p o r a t i o n a c c o m p a n y in g t h e s e c o n d e v e n t . L a r g e d e u t e r i u m e x c e s s e s a r e c o r r e l a t e d w i t h h i g h r a i n f a l l b e c a u s e u n d e r t h e s e c o n d i t i o n s , r e l a t i v e l o s s e s d u e t o e v a p o r a t i o n a r e s m a l l e r . I t i s r e l e v a n t t o n o t e t h a t a D /H r a i n f a l l m o d e l d e v e l o p e d f r o m b a s i c p r i n c i p l e s b y H a r t l e y [7 ] w h ic h a s s î m e s t h a t a l l v a p o u r i s o f o c e a n i c o r i g i n b u t i s o t h e r w i s e q u i t e f l e x i b l e , c a n b e m a d e c o n s i s t e n t w i t h a l l A u s t r a l i a n d a t a e x c e p t t h o s e r e c o r d e d a t A l i c e S p r i n g s f o r tw o m o n th s i n w h ic h t h e ÔD v a l u e s a r e m o r e d e p l e t e d t h a n - 1 0 0 p e r m i l l e . T h e c l e a r i m p l i c a t i o n i s t h a t v a p o u r o r i g i n a t i n g f r o m r e - e v a p o r a t i o n p r o c e s s e s w a s i n v o l v e d .
T h e v a r i a b i l i t y o f 6D v a l u e s
I t i s co m m o n ly o b s e r v e d t h a t ' t h e s t a n d a r d d e v i a t i o n o f t h e g r o u n d w a t e r D /H r a t i o s i s s u b s t a n t i a l l y l e s s t h a n t h a t o f t h e w e i g h t e d v a l u e s o f t h e s t a b l e i s o t o p e r a t i o s o f m o n t h ly r a i n f a l l s a m p l e s i n t h e r e c h a r g e a r e a . E x a m p l e s a r e l i s t e d i n T a b l e 1 . F o r A l i c e S p r i n g s , o n l y t h o s e m o n th s i n w h ic h t h e a m o u n t o f r a i n f a l l e x c e e d e d t h e t h r e s h o l d v a l u e f o r r e c h a r g e w e r e i n c l u d e d . A c o m b i n a t i o n o f h y d r a u l i c c o n s i d e r a t i o n s a n d c a r b o n - 1 4 d a t i n g h a v e sh o w n t h a t t h e g r o u n d w a t e r a g e s i n c r e a s e f r o m c o m p a r a t i v e l y m o d e r n t o a b o u t 3 5 0 0 0 0 a i n t h e o r d e r sh o w n i n T a b l e 1 . T h e s t a n d a r d d e v i a t i o n s d e c r e a s e i n t h e s a m e o r d e r .No a t t e m p t h a s b e e n m a d e t o s u b t r a c t t h e c o n t r i b u t i o n d u e t o m e a s u r e m e n t e r r o r , w h ic h i s e s t i m a t e d t o b e 0 .6 p e r m i l l e , a s t h e s a m e m a s s s p e c t r o m e t e r w a s u s e d i n e a c h c a s e . T h e e f f e c t w o u ld n o t b e e x p e c t e d i f t h e s p r e a d o f ÔD a n d <5-*-®0 v a l u e s w e r e d u e p r i m a r i l y t o t h e m i x i n g o f g r o u n d w a t e r f r o m d i f f e r e n t s o u r c e a r e a s .
I n a s s e s s i n g t h i s r e s u l t , a c c o u n t m u s t b e t a k e n o f t h e t i m e o v e r w h ic h t h e s t a b l e i s o t o p e r a t i o s o f t h e p r e c i p i t a t i o n i s a v e r a g e d t o d e t e r m i n e t h e s t a n d a r d d e v i a t i o n s . I t i s s i g n i f i c a n t t h a t t h e s t a n d a r d d e v i a t i o n o f t h e i s o t o p e r a t i o s o f s a m p l e s f r o m t h e N e w e r B a s a l t s A q u i f e r i s c o m p a r a b l e t o t h a t a s s o c i a t e d w i t h . m o n t h ly r a i n f a l l m e a s u r e m e n t s b u t t h e s t a n d a r d d e v i a t i o n s o f s a m p l e s f r o m a q u i f e r s w i t h l a r g e r r e s i d e n c e t i m e s a r e s u b s t a n t i a l l y l e s s ( T a b l e 1 ) . S m o o t h in g a s s o c i a t e d w i t h t i m e a v e r a g i n g m ay w e l l b e t h e d o m i n a t i n g e f f e c t , e s p e c i a l l y f o r v e r y o l d g r o u n d w a t e r . T h e t i m e o v e r w h ic h t h e i n p u t d a t a s h o u l d b e a v e r a g e d i s c o m p a r a b l e t o t h e r e s o l u t i o n o f t h e g r o u n d w a t e r r e s i d e n c e t i m e s .
IA EA -A G -158/8 101
CARBON ISO TO PE DATA
M a t h e m a t i c a l f o r m u l a t i o n
T h e f o l l o w i n g a n a l y t i c a l t r e a t m e n t i s p r e s e n t e d , n o t b e c a u s e i t c a n b e d i r e c t l y u s e d t o d e s c r i b e t h e g e o c h e m i s t r y o f m an y r e a l s y s t e m s , b u t b e c a u s e i t f o r m s a b a s i s f o r d i s c u s s i o n o f t h e t r e a t m e n t o f d a t a i n a r i d z o n e s w h e r e s e l e c t i o n p r o c e s s e s m ay b e i m p o r t a n t . So m e s i m p l i f y i n g a s s u m p t i o n s a r e t h e r e f o r e m a d e :
( i ) t h e s t e a d y s t a t e ' p i s t o n f l o w ' a p p r o x i m a t i o n a d e q u a t e l y d e s c r i b e s g r o u n d w a t e r t r a n s p o r t i . e . d i s p e r s i o n t e r m s a r e n e g l e c t e d ;
( i i ) C a r t e s i a n c o - o r d i n a t e s ( x , y , z ) m ay b e t r a n s f o r m e d i n t o a t i m e c o o r d i n a t e s y s t e m ( s , t ) d e f i n e d b y t h e m o v e m e n t o f w a t e r p a r t i c l e s a l o n g g r o u n d w a t e r s t r e a m l i n e s [ 1 2 , 1 3 ] ; a n d
( i i i ) t h e m ean p a r t i c l e v e l o c i t y Vs i s i n d e p e n d e n t o f t h e p o s i t i o n s a l o n g t h e f l o w l i n e .
T h e t h i r d a s s u m p t i o n i m p l i e s t h a t e i t h e r t h e f l o w l i n e s a r e p a r a l l e l a n d t h e p o t e n t i o m e t r i c s u r f a c e i s s t a b l e , o r t h e g r o u n d w a t e r v e l o c i t i e s a r e e s s e n t i a l l y z e r o .
W ith t h e s e s i m p l i f i c a t i o n s , t h e g e n e r a l i s e d d i s p e r s i o n - c o n v e c t i o n e q u a t i o n m ay b e s i m p l i f i e d t o
C ( S , t ) / 3 t + U ( 9 C ( S , t ) / 9 S = Q (2 )
w h e r e C ( S , t ) i s t h e s p e c i e s c o n c e n t r a t i o n a t ( s , t ) ;
U i s t h e m ean p a r t i c l e v e l o c i t y ; a n d
Q = Q'/p is the ratio of the source term Q ' to the groundwater density, p.
T h e s o l u t i o n o f e q u a t i o n (2 ) d e p e n d s o n t h e n a t u r e o f t h e s o u r c e / s i n k t e r m Q w h ic h i s a f u n c t i o n o f t h e t i m e s c a l e s i n v o l v e d .
I n t h e s t u d y o f a n y h y d r o g e o c h e m i c a l s y s t e m s , a t l e a s t t h r e e t i m e s c a l e s n e e d t o b e c o n s i d e r e d :
( a ) t h e t i m e f o r t h e e s t a b l i s h m e n t o f c h e m i c a l e q u i l i b r i a w i t h a c c e s s i b l e s u r f a c e m i n e r a l s ;
(b ) t h e t im e f o r t h e t r a n s p o r t o f w a t e r t h r o u g h a f l o w i n g a q u i f e r s y s t e m ; a n d
1 0 2 A IR EY et al.
( c ) t h e t i m e f o r c h a n g e s t o o c c u r i n t h e c o m p o s i t i o n o f s u r f a c e m i n e r a l s .
I t w i l l b e a r g u e d b e l o w t h a t i n m any s e t t i n g s , t h e s e t i m e s c a l e s a r e s e p a r a b l e .
( a ) C h e m ic a l t i m e s c a l e s T h e r a t e o f c h a n g e o f c o n c e n t r a t i o n s o f c h e m i c a l s p e c i e s i s d i r e c t l y r e l a t e d t o t h e m a g n i t u d e o f t h e d i s p l a c e m e n t f r o m t h e r m o d y n a m ic e q u i l i b r i u m . H o w e v e r , c h e m i c a l r e a c t i o n s a s s o c i a t e d w i t h t h e e s t a b l i s h m e n t o f b i c a r b o n a t e e q u i l i b r i a a r e n o r m a l l y r a p i d c o m p a r e d w i t h t h e l^ C d e c a y r a t e . C o n c e n t r a t i o n s a r e t h e r e f o r e d e t e r m i n e d n o t b y c h e m i c a l k i n e t i c e f f e c t s b u t b y t h e s u r f a c e f r e e e n e r g y o f m i n e r a l s a c c e s s i b l e t o t h e g r o u n d w a t e r U n d e r t h e s e c o n d i t i o n s , t h e s o u r c e t e r m Q t a k e s t h e fo r m
Q ( S , t ) = Q0 ( S ) - k ' C ( S , t ) (3 )
w h e r e Q o ( S ) i s d e t e r m i n e d b y c h a n g e s i n t h e f r e e e n e r g y d i f f e r e n c e s b e t w e e n t h e m i n e r a l a n d g r o u n d w a t e r n e a r t h e p o i n t S , a n d t h e f i r s t o r d e r r a t e c o n s t a n t k 1 i n c l u d e s t h e r a d i o a c t i v e d e c a y c o n s t a n t .
C o m p l i c a t i o n s w h ic h c a n a r i s e a r e w e l l i l l u s t r a t e d b y t h e t r a n s p o r t o f g r o u n d w a t e r t h r o u g h t h e J u r a s s i c s a n d s t o n e a q u i f e r o f t h e G r e a t A r t e s i a n B a s i n [ 1 0 ] . T h e m e a s u r e d r a t e o f c a r b o n a t e s o l u t i o n i n lo w t e m p e r a t u r e b o r e s (T < 6 0 ° C ) i s c o n s i s t e n t w i t h a c u m u l a t i v e u p t a k e o f c a r b o n a t e s i n c e t h e m id T e r t i a r y o f 0 . 1 p e r c e n t o f t h e a q u i f e r m a s s w h ic h i s m o r e t h a n a n o r d e r o f m a g n i t u d e l e s s t h a n t h e c a r b o n a t e m i n e r a l c e m e n t i n t h e s a n d s t o n e p o r e s .O ne c o n c l u s i o n i s t h a t t h e b u l k o f t h e w a t e r f l o w s a l o n g p r e f e r r e d p a t h s i n w h ic h s o l u t i o n i s v i r t u a l l y c o m p l e t e . T h e r a t e d e t e r m i n i n g s t e p w o u ld t h e n b e t h e t r a n s p o r t o f c a r b o n a t e f r o m " s t a g n a n t " r e g i o n s . O t h e r f a c t o r s b e i n g e q u a l , t h e h i g h e r t h e c o n c e n t r a t i o n o f c a r b o n a t e i n t h e f l o w i n g g r o u n d w a t e r , t h e l o w e r w i l l b e t h e c o n c e n t r a t i o n g r a d i e n t a n d t h e l o w e r t h e r e a c t i o n r a t e . T h e f i r s t o r d e r c o n s t a n t k ' ( e q u a t i o n ( 3 ) ) s h o u l d t h u s t a k e t h e f o r m
к ' = X + к (4 )
w h e r e A i s t h e r a d i o a c t i v e d e c a y c o n s t a n t a n d к r e p r e s e n t s a l l t h e o t h e r f i r s t o r d e r p r o c e s s e s .
(b ) H y d r o d y n a m ic t i m e s c a l e I n a n y s t u d y i n v o l v i n g g r o u n d w a t e r d a t i n g , t h e h y d r o d y n a m ic t i m e s c a l e i s c o m p a r a b l e t o t h e d e c a y c o n s t a n t o f t h e i s o t o p e . I n t h e c a s e o f t r i t i u m i t i s o f t h e o r d e r o f d e c a d e s ; f o r ^ C i t i s i n t h e r a n g e o f 10^ t o 10^ y e a r s . A s t h e t i m e s c a l e f o r t h e e s t a b l i s h m e n t o f c a r b o n a t e e q u i l i b r i a i s v e r y r a p i d c o m p a r e d w i t h t h e h y d r o d y n a m ic t i m e s c a l e t h e tw o a r e s e p a r a b l e . H o w e v e r , p r o c e s s e s a s s o c i a t e d w i t h t h e
IAEA-AG-15 8 /8 1 0 3
t r a n s p o r t o f c a r b o n a t e s p e c i e s f r o m " s t a g n a n t " p o r e s t o t h e f l o w i n g r e g i o n s o f t h e a q u i f e r m ay o c c u r w i t h a r a t e c o m p a r a b l e t o
d e c a y .
( c ) G e o l o g i c a l t i m e s c a l e O v e r a l o n g p e r i o d t h e c o n t i n u o u s i n p u t o f a c t i v e s o l u t e s a t r e c h a r g e w i l l r e s u l t i n s y s t e m a t i c c h a n g e s t o t h e s u r f a c e c o m p o s i t i o n o f a c c e s i b l e m i n e r a l s . T h e s e e f f e c t s c a n n o t n o r m a l l y b e d i r e c t l y o b s e r v e d a s t h e r e s i d e n c e t im e o f t h e w a t e r i s i n e v i t a b l y s h o r t c o m p a r e d w i t h t h e g e o l o g i c a l a g e o f t h e a q u i f e r .
A s p e c i a l c a s e h a s b e e n i d e n t i f i e d i n t h e u n s a t u r a t e d z o n e a s s o c i a t e d w i t h a q u i f e r s i n lo w r a i n f a l l a r e a s . U n d e r t h e s e c o n d i t i o n s , c y c l e s o f s u r f a c e w a t e r i n f i l t r a t i o n f o l l o w e d b y e v a p o r a t i v e c o n c e n t r a t i o n h a v e l e d t o t h e a c c u m u l a t i o n o f m o d e r n c a r b o n a t e m i n e r a l i n t h e p e r m e a b l e m a t r i x . T h i s e f f e c t h a s b e e n p r e d i c t e d b y I n g e r s o n a n d P e a r s o n [ 1 4 ] ; h a s b e e n o b s e r v e d d i r e c t l y b y W a l l i c k [1 5 ] ; a n d h a s b e e n i n f e r r e d f r o m t h e r e s u l t s o f C a l f[ 3 ] a n d V e r h a g e n e t a l [ 1 6 ] b u t n o t f r o m t h o s e o f P e a r s o n a n d
S w a r z e n k i [ 1 7 ] .
A n a l y t i c a l s o l u t i o n s o f t h e t r a n s p o r t e q u a t i o n
T h e t r a n s p o r t e q u a t i o n s m ay b e s o l v e d s u b j e c t t o t h e f o l l o w i n g b o u n d a r y c o n d i t i o n s :
i . e . t h e c o n c e n t r a t i o n o f b i c a r b o n a t e a t i n p u t (S = O , t im e = t - S / U ) i s c o n s t a n t ; a n d
i . e . t h e i n i t i a l c o n c e n t r a t i o n a t p o s i t i o n S i s c o n s t a n t . T h e g e n e r a l s o l u t i o n s o f e q u a t i o n s ( 2) t o (6 ) w h ic h c a n b e v e r i f i e d b y s u b s t i t u t i o n a r e
C ( 0 , t - S / U ) = A ( a c o n s t a n t ) (5 )
C ( S ; 0 ) = В ( a c o n s t a n t ) (6)
C ( S , t ) = A e x p ( - ( к + X) S / U ) H ( t - S /U )
+ В e x p ( - (к + X) t ) + (Q0 /(k + X) ) (1 - е з ф -(к + X) t )
- В e x p ( - ( к + X) t ) H ( t - S /U ) (7 )
( Q o / ( k + X ) ) [ e x p ( - ( к + X) S / U ) - e x p -(k + X ) t ] H ( t - S /U )
W h ere H i s t h e H e a v y s i d e f u n c t i o n
H ( t - S / U ) = 1 t > S / U
= 0 t í S /U
( 8 )
1 0 4 AI R E Y et al.
E q u a t i o n ( 8) a l l o w s a u s e f u l m a t h e m a t i c a l d i s t i n c t i o n t o b e m ad e b e t w e e n ' f l o w i n g 1 a n d 'n o n - f l o w i n g ' a q u i f e r s . An a q u i f e r c a n b e s a i d t o b e f l o w i n g i f t h e t i m e t , s i n c e a d e f i n a b l e z e r o p o i n t ( t = 0 ) i s g r e a t e r t h a n t i m e S / U f o r a p a r t i c l e o f w a t e r t o b e t r a n s p o r t e d f r o m t h e r e c h a r g e a r e a t o t h e o b s e r v a t i o n p o i n t . I n t h i s m a t h e m a t i c a l s e n s e i t i s n o n f l o w i n g i f t < S / U .
B o t h t h e f l o w i n g a n d n o n f l o w i n g s o l u t i o n s t a k e t h e f o l l o w i n g fo r m
С ( S , t ) = (D - Q0/ (k + X )T e x p ( - (к + X) T ) + Q0 / C k + X ) ( 9 a )
= D + Q0 T ( i f к + X = 0 ) (9 b )
W h ere T = S / U ( f l o w i n g )
= t (n o n f l o w i n g )
a n d D = A ( f l o w i n g )
= В (n o n f l o w i n g )
T h e d i s t i n c t i o n b e t w e e n f l o w i n g a n d n o n - f l o w i n g s y s t e m s i s a r b i t r a r y u n l e s s t h e p o i n t o f z e r o t im e c o r r e s p o n d s t o a d i s c o n t i n u i t y ( o r r a p i d c h a n g e ) i n t h e h y d r o d y n a m ic s o r g e o c h e m i s t r y .I t i s l i k e l y t o b e o f s p e c i a l v a l u e i n lo w r a i n f a l l a r e a s w h e r e r e c h a r g e o c c u r s o n l y a f t e r p e r i o d s h i g h e r t h a n a v e r a g e p r e c i p i t a t i o n . A s p o i n t e d o u t b y P e a r s o n a n d S w a r z e n k i [ 1 7 ] a n d c o n f i r m e d b y C a l f [3 ] n e t r e c h a r g e t o a q u i f e r s i n a r i d a n d s e m i - a r i d a r e a s i s n o t a s t e a d y p r o c e s s . P e r i o d s o f h i g h e r t h a n a v e r a g e i n f i l t r a t i o n w h ic h c o u l d b e c o n s i d e r e d a s p o i n t s o f z e r o t i m e i n n o n f l o w i n g s y s t e m s c a n b e s e p a r a t e d b y w e l l d e f i n e d t i m e i n t e r v a l s .
E q u a t i o n ( 9 ) c a n b e u s e d t o a s s e s s f a c t o r s i n v o l v e d i n t h e p r e s e n t a t i o n o f c a r b o n i s o t o p e d a t a . S i n c e t h e e x p r e s s i o n a p p l i e s o n l y t o t h e s a t u r a t e d z o n e i n w h ic h t h e a q u i f e r c a r b o n a t e i s " d e a d " t o 14c, so m e s i m p l i f i c a t i o n s a r e p o s s i b l e .
1 2 c = (D - Q0 / k ) e x p ( - k T ) + Q0 / k ( 1 0 )
1 3 C = ( 1 3 I r D - 1 3 I a Q0 / a k ) e x p ( - a k T ) + 1 3 I a Q0 / a k (1 1 )
1 4 1 4С = I r D e 'x p (-X T ) (1 2 )
w h e r e 1 4 , 13I r , 1 4 , 13I a a r e t h e 14C / 1 2 C a n d 13C / 1 2 C r a t i o s a s t h e i n f i l t r a t i n g w a t e r e n t e r s t h e s a t u r a t e d z o n e , a n d i n t h e g r o u n d w a t e r s a m p l e r e s p e c t i v e l y ; a n d a i s t h e 1 3 k / 1 2 k r a t i o .
IAEA-AG-15 8 /8 1 0 5
FIG.4. Spatial distribution o f: A, absolute 14C activity (mBq ltr~l: 1 Bq = 1 dis/s);В, 14C, %mc; C, S13C (per mille PDB), and SD (per mille, SMOW); D, [carbonate] and [Cl~] (mmol Itr“V; E, temperature ( C). The data is related to the appropriate water well on the sketch map (part F) by the vertical dotted lines. The location o f the Mereenie Sandstone aquifer [19 ] and the R oe Creek is shown. The water-well classification [3 ] from the A14С - [HCO3'1 ] relationship, i.e. from the absolute 14 С activity (see Part A j is shown.
6D
(SM
OW
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er
mill
e
1 0 6 A IR EY et al.
A s p e c t s o f t h e i n t e r p r e t a t i o n o f c a r b o n i s o t o p e d a t a f r o ma r i d z o n e s a n d s t o n e
S p e c i f i c r e f e r e n c e i s now m a d e t o d a t a f r o m t h e p o r t i o n o f t h e M e r e e n i e S a n d s t o n e a q u i f e r sh o w n i n F i g u r e 4 F . A p o s s i b l e s o u r c e o f r e c h a r g e i s t h e R o e C r e e k f l o o d o u t . T h e a q u i f e r h a s b e e n c l a s s i f i e d i n t o r e g i o n s o n t h e b a s i s o f t h e s l o p e o f d i s c r e t e l i n e a r ^ C (% m . c . ) - [H C O g]- ! r e l a t i o n s h i p s i . e . t h e a b s o l u t e [Н - ^ С О з ] . Two p a r t i c u l a r f e a t u r e s o f t h e r e s u l t s w i l l b e d i s c u s s e d :
( i ) t h e i n t e r p r e t a t i o n o f d i f f e r e n c e s i n t h e a b s o l u t e 1 4 c c o n c e n t r a t i o n i n w a t e r , a n d t h e l^ C (% m . c . ) v a r i a b i l i t y ; a n d
( i i ) t h e t e n d e n c y f o r t h e f r e q u e n c y d i s t r i b u t i o n o f c a r b o n - 1 4 l e v e l s t o b e m u l t i - m o d a l .
( a ) T h e p r e s e n t a t i o n o f c a r b o n - 1 4 d a t a
T h e v a r i a t i o n o f t h e a b s o l u t e - ^ C a c t i v i t y w i t h d i s t a n c e i s sh o w n i n F i g u r e 4 A . A s s e e n f r o m e q u a t i o n ( 1 2 ) t h e d a t a a r e i n d e p e n d e n t o f t h e c o n c e n t r a t i o n o f d e a d c a r b o n a t e d i s s o l v e d w i t h i n t h e a q u i f e r , b u t i s s t r o n g l y d e p e n d e n t o n t h e i s o t o p e a c t i v i t y a t r e c h a r g e I r w h ic h i s d e t e r m i n e d b y a n u m b e r o f f a c t o r s [ 1 8 ] i n c l u d i n g
( i ) t h e c o n c e n t r a t i o n o f d i s s o l v e d a t m o s p h e r i c ^ C C ^ ;
( i i ) t h e i n p u t o f l ^ C c a r b o n a t e b y r e c h a r g e f r o m R o e C r e e k d u r i n g p e r i o d s o f h i g h r a i n f a l l ;
t h e u p t a k e o f 14C02 f r o m s o i l a i r ; a n dt
c o n c e n t r a t i o n e f f e c t s d u e t o s u b - s u r f a c e e v a p o r a t i o n a n d t r a n s p i r a t i o n .
I n t h e a b s e n c e o f e v a p o r a t i v e e f f e c t s , t h e c o n c e n t r a t i o n s o f 1 4 C w i l l b e d e t e r m i n e d p r i n c i p a l l y b y t h e p a r t i a l p r e s s u r e o fCC>2 i n t h e s o i l a i r , t h e c h e m i s t r y o f t h e i n f i l t r a t i n g w a t e r a n dt h e a v a i l a b i l i t y o f c a l c i t e f o r t h e r e a c t i o n
H ,0 + +C a C 0 3 + C 0 2 i C a + 2 HCO3- ( 1 3 )
T h e e f f e c t o f t h e r e l a t i v e l y lo w s p e c i f i c a c t i v i t y o f ^ C i n s o i l c a l c i t e o n t h e g r o u n d w a t e r w o u ld b e c o m p a r a t i v e l y s m a l l . Af a c t o r w h ic h i s p o t e n t i a l l y m uch m o r e i m p o r t a n t i s t h e e f f e c t o fc o n c e n t r a t i o n d u e t o s u b - s u r f a c e e v a p o r a t i o n a n d t r a n s p i r a t i o n . T h e a b s o l u t e ^ C c o n c e n t r a t i o n a t i n p u t w o u ld r e s p o n d d i r e c t l y t o t h i s e f f e c t .
( i i i )
( i v )
IA EA -A G -158/8 1 0 7
D e s p i t e t h e h i g h p o t e n t i a l e v a p o r a t i o n r a t e s r e l a t i v e t o m e an a n n u a l r a i n f a l l , t h e i n f l u e n c e o f é v a p o t r a n s p i r a t i o n o n t h e a b s o l u t e c o n c e n t r a t i o n i n t h e g r o u n d w a t e r i s t h o u g h t n o t t o b e i m p o r t a n t f o r t h e f o l l o w i n g r e a s o n s :
( i ) t h e [H ^ C O ^ ] d e c r e a s e s i n a r e g u l a r s t e p w i s e m a n n e r f r o m t h e r e c o g n i s e d r e c h a r g e a r e a n e a r t h e R o e C r e e k f l o o d o u t ( F i g u r e 4A a n d 4 F ) ; a n d
( i i ) t h e c h l o r i d e l e v e l s ( F i g u r e 4D) a r e n o t p a r t i c u l a r l y h i g h . E x t e n s i v e é v a p o t r a n s p i r a t i o n w o u ld l e a d t o c o n c e n t r a t i o n .
B o t h r e s u l t s s u g g e s t t h a t e f f e c t i v e r e c h a r g e o c c u r s o n l y d u r i n g p e r i o d s o f h i g h e r t h a n a v e r a g e r a i n f a l l w hen s u b - s u r f a c e l o s s e s m ay n o t b e g r e a t e r t h a n t h o s e i n l e s s e x t r e m e c l i m a t e s .
I n c o n t r a s t t o t h e a b s o l u t e -*-4 C c o n c e n t r a t i o n s , t h e ^ 4 C (% m . c . ) r a t i o s a r e i n d e p e n d e n t o f d i l u t i o n e f f e c t s b u t a r e
s e n s i t i v e t o t h e s o l u t i o n o f i n a c t i v e c a r b o n a t e . T h a t t h e s p a t i a l v a r i a b i l i t y o f b o t h p a r a m e t e r s i s s i m i l a r ( F i g u r e s 4A a n d 4 B ) s u g g e s t s t h a t t h e s o l u t i o n o f c a r b o n a t e w i t h i n t h e s a t u r a t e d z o n e i s a m in o r p r o c e s s . T h i s i s s u p p o r t e d b y t h e g e n e r a l c o n s t a n c y o f t h e ó - ^ C v a l u e s a n d t h e g o o d c o r r e l a t i o n b e t w e e n t h e g r o u n d w a t e r c h l o r i d e a n d b i c a r b o n a t e c o n c e n t r a t i o n s .
(b ) T h e d i s t r i b u t i o n o f c a r b o n —1 4 a b u n d a n c e s i n a r i d z o n e s
A s s u m in g a u n i f o r m , s p a t i a l d i s t r i b u t i o n o f s a m p l i n g w e l l s , a c o n s t a n t t r a n s p o r t r a t e a n d a o n e d i m e n s i o n a l g e o m e t r y , e q u a l n u m b e r s o f s a m p l e s s h o u l d b e i n e a c h ag e i n t e r v a l . T h e n u m b e r s w o u ld i n c r e a s e m o n o t o n i c a l l y i f t h e f l o w l i n e s r a d i a t e d f r o m t h e r e c h a r g e a r e a . H o w e v e r , t h e r e i s i n c r e a s i n g e v i d e n c e t h a t t h e d i s t r i b u t i o n o f r e s i d e n c e t i m e s i n a r i d z o n e s i s m u l t i - m o d a l i . e . t h e p r o b a b i l i t y o f r e c h a r g e i s g r e a t e r i n so m e p e r i o d s o f t i m e t h a n i n o t h e r s . E x a m p l e s c a n b e s i t e d f r o m N o r t h - A f r i c a [ 1 1 ] , K e n y a[1 7 ] a n d C e n t r a l A u s t r a l i a [ 3 ] . E x p l a n a t i o n s a r e s o m e t i m e s s o u g h t
i n t e r m s o f t h e c l i m a t e c h a n g e s i n r e c h a r g e a r e a s t h r o u g h o u t t h e a c c e s s i b l e t i m e - s c a l e . H o w e v e r , i t h a s b e e n r e c e n t l y p o i n t e d o u t t h a t t h e s u b s u r f a c e m i x i n g o f m o d e r n a n d o l d w a t e r c a n l e a d t o a l o w e r v a r i a b i l i t y i n t h e 1 ^C (% m . c . ) l e v e l s t h a n p r e d i c t e d f r o m t h e a g e d i s t r i b u t i o n o f t h e o l d e r s a m p l e s [ 20] .
N e v e r t h e l e s s t h e o b s e r v a t i o n s a r e c o n s i s t e n t w i t h t h e g e n e r a l h y p o t h e s i s t h a t e f f e c t i v e r e c h a r g e o c c u r s o n l y w h en t h e r a i n f a l l i n t e n s i t y e x c e e d s a t h r e s h o l d v a l u e . I f t h e t h r e s h o l d v a l u e i s h i g h s m a l l c h a n g e s i n t h e m e an o r t h e s t a n d a r d d e v i a t i o n o f t h e r a i n f a l l d i s t r i b u t i o n w i l l l e a d t o m u ch l a r g e r c h a n g e s i n t h e
1 0 8 AI R E Y et al.
r e c h a r g e r a t e . S t a t i s t i c a l f o r m u l a t i o n s d e v e l o p e d f o r f l o o d r e c u r r e n c e a n a l y s i s a r e a p p l i c a b l e . O f t h e v a r i o u s p r o c e d u r e s a v a i l a b l e , u s e w i l l b e m a d e o f t h e d i s t r i b u t i o n o f e x t r e m e v a l u e s f o l l o w i n g G u m b el [ 2 1 ] . I f X ^ , X2 . . . . Xn a r e e x t r e m e v a l u e s o f , s a y , m o n t h ly r a i n f a l l a m o u n ts i n n s a m p l e s o f e q u a l s i z e N a n d i f X ( t h e t h r e s h o l d v a l u e f o r r e c h a r g e , s a y ) i s a n u n l i m i t e d e x p o n e n t i a l l y d i s t r i b u t e d v a r i a b l e , t h e n t h e c u m u l a t i v e p r o b a b i l i t y P t h a t a n y o f t h e n e x t r e m e s w i l l e x c e e d t h e v a l u e X a p p r o a c h e s t h e e x p r e s s i o n
P = 1 - e x p [ - ( e x p ( - y ) ) ]
w h e r e y = a (X - X + b ô x )
X i s t h e m ean m o n t h ly r a i n f a l l ;
ÔX i s t h e s t a n d a r d d e v i a t i o n ; a n d
a a n d b a r e c o n s t a n t s .
A s m a l l d e c r e a s e i n t h e v a l u e o f t h e r e d u c e d v a r i a b l e y c o u l d h a v e a v e r y m uch l a r g e r e f f e c t o n P . T h e d e c r e a s e w o u ld r e s u l t e i t h e r f r o m a n i n c r e a s e i n t h e m e an m o n t h ly r a i n f a l l , o r a n i n c r e a s e i n <5x i . e . i n t h e r a i n f a l l v a r i a b i l i t y . F o r i n s t a n c e , i fP w e r e o f t h e o r d e r 0 . 1 , a r e d u c t i o n o f 2 0 p e r c e n t i n t h e v a l u eo f y w o u ld l e a d t o a 5 0 p e r c e n t i n c r e a s e i n P ; a g a i n , i f P w e r e0 . 0 1 t h e sa m e p r o p o r t i o n a l d e c r e a s e i n Y w o u ld l e a d t o a 2 . 5 f o l d i n c r e a s e i n t h e p r o b a b i l i t y t h a t t h e t h r e s h o l d v a l u e w o u ld b e e x c e e d e d .
T h u s , i n a r i d z o n e s , m o d e s t c h a n g e s i n c o n d i t i o n s i n t h e i n p u t a r e a s c a n l e a d t o m uch l a r g e r c h a n g e s i n t h e p r o b a b i l i t y o f r e c h a r g e ' . T o u n d e r s t a n d t h e i m p l i c a t i o n s o f t h i s f u l l y i s o n e o f t h e c h a l l e n g e s o f a r i d z o n e h y d r o l o g y .
SUMMARY AND CONCLUSIONS
1 ft1 . T h e s t a b l e i s o t o p e r a t i o s ÔD a n d 6 0 i n t h e M e r e e n i e
S a n d s t o n e a q u i f e r , C e n t r a l A u s t r a l i a a r e s u b s t a n t i a l l y d e p l e t e d c o m p a r e d w i t h t h e w e i g h t e d m ean v a l u e s o f m o d e r n r a i n f a l l , i n d i c a t i n g t h a t r e c h a r g e i n t h i s r e g i o n o f t h e a r i d z o n e s i s a b i a s e d s a m p l e o f t h e p r e c i p i t a t i o n .
2 . C a r e m u s t b e e x e r c i s e d i n i n t e r p r e t i n g v a l u e s o f t h e d e u t e r i u m e x c e s s i n a r i d z o n e g r o u n d w a t e r s . A s w i t h m an y o t h e r m e t e o r o l o g i c a l p a r a m e t e r s , v a l u e s o f t h e d e u t e r i u m e x c e s s i n lo w r a i n f a l l a r e a s a r e v e r y v a r i a b l e . A t A l i c e S p r i n g s t h e y t e n d t o i n c r e a s e w i t h i n c r e a s i n g r a i n f a l l . T h e o b s e r v e d v a l u e s i n g r o u n d w a t e r s a m p l e s a r e t h e n e t r e s u l t o f a n i n c r e a s e d u e t o b i a s i n g a n d a d e c r e a s e d u e t o s u r f a c e e v a p o r a t i o n .
IAEA-AG-158 /8 1 0 9
3 . T h e s t a n d a r d d e v i a t i o n s a s s o c i a t e d w i t h t h e ÔD v a l u e s d e c r e a s e w i t h . i n c r e a s i n g r e s i d e n c e t i m e .
4 . A s i m p l i f i e d v e r s i o n o f t h e g e n e r a l i s e d t r a n s p o r t e q u a t i o n w a s s o l v e d a n a l y t i c a l l y . T h e s o l u t i o n a l l o w e d a c l e a r m a t h e m a t i c a l d i s t i n c t i o n t o b e m a d e b e t w e e n a f l o w i n g a n d n o n f l o w i n g a q u i f e r s y s t e m . A " n o n f l o w i n g " s y s t e m i s o n e i n w h ic h t h e i s o t o p i c c o m p o s i t i o n o f t h e w a t e r i s i n h e r e n t l y u n p r e d i c t a b l e f r o m m o d e rn c o n d i t i o n s a t r e c h a r g e . T h i s c o n c e p t m ay b e u s e f u l i n a r i d z o n e s w h e r e t h e r a t e o f r e c h a r g e m ay v a r y s u b s t a n t i a l l y o v e r t h e t i m e s c a l e o f i n t e r e s t .
5 . T h e r e l a t i v e a d v a n t a g e o f p r e s e n t i n g С d a t a a s p e r c e n t m o d e rn c a r b o n , o r a s a b s o l u t e c o n c e n t r a t i o n s i s d i s c u s s e d .1 4 C (% m . c . ) v a l u e s a r e s e n s i t i v e t o v a r i a t i o n s i n e x t e n t o f s o l u t i o n o f i n a c t i v e c a r b o n a t e ; a b s o l u t e c o n c e n t r a t i o n s r e s p o n d t o f a c t o r s s u c h a s é v a p o t r a n s p i r a t i o n w h ic h l e a d t o c o n c e n t r a t i o n c h a n g e s i n t h e u n s a t u r a t e d z o n e .
6 . T h e s t a b l e i s o t o p e r e s u l t s s u g g e s t t h a t e f f e c t i v e r e c h a r g e t o t h e M e r e e n i e S a n d s t o n e a q u i f e r o c c u r w hen t h e m o n t h ly r a i n f a l l e x c e e d s t h e 9 0 p e r c e n t i l e v a l u e . U n d e r t h e s e c o n d i t i o n s , s m a l l c h a n g e s t o t h e m e an o r t h e s t a n d a r d d e v i a t i o n o f t h e r a i n f a l l d i s t r i b u t i o n l e a d t o p r o p o r t i o n a l l y l a r g e r c h a n g e s i n t h e p r o b a b i l i t y o f r e c h a r g e .
ACKNOWLEDGMENTS
T h e i n v a l u a b l e a s s i s t a n c e o f D r . M. A . H a b e r m e h l o f t h e B u r e a u o f M i n e r a l R e s o u r c e s , G e o l o g y a n d G e o p h y s i c s , C a n b e r r a , i n t h o s e a s p e c t s o f t h e w o rk i n v o l v i n g t h e G r e a t A r t e s i a n B a s i n , i s g r a t e f u l l y a c k n o w l e d g e d . D i s c u s s i o n s w i t h M r . W a l l a n d M r s . K o m a ro w e r o n t h e N e w e r B a s a l t s A q u i f e r w e r e a p p r e c i a t e d .
REFEREN CES
[1 ] SL A T Y E R , R . O . , P E R R Y , R . A . , A r i d L a n d s o f A u s t r a l i a , A u s t r a l i a n N a t i o n a l U n i v e r s i t y , C a n b e r r a , ( 1 9 6 9 ) .
[2 ] F IS H E R , N . H . , W a te r r e s o u r c e s , i b i d c h a p . 4 , p . 5 5 .
[ 3 ] C A L F , G . E . , " I s o t o p e h y d r o l o g y o f t h e M e r e e n i e S a n d s t o n e A q u i f e r , A l i c e S p r i n g s , A u s t r a l i a . J . H y d r o l o g y , 38^ ( 1 9 7 8 ) 3 4 3 .
[4 ] KOMAROWER, P . , W ALL, V . S . , "T h e c h e m i c a l e v a l u a t i o n o f g r o u n d w a t e r s i n t h e t e r t i a r y b a s a l t s o f V i c t o r i a " , P r o c . AWRC G r o u n d w a t e r P o l l u t i o n C o n f e r e n c e , P e r t h ( 1 9 7 9 ) .
1 1 0 A IR EY et al.
[5 ] ( a ) INTERNATIONAL ATOMIC ENERGY AGENCY, ' E n v i r o n m e n t a lI s o t o p e D a t a N o . l : W o rld S u r v e y o f I s o t o p e C o n c e n t r a t i o ni n P r e c i p i t a t i o n ( 1 9 5 3 - 1 9 6 3 ) , T e c h n i c a l R e p o r t S e r i e sN o . 9 6 , IA E A , V i e n n a ( 1 9 6 9 ) .
(b ) E n v i r o n m e n t a l I s o t o p e D a t a N o . 2 : W o r ld S u r v e y o f I s o t o p e C o n c e n t r a t i o n i n P r e c i p i t a t i o n ( 1 9 6 4 - 1 9 6 5 ) ,T e c h n i c a l R e p o r t S e r i e s N o . 1 1 7 , IA E A , V i e n n a ( 1 9 7 0 ) .
( c ) E n v i r o n m e n t a l I s o t o p e D a t a N o . 3 : W o rld S u r v e y o f I s o t o p e C o n c e n t r a t i o n i n P r e c i p i t a t i o n ( 1 9 6 6 - 1 9 6 7 ) . T e c h - n i c l a R e p o r t S e r i e s N o . 1 2 9 , IA EA V i e n n a ( 1 9 7 1 ) .
(d ) E n v i r o n m e n t a l I s o t o p e D a t a N o . 5 : W o r ld S u r v e y o f I s o t o p e C o n c e n t r a t i o n i n P r e c i p i t a t i o n ( 1 9 7 0 - 1 9 7 1 ) . T e c h n i c a l R e p o r t S e r i e s N o . 1 4 7 IA EA V i e n n a ( 1 9 7 3 ) .
[6 ] AUSTRALIAN WATER RESOURCES C O U N C IL, R e v ie w o f A u s t r a l i a ' s W a te r R e s o u r c e s ; M o n th ly R a i n f a l l a n d E v a p o r a t i o n .P a r t 1 , D a t a T a b u l a t i o n s . B u r e a u o f M e t e o r o l o g y ,M e l b o u r n e , 1 9 6 8 .
[7 ] HARTLEY, P . E . D e u t e r iu m h y d r o g e n r a t i o s i n A u s t r a l i a n r a i n a n d g r o u n d w a t e r , M .S c . T h e s i s , U n i v e r s i t y o f New S o u t h W a le s ( 1 9 7 8 ) .
[8 ] HABERMEHL, M . P . , H y d r o g e o l o g y o f t h e G r e a t A r t e s i a n B a s i n , A u s t r a l i a . J . B u r e a u o f M i n e r a l R e s o u r c e s , A u s t . , i n p r e p .
[9 ] S E I D E L , G . E . , G a b h y d d i g i t a l c o m p u t e r m o d e l o f t h e G r e a t A r t e s i a n B a s i n , A u s t r a l i a . J . B u r . M in . R e s o u r . A u s t . , i n p r e p .
[1 0 ] A IR E Y , P . L . , C A L F , G . E . , CAM PBELL, B . L . , HARTLEY, P . E . ,ROMAN, D . / ‘ A s p e c t s o f t h e i s o t o p e h y d r o l o g y o f t h e G r e a t A r t e s i a n B a s i n , A u s t r a l i a ” , I s o to p e H y d r o lo g y 1 9 7 8 (P ro c . S y m p . N e u h e rb e rg 1 9 7 8 ) 1, IA E A , V ie n n a ( 1 9 7 9 ) 2 0 5 - 1 9 .
[ 1 1 ] SONNTAG, C . , K L IT S C H , E . , LOEHNERT, P . , MÜNNICH, K .O . ,E L SH A ZLY, E . M . , K A LIN K E , G . , THORWEIHE, U . , W E IST R 0 F F E R , K . , SWAYLEM, F .M . “ P a l e o c l i m a t e i n f o r m a t i o n f r o m d e u t e r i u m a n d o x y g e n - 1 8 i n c a r b o n - 1 4 d a t e d N o r t h S a h a r a n g r o u n d w a t e r s ; g r o u n d w a t e r f o r m a t i o n i n t h e p a s t ” , ib id . 2 , p p . 5 6 9 —8 1 . • -
[1 2 ] MERCADO, A . , B I L L I N G S , G . K . , K i n e t i c s o f m i n e r a l d i s s o l u t i o n i n c a r b o n a t e a q u i f e r s a s a t o o l f o r h y d r a u l i c i n v e s t i g a t i o n s .I c o n c e n t r a t i o n - t i m e r e l a t i o n s h i p s . J . H y d r o l o g y 2 £ ( 1 9 7 5 )3 0 3 .
IAEA-AG-158 /8 111
[ 1 3 ] SHAM IN, U . Y . , HARLEMAN, D . R . F . , N u m e r i c a l d i s p e r s i o n s f o r s o l u t i o n s i n p o r o u s m e d i a , W a te r R e s o u r c e s R e s e a r c h J3 ( 1 9 6 7 ) 5 5 7 .
[ 1 4 ] IN G ER SO N , E . , PEA RSO N , F . J . , J r . E s t i m a t i o n o f a g e a n d r a t eo f m o t i o n o f g r o u n d w a t e r b y t h e - ^ C m e t h o d , R e c e n t R e s e a r c hi n t h e F i e l d s o f H y d r o s p h e r e , A t m o s p h e r e a n d N u c l e a r G e o c h e m i s t r y , M a r u z e n C o . , T o k y o ( 1 9 6 4 ) 2 6 3 .
[ 1 5 ] W A LLICK, E . I . I s o t o p i c a n d c h e m i c a l c o n s i d e r a t i o n s i n r a d i o c a r b o n d a t i n g o f g r o u n d w a t e r w i t h i n t h e s e m i - a r i d T u x o n B a s i n , A r i z o n a . I n t e r p r e t a t i o n o f E n v i r o n m e n t a l I s o t o p e a n d H y d r o c h e m i c a l D a t a i n G r o u n d w a t e r H y d r o l o g y ( P r o c . A d v i s o r y G r o u p M e e t i n g , V ie n n a ) IA E A , V i e n n a , 1 9 7 6 .
[1 6 ] VERHAGEN, B . T h . , SM IT H , P . E . , McGEORGE, I . , D ZIEM BO W SK I,Z . ,“Tritium profiles in Kalahari sands as a measure of rain-water recharge”, Isotope Hydrology 1978 (Proc. Symp. Neuherberg 1978) 2, IAEA, Vienna (1979) 7 3 3 - 5 1 .
[ 1 7 ] PEA RSO N , F . J . J r . , SW ARZENKI, W .V . , 1 4 C e v i d e n c e f o r t h e o r i g i n o f a r i d r e g i o n g r o u n d w a t e r . N o r t h e a s t e r n P r o v i n c e , K e n y a , I s o t o p e T e c h n i q u e s i n G r o u n d w a t e r H y d r o l o g y 1 9 7 4 ( P r o c . S y m p . V i e n n a , 1 9 7 4 ) 1 IA E A , V ie n n a ( 1 9 7 4 ) 9 5 - 1 0 8 .
[ 1 8 ] MOOK, W .G . T h e d i s s o l u t i o n - e x c h a n g e m o d e l f o r d a t i n gg r o u n d w a t e r w i t h I n t e r p r e t a t i o n o f E n v i r o n m e n t a lI s o t o p e a n d H y d r o c h e m i c a l D a t a i n G r o u n d w a t e r H y d r o l o g y ( P r o c . A d v i s o r y G r o u p M e e t i n g , V ie n n a ) IA E A , V i e n n a , 1 9 7 6 .
[1 9 ] WOOLEY, D . , G e o h y d r o l o g y o f t h e E m i l y a n d B r e w e r P l a i n s A r e a , A l i c e S p r i n g s , N .T . W a te r R e s o u r c e s B r a n c h ,D e p a r t m e n t o f N o r t h e r n T e r r i t o r y 1 9 6 6 .
[ 2 0 ] GEYH, M .A . I n t e r p r e t a t i o n p r o b l e m s o f g r o u n d w a t e r i s o t o p e d a t a f r o m a r i d a n d s e m i - a r i d z o n e s ( 1 9 7 8 ) IA EA A d v i s o r y G r o u p M e e t in g o n t h e A p p l i c a t i o n o f I s o t o p e T e c h n i q u e s t o A r i d Z o n e s H y d r o l o g y , p a p e r 8 .
[2 1 ] L IN S L E Y , R . J . J r . , KOHLER, M .A . , PAULH US, J . L . H . , H y d r o l o g y f o r E n g i n e e r s , M c G r a w - H i l l B o o k C o . New Y o r k ( 1 9 5 8 ) , c h a p . 1 1 , p . 2 4 5 .
IAEA-AG -15 8 /9
S T U D Y O F T H E L E A K A G E B E T W E E N
T W O A Q U IF E R S I N H E R M O S IL L O , M E X IC O ,
U S IN G E N V I R O N M E N T A L IS O T O P E S
B.R. PAYNE, L. QUIJANO Isotope Hydrology Section,International Atomic Energy Agency, Vienna
C. LATORRE D.Grupo de Física,Secretaría de Agricultura y
Recursos Hidráulicos,Mexico
Abstract
STUDY OF TH E LE A K A G E BETW EEN TWO AQ U IFER S IN H E R M O S ILLO , M EXIC O , USING E N V IR O N M E N T A L ISOTOPES.
The Coast o f Hermosillo is located in the G u lf o f California, M exico. I t is a Quaternary
alluvial plain o f continenta l origin. Underlying these deposits is a layer o f blue clay about
100 m th ick which imposes confinem ent to a deep aquifer in basaltic and pyroclastic rocks.
O xygen-18 and deuterium data support the occurrence o f an upwards leakage. The amount
o f the leakage was evaluated, on the basis o f 14C data, to a m axim um o f 20% o f the water
pumped by the irriga tion wells in the upper aquifer. The stable isotope data also support the
occurrence o f sea-water in trus ion by preferentia l channels in the south and in the area o f
K ino Bay.
1. INTRODUCTION
1.1. Description of the area
The irrigation area, known as “Coast of Hermosillo” , is located between the parallels 28° 20 ' and 29° 55' north and the meridians 11 Io 0 ' and 112° 30' west (F ig .l). It shares its surface between two basins, Hermosillo and Bacoachi. The irrigation district has an area of about 14 0 00 km2, but only 1250 km2 are cultivated.
The climate is extreme, semi-arid, with deficient precipitation during the whole year. The mean annual temperature is 24°C, the maximum being 50°C and the minimum -5 °C . From November to March the mean temperature is about 15°C. The mean annual precipitation is 221 mm and the mean potential
1 1 3
1 1 4 PAYNE et al.
FIG. I. Location o f the study area.
evaporation is 2440 mm. During June and July the monthly potential evaporation exceeds 300 mm [ 1 ].
Two surface streams reach the area. The river Sonora has an annual discharge of about 200 X 106 m3, including the river San Miguel. All this volume is stored in the Abelardo Rodríguez Reservoir by the city of Hermosillo and is used entirely for municipal purposes. Before 1948 the river flowed through the coastal plain and discharged to the sea. The river Bacoachi is rather a wadi, located to the north of the area, and discharges to Laguna Noriega, a small temporal lake known as a playa, where water remains for a few weeks during which time it is lost by evaporation and seepage. The discharge is not measured. During the scarce but heavy rain events, run-off occurs to some extent in the area [2].
IAEA-AG-1 5 8 /9 1 1 5
PROFILE A -A '
WELLPHO-1260.
30 GROUND LEVEL
0-
WELLPH0-10
WELLPHB-9
WELL WELL WELL WELLPHB-15 PHO-3 PHO-8 PHB-7
WELL PHO-19 fin
.30
0
FIG.2. Lithological profile along a NW-SE direction.
1.2. Geohydrological setting
The Coast of Hermosillo is a Quaternary alluvial plain of continental origin. The deposits comprise gravel and sand with intercalations of clay. The stratigraphie sequences are horizontal with a gentle slope towards the Gulf of California. The deposits become more clayey in the coastal zone where permeability decreases with depth and a horizontal layer of brown clay, a few metres thick, occurs at a depth of about 60 m. This clay layer imposes a semiconfinement to the deposits lying below. The thickness of the alluvial fill varies between 100 and 190 m. The fill rests on a layer of blue clay of marine origin with a slope similar to the fill. The thickness is variable. It is about 200 m at the coast and in the centre of the irrigation area. Towards the northeast it becomes thinner, disappearing near the city of Hermosillo. Under the blue clay pyroclastic and basaltic rocks occur, resting on the crystalline basement, located at a depth between 500 and 1200 m as a result of the tectonism of the zone [ 1 ]. Both the blue clay and this formation are not known in detail.Figure 2 shows a lithological profile along a NW-SE direction in the centre of the area of study.
The water-level contours for the upper aquifer in 1954 show a northeast- southwest direction of flow, that is from the city of Hermosillo to the sea. The
1 1 6 PAYNE et al.
water-level contours in 1975 show that the water table has been lowered down to 40 m below sea level in the centre of the area. The groundwater balance for the upper aquifer in 1969 showed a total annual recharge of about 350 X 106 m3, distributed in the following manner: 270 X 106 from infiltration of local precipitation, return flow of excess of irrigation water and a possible recharge through the blue clay, the remaining 80 X 106 came from the lateral underground flow from the periphery of the area [3]. The piezometry of the deep aquifer before groundwater exploitation commenced is not known. Even now, the only evidence at hand is the static level of well PHB-15 located in the centre of the depressed zone. In 1969 it was about 3 m below sea level and 7 m lower in 1974. Nevertheless, the existence of submarine springs a few decades ago, which no longer flow, suggests that the head of the confined aquifer was above sea level. (Sainz Oritz, personal communication.) This aquifer is recharged in that area where the blue clay disappears.
1.3. The problem
Leakage from the deep aquifer
Nothing is known about the relationship between the hydraulic head of both aquifers before water development in the area, but probably the deep aquifer had a higher head as suggested by the existence of submarine springs and the fact that in 1954 the water table of the upper aquifer was less than 10 m above sea level in the area of interest. Therefore, the existence of a steady flow from the deep to the upper aquifer before exploitation was possible. At present the head of the confined aquifer is about 30 m higher than the head of the upper one, in the most depressed area.
The fact that, as exploitation of the upper aquifer proceeded, the submarine springs disappeared and the water level of well PHB-15 decreased, would suggest that the flow through the blue clay has increased significantly as a consequence of the drawdown of the water table of the upper aquifer. Nevertheless, the severe exploitation of groundwater has necessarily induced changes up-gradient where recharge to the deep aquifer takes place and probably the amount of recharge to the latter has been reduced, so that the confined'aquifer has lost pressure.
Saline intrusion
Enough evidence is present in the area of the study to affirm that sea-water is intruding into the upper aquifer [4]. Nevertheless, some samples near the coast were taken to show how the.sea-water intrusion modifies the stable isotope content of the upper aquifer which thus causes some difficulty in determining the isotope index of the upper aquifer.
IAEA-AG-15 8 /9 1 1 7
2.1. Sampling description
In 1973 a reconnaissance sampling was carried out. This included 12 wells from the irrigation area and three wells located along the course of the River Sonora. All these samples were analysed for deuterium,180 and tritium. In 1975 a second sampling was performed — 47 wells from the irrigation area were chosen, covering the cone of depression and the area nearer the coast. These wells were analysed for deuterium and 180 and three of them also for tritium,13C and 14C. At that time the well PHB-15, which taps water only from the deep aquifer, was sampled and analysed for stable isotopes, tritium and 14C.A third sampling was carried out in 1978, including the following: three wells located outside the occurrence of the blue clay, the same three wells located along the river Sonora, which were sampled in 1973, the river itself before the reservoir, the precipitation of July 1978 at the city of Hermosillo, sea-water and seven holes drilled along the shore which can tap water independently from two different horizons. All these samples were analysed for deuterium and 180 . Isotopic analyses were carried out by the IAEA Isotope Hydrology Section.The results are expressed in the conventional delta units. The overall analytical error (1 a) for 6D is 1.0%o and 0.1%o for 5 180 . Samples from 1975 and 1978 were also analysed for major ions. No field measurements of pH were made. These analyses were carried out by the Commission for Water of the Valley of Mexico. The results are given in Tables I and II.
2.2. Approach to the problem
Leakage through the blue clay
Considering the confined nature of the deep aquifer, it was reasonable to expect a large difference between water from this aquifer and water from the upper aquifer. Hence a difference in 180 and deuterium content could be expected. This difference could then be used to investigate the leakage.
The problem of flow through the blue clay can be approached in two ways:
(i) It is possible that under original conditions the flow through the blue clay was downwards and exploitation has caused an inverse flow. On the basis of hydraulic consideration isotope techniques would probably not be applicable for the following reason. Since the permeability of the blue clay should be low and the thickness is about 200 m in the area of interest, it would take hundreds or even thousands of years for the isotopic signal from the confined aquifer to reach the upper one.
2 . M E T H O D O L O G Y
T A B L E I. S A M P L IN G P O IN T S A N D R E S U L T S
Code Name Samplingdate
Total depth or screen location (m )
5 180('%o)
6D(%o)
Tritium(TU )
Ca2+(ppm)
Mg2+(ppm )
Na+(ppm )
С Г(ppm )
s o 3 _(ppm )
HCO3-(ppm)
3 6 4 Well 15-02 2 1 .6 .7 3 12 2 - 6 .8 2 - 4 6 .9 2.1 ± 0 .3 — — — — — _
365 Well 3 2 -1 4 2 1 .6 .7 3 - - 6 .4 8 - 4 7 .7 0.1 ± 0 .3 - - - - - -
36 6 Well 0 4 -09 2 1 .6 .7 3 - - 6 .3 4 - 4 2 .3 0.1 ± 0.2 - - - - - -
367 Well 20-03 2 1 .6 .7 3 - - 6 .3 5 - 4 5 .2 0 .4 + 0 .2 - - - - - -
368 Well 29-15 2 1 .6 .7 3 - - 6 .7 4 - 4 6 .8 1 .1 ± 0.2 - - - - -
369 Well 4 2 -06 2 1 .6 .7 3 - - 6 .2 6 - 4 5 .9 0 .7 ± 0 .2 - - - - - -
37 0 Well 4 9 -09 2 1 .6 .7 3 50 - 6 .8 5 - 4 8 .9 0 .7 ± 0 .2 - - - - - -
371 Well 4 4 -08 2 1 .6 .7 3 - - 6 .8 7 - 4 8 .2 0 .3 ± 0 .3 - - - - - - .
372 Well 51-12 2 1 .6 .7 3 - - 6 .3 4 - 4 5 .8 1 . 1 ± 0.2 - - - - - -
37 4 Well 26-03 2 1 .6 .7 3 40 - 5 .4 7 -3 7 .1 6 6 .4 ± 4 - - - - - -
375 Well 0 9 -02 2 1 .6 .7 3 - - 6 .1 1 - 4 3 .1 0.8 ± 0.2 - - - - - -
376 Well A P-Aconchi 2 1 .6 .7 3 - - 6 .3 2 - 4 3 .0 100 ± 6 - - - - -
377 Well AP-Ures 2 1 .6 .7 3 20 - 6 .0 6 - 4 1 .9 9 5 .2 ± 5.7 - - - - - -
378 Well HAP-7 2 1 .6 .7 3 - - 5 .5 2 - 3 7 .8 86 ± 5.5 - - - - - -
303 Well P H B-15 2 6 .7 .7 5 2 9 8 - 7 3 2 -8 .4 1 - 6 1 .5 0.2 ± 0.2 1.7 0.5 188 108 8 2 .4 2 2 4
3 0 4 Well 15-02 2 2 .7 .7 5 12 2 - 6 .4 5 - 4 8 .5 - 6 0 .3 11.7 70 .5 68.2 6 8 .7 229
305 Well 30-23 2 2 .7 .7 5 98 - 6 .5 5 - 4 9 .4 — 4 9 .0 1 1 . 1 6 1 .3 45 .5 6 5 .9 2 1 1
PA
YN
E
et al.
T A B L E I ( c o n t . )
Total depthCode Name Sampling
dateor screen location (m)
6 18o(%o)
5D
(%o)
Tritium(TU )
Ca2+(ppm )
Mg2+(PPm)
Na+(ppm )
С Г(ppm )
s o 3 '(ppm )
HCO3-(ppm)
3 0 7 Well 30-17 22 .7 .7 5 122 -6 .6 1 - 4 6 .6 - 32 .2 4 .2 49 .3 20 .8 3 4 .6 175
3 0 8 Well 3 7 -14 22 .7 .7 5 152 -6 .6 1 - 4 6 .9 - 3 5 .0 5 .8 4 2 .7 20 .8 31.1 178
3 0 9 Well 37-17 22 .7 .7 5 114 - 6 .5 5 - 4 7 .2 - 3 1 .5 4 .2 45 .5 20 .8 34.1 162
3 1 0 Well 37-21 22 .7 .75 119 - 6 .6 3 - 4 9 .4 - 39.1 8.5 4 6 .4 - 22 .7 3 8 .0 187 '
311 Well 37-23 2 1 .7 .7 5 131 - 6 .5 9 - 4 7 .2 0.1 ± 0 .2 11 .4 19.6 4 4 .6 19 .0 4 0 .4 175
3 1 2 Well 44-01 22 .7 .7 5 114 - 6 .6 5 - 4 9 .0 - 3 1 .5 4 .8 4 5 .0 19.0 33.1 169
313 Well 4 4 -07 2 2 .7 .7 5 ’ 114 - 6 .8 3 - 4 9 .4 - 3 9 .3 5.3 39 .3 20 .8 2 0 .2 187
3 1 4 Well 4 4 -1 0 22 .7 .75 119 -6 .8 1 - 4 8 .2 - 41.1 5 .8 4 6 .0 28.1 3 9 .9 181
315 WeM 4 4-17 25 .7 .7 5 122 - 6 .5 6 - 5 3 .1 - 4 2 .0 6 .9 4 4 .2 30 .3 3 9 .9 178
3 1 6 Well 4 4 -23 25 .7 .7 5 122 - 6 .8 7 - 5 0 .2 - 41 .1 5.3 4 1 .7 2 8 .4 3 2 .6 172
3 1 7 Well 50-12 25 .7 .75 65 - 6 .7 3 - 5 0 .0 - 3 7 .6 5.8 42 .5 24 .6 3 7 .0 172
3 1 8 Well 5 0 -04 2 5 .7 .7 5 108 - 6 .7 0 - 5 0 .4 0 .6 ± 0 .2 34.1 5.3 44.1 22 .7 31.1 175
3 1 9 Well 4 3 -0 4 2 2 .7 .7 5 118 - 6 .7 0 - 4 8 .7 - 3 5 .8 6.1 4 3 .1 20 .8 34.1 181
3 2 0 Well 3 6 -2 0 22 .7 .75 137 - 6 .6 5 - 4 7 .2 - 3 7 .6 5 .3 50 .3 30 .3 4 5 .0 169
321 Well 36-11 22 .7 .7 5 128 - 6 .7 6 - 4 9 .5 - 3 5 .0 10.1 51 .9 30 .3 5 4 .6 175
3 2 2 Well 11-02 2 4 .7 .7 5 61 - 6 .5 0 -4 4 .5 - 3 0 .6 13.3 8 3 .5 70.1 2 9 .2 2 3 0
323 Well 19-01 2 4 .7 .7 5 34 - 6 .4 1 - 4 4 .9 - 5 6 .8 6 2 .6 163 3 0 0 118 261
3 2 4 Well 19-03 24 .7 .7 5 55 - 6 .4 6 -4 6 .8 — 52 .5 14.3 119 9 2 .9 133 2 2 4
IAEA
-AG
-158/9 119
T A B L E I ( c o n t . )
Code Name Sampling
date
T ota l depth
o r screen
location (m )
6 ls O
i%0)5D
(%o)T rit iu m
(TU )
Ca2+
(PPm)
Mg2+
(ppm )
Na+
(ppm )
cr(ppm )
sol‘(ppm )
HCO3
(ppm )
325 W ell 19-05 24.7.75 107 -6 .3 1 -47 .1 - 32.3 5.3 59.9 26.5 41.4 187
326 Well 12-06 24.7.75 91 -6 .3 2 -4 4 .8 - 23.6 3.2 67.1 24.6 31.6 190
327 W ell 20-02 24.7.75 91 -6 .3 7 -4 7 .2 - 91.8 25.0 125 2 1 0 114 230
328 Well 19-04 24.7.75 1 0 2 -6 .3 0 -4 4 .5 - 248 43.0 215 502 311 245
329 Well 20-10 24.7.75 76 -6 .5 9 -4 7 .4 - 157 27.1 81.2 258 156 187
330 Well 35-13 24.7.75 113 -6 .2 8 -48 .1 - 240 45.1 129 675 31.1 116
331 Well 35-15 24.7.75 91 -6 .2 5 -4 5 .9 - 47.2 6.9 60.9 1 0 2 23.7 141
332 Well 35-12 24.7.75 130 -6 .5 3 -4 8 .4 - 31.5 17.5 51.9 43.6 55.7 181
333 Well 35-09 24.7.75 . 98 -6 .2 8 -4 7 .6 - 115 35.6 1 2 0 387 39.9 141
334 Well 35-11 24.7.75 6 6 -6 .5 1 -4 8 .6 - 36.7 1 2 . 2 60.3 43.6 60.2 184
335 Well 35-04 24.7.75 85 -6 .4 9 -4 6 .9 - 33.2 19.1 60.1 51.2 85.6 162
336 Well 49-07 23.7.75 92 -6 .8 0 -5 0 .0 - 28.9 19.1 55.4 6 8 . 2 37.0 172
337 Well 49-09 23.7.75 50 -6 .7 4 -5 1 .4 - 35.8 41.4 78.9 173 52.9 162
338 Well 49-17 23.7.75 1 1 2 -6 .7 7 -4 8 .4 - 30.6 27.1 59.9 1 1 2 33.1 159
339 Well 49-14 23.7.75 107 -6 .71 -50 .1 - 37.6 41.9 138 248 8 6 . 2 159
340 Well 49-03 23.7.75 117 -6 .2 9 -4 5 .7 - 612 104 370 1 820 1 2 2 113
341 Well 49-04 23.7.75 119 -6 .5 8 -4 9 .2 - 30.6 4.8 50.8 34.1 33.1 157
342 Well 55-02 23.7.75 109 -6 .3 6 -4 3 .4 — 490 94.3 406 1 540 187 95.0
120 PAYNE
et al.
T A B L E I ( c o n t . )
Code Name Samplingdate
T otal depth or screen location (m)
5 180(%o)
SD(%c)
Tritium(T U )
Ca2+(ppm )
Mg2+(ppm )
Na+(ppm )
С Г(ppm )
so|~(ppm)
HCO3-(PPm)
343 Well 4 9 -05 2 3 .7 .7 5 96 - 6 .7 5 - 4 9 .0 - 46 .3 20.2 6 0 .9 12 1 38 .4 158
3 4 4 Well 4 9 -1 0 2 3 .7 .75 96 - 6 .5 2 - 4 7 .5 - 3 1 9 85.1 179 861 77.9 1 1 0
345 Well 49-11 2 3 .7 .7 5 91 - 6 .4 4 - 4 8 .5 - 34.1 7 .4 6 3 .0 73 .9 3 1 .4 135
346 Well 5 0 -10 2 3 .7 .75 183 - 6 .4 1 - 4 6 .0 - 3 0 .6 6 .4 52 .3 56 .9 30.2 132
347 Well 50-16 2 3 .7 .7 5 93 - 6 .6 5 - 4 7 .9 - 2 4 4 4 7 .8 10 1 635 55.7 101
348 Well 50-09 2 3 .7 .7 5 108 - 6 .4 5 - 4 6 .8 - 6 4 7 106 345 1 880 136 76 .6
3 4 9 Well 50-07 2 3 .7 .7 5 102 - 6 .8 9 -4 8 .1 - 15.7 18.1 4 6 .7 3 2 .2 37 158
3 5 0 Well 55-01 2 3 .7 .7 5 107 - 6 .7 4 - 4 6 .7 - 20 .1 10.6 4 3 .4 30 .3 31.1 138
351 Well 50-19 2 3 .7 .7 5 91 - 6 .7 4 - 5 0 .4 - 34.1 30 .8 69 .7 108 92 141
3 8 0 Well HAP-7 6.78 - - 3 .9 7 - 3 0 .3 - 8 1 .0 18.1 108 43 .8 123 389
381 Well P SB -10 6 .78 1 3 8 -2 9 9 - 5 .9 3 - 3 9 .9 - 2.1 1.9 127 60 .2 32 185
3 8 2 Well 2 6 -03 6 .78 40 - 5 .1 8 - 3 6 .8 - 76 .9 12.5 22 .3 3 1 .0 3 8 .4 257
383 Well CH -16 6 .7 8 150 - 4 .6 4 - 3 6 .3 - 1 1 0 2 2 .4 177 139 242 3 72
3 8 4 Well AP-Ures 6 .78 20 - 5 .3 9 - 3 6 .4 - 76 14.3 45 .5 18.2 125 235
385 Well AP-Aconchi 6 .78 - - 5 .9 0 - 4 2 .2 - 10 1 17.4 52 .9 20 .1 150 312
386 River Sonora (Puente el Gavilán) 6 .78 _ - 4 .4 9 - 3 6 .2 _ 6 8 .7 18.7 102 3 4 .7 2 2 1 2 4 0
387 Rain (Herm osillo C ity) 3 6 .4 mm 7.78 - 0 . 1 2 + 0.1
T A B L E I ( c o n t .)
Code Name Samplingdate
Total depth or screen location (m )
s18o('%0)
8D
(%o)
Tritium(TU )
Ca2t(ppm )
Mg2+(ppm )
Na+(ppm )
С Г(ppm )
SO|~(ppm)
HCC(ppn
388 Sea water (K ino) 6 .7 8 - + 0 .33 + 2.3 - - - - — - -
3 8 9 Well PCH-2 6 .7 8 6 4 - 9 6 -5 .5 5 - 4 0 .3 - 815 143 41 5 2 2 4 0 3 1 2 9 6 .
3 9 0 Well PCH-2 6 .7 8 1 2 7 - 2 0 4 - 5 .5 5 - 4 3 .5 - 18.5 2.5 50.5 32 .8 2 4 .8 116
391 Well PCH-4 6 .7 8 6 0 - 1 2 0 - 7 .3 2 - 5 3 .5 - 27 .7 3 .7 396 503 154 105
392 Well PCH-4 6 .7 8 1 4 4 - 1 9 6 - 7 .5 8 - 5 2 .2 - 2 0 .5 4 .4 39 4 4 9 4 146 105
3 9 4 Well PCH-5 6 .7 8 1 2 0 - 2 0 0 -6 .4 5 - 4 6 .7 - 282 4 6 .7 144 783 55 77
3 9 5 Well PCH-6 6 .7 8 3 1 - 9 7 - 1 .3 9 - 1 2 .6 - 1820 8 28 6 9 3 0 14 3 0 0 2 7 1 0 121
396 Well PCH-6 6 .7 8 1 1 0 - 2 0 0 - 1 .5 0 - 1 0 .7 - 1110 863 7 3 9 0 14 0 0 0 2 6 1 0 135
397 Well PCH-7 6 .7 8 8 2 - 1 0 2 - 4 .2 9 - 3 2 .2 - 9 9 0 368 2 7 3 0 6 2 9 0 9 5 2 113
398 Well PCH-7 6 .7 8 1 1 9 - 1 9 0 - 3 .2 7 - 2 5 .9 - 1830 6 2 0 4 1 9 0 10 3 0 0 1660 110
399 Well PCH-8 6 .7 8 9 3 - 1 1 2 - 1 .9 0 - 1 5 .1 - 1520 791 6 5 5 0 13 100 2 6 1 0 132
4 0 0 Well PCH-8 6 .7 8 1 2 7 - 1 9 8 - 1 .8 3 - 1 3 .9 - 1500 825 6 5 7 0 13 500 2 3 2 0 132
401 Well PCH-9 6 .7 8 7 6 - 1 2 1 - 6 .4 0 - 4 5 .4 - 35 .9 9.3 337 4 5 2 67 185
4 0 2 Well PCH-9 6 .7 8 1 5 3 -1 9 8 - 6 .4 0 - 4 5 .9 —
122 PAYN
E etal.
IAEA-AG-15 8 /9 1 2 3
T A B L E II. C A R B O N C H E M IS T R Y
Code Name Samplingdate
Water temp. (°C )
pH(lab)
5 13C
(%■>)
14C(pm c)
303 Well PHB-15 2 6 .7 .7 5 50 8.1 - 1 0 .6 9 2 .1 2 ± 0 .7 6
311 Well 37-23 2 1 .7 .7 5 32 7.7 - 1 0 .4 3 7 8 .9 6 ± 2 .4
315 Well 44-17 2 5 .7 .7 5 32 7.6 - 9 .8 0 8 1 .0 ± 2 .4
318 Well 50-04 2 5 .7 .7 5 31 7.7 - 9 .3 9 77 .9 ± 2 .5
(ii) On the contrary, before exploitation of groundwater commenced, the piezometric head of the confined aquifer may have been higher than the head of the upper one. If that was the case, an upwards flow has taken place since millenia and the isotopic composition of the upper aquifer should be influenced by this steady but small recharge.
The sampling was oriented to characterize the isotopic content of both aquifers as well as the content of the waters entering the area, that is, underground flow, local precipitation and the river Sonora.
Saline intrusion
To investigate the nature of the saline intrusion a simple linear model of mixing with sea-water is assumed, combining 180 and chloride. This allows the proportion of sea-water present in each sample to be estimated.
Wells PCH-2 to 9 (samples 389 to 402) each have two independent tubes with screens at different depths.
3. RESULTS AND DISCUSSION
3.1. The problem of leakage
Isotopic content o f the deep aquifer
Figure 3 shows the 180 and deuterium content of all samples. A meteoric line through the jnean content of the upper aquifer has been drawn. The most depleted sample in heavy isotopes is from the deep aquifer. The delta values are quite distinct from those of the upper aquifer. As éxpected, the tritium content is below the detection limit and the 14C concentration is very low.
1 2 4 PAYNE et al.
óD%o
FIG.3. 180 and deuterium content o f all samples. The meteoric line passes through the mean content o f the upper aquifer. The delta values o f the upper aquifer are situated between those o f the deep aquifer and those o f the groundwater in the periphery.
Considering the confined nature of the aquifer, sample 303 (well PHB-15) can be taken as representative of the central part of the deep aquifer, located under the area where the water level of the upper aquifer has decreased markedly. Anyway, no more wells penetrating the deep aquifer exist in that area.
Age o f waters from the deep aquifer
The WATEQF Model [5] and ISOTOPE Subroutine [6] were applied to interpret the carbon chemistry. The lack of pH measurement in the field, as well as a knowledge of the partial pressure and 13C content of soil carbon dioxide in the recharge area make the model indicative rather than conclusive.
Assuming a 5 13C of the soil carbon dioxide equal to - 1 8 %o„ which seems reasonable for arid regions [7], and a 6 13C equal to ~0.6%o for rock carbonates [8], the best fit between the measured 6 13C of sample 303 ( -1 0 .6 9 %o) and the calculated ( -1 0 .1 0 %o) correspond to an assumed recharge pH of 7.5 and an assumed partial pressure of soil carbon dioxide equal to 10-215 atm. According to the model, the recharge water has a total dissolved biogenic carbon equal 3 .50 mmoles and a 5 13C equal to -1 0 .8 2 %o. On the other hand, the laboratory pH of sample 303 is 8.1 and the total carbon content 3.76 mmoles. The corresponding partial pressure of carbon dioxide is 10 -2-57 atm.
It seems that 93% of the total carbon content of sample 303 is biogenic.That means that the 14C concentration has decreased mostly by radioactive
IAEA-AG -15 8 /9 1 2 5
FIG. 4. The stable isotope content o f the upper aquifer is shown in detail.
decay and not by dissolution of dead carbonate. The chemical composition supports this interpretation. Since the water is almost free of calcium and magnesium, the aquifer should be very poor in carbonate minerals. This agrees also with the volcanic nature of the rock. Taking into account the above considerations it is likely that water represented by sample 303 has an average age of about 30 thousand years. The lighter 5D and 5 180 of this sample as compared with samples from the upper aquifer support the palaeo-origin of waters from the confined aquifer, recharged during a colder and more humid climate than the present.
Isotopic and chem ical content o f the upper aquifer
The mean 0 180 and 5D of all samples from the upper aquifer (numbers304, 305, 307—351, 3 6 4 —373) are the following:
5 180 = -6.58% o ± 0 . 1 9d = 4 .9 (1)
§D = -47.7% o ± 2.0
The stable isotope content is rather homogeneous (Fig.4). The standard deviation is twice the analytical error.
Figure 5 is a graph of chloride vs 5 180 . Samples 3 6 4 —373 are missing because chemical analyses are not available. Two groups can be distinguished.
1 2 6 PAYNE et al.
Cl",O O
o
oo o
o o
OO
□ GROUP 1 O GROUP 2
л325326
6180%
FIG.5. Chloride content vs 5 180 of samples from the upper aquifer. Group one represents waters from the central part and shows an homogeneous composition. Group two represents waters from the coastal areas arid shows an increasing chloride content.
The first group comprises 18 wells from the central and southern parts of the area (samples 307—321, 341, 349, 350). This group has a very homogeneous chemical and isotopic composition:
Ca2+= 32.8 ± 8.7 ppm СП = 2 5 . 3 ±5 . 1 ppm§ 180 = -6.69% o ± 0 . 1 1 Mg2+= 7.6 ± 4 . 5 ppm SO4- = 35 .9 ± 7 .0 ppm (2).5D = -48.7% o ± 1.7 Na+ = 45 .4 ± 3.4 ppm HCO3 = 172 ± 1 2 ppm
Samples 365, 368, 371 and 373, located in the same area, have a similar stable isotope composition to that given in Eq.(2). Sample 372 is slightly enriched. The chloride content of these samples is not available.
The tritium measurements obtained on this group ( 3 1 1 , 3 1 8 , 365, 368, 371, 372, 373) show a content equal or less than 1 TU. The 13C and 14C concentrations of wells 311, 315 and 318 are similar. The 14C and tritium results are discussed later.
IAEA-AG-1 5 8 /9 1 2 7
Considering the location and the homogeneity in composition of the above samples, it is concluded that the mean values given in Eq.(2) characterize the upper aquifer.
Samples 325 and 326 , located in the north of the area, have a chemical composition similar to that of group one, but the stable isotope composition is slightly enriched in 180 .
The second group (samples 322—340, 3 4 2 —348, 351) shows an increase in chloride content. This group will be discussed under the topic of saline intrusion.
Iso topic com position o f recharge waters
The stable isotope content of the wells located outside the occurrence of the blue clay (374 , 375, 378, 380, 381, 382, 383) show a more positive 6D and S180 in comparison to Eq.(2). The pairs of samples 3 7 4 —382 and 3 7 8 —380 were obtained from the same wells on different dates. Sample 380 is enriched in heavy isotopes as compared with sample 378, along an evaporation line.This enrichment is probably a consequence of seepage from A. Rodriguez Reservoir. By contrast, the other pair shows no significant variation.
Samples 376 and 385 were taken from well AP Aconchi and samples 377 and 384 from well AP Ures, on different dates. Both wells are located close to the river Sonora and most probably are recharged by it. The stable isotope content shows variations according to the date of sampling as would be expected in view of the high tritium, content. All these samples also have a higher content of deuterium or 180 than samples from the group defined by Eq.(2). Well 366, located further north, has a similar enrichment. These facts suggest again that waters flowing towards the centre of the area are enriched in deuterium and 180 as compared with waters from the centre.
Assessmen t o f the leakage
Water samples obtained outside the area overlying the blue clay and taken as representative of the inflow to the area show systematically a more positive 5D and 5 180 value in comparison to the mean value for that area (2 ), suggesting that, in fact, upwards flow through the blue clay occurs, since waters from the deep aquifer are characterized by a more negative delta value.
Well PCH-4 (samples 391, 392), located to the south, has a stable isotope content between the figures given in (2 ) and those for the confined aquifer.Sample 391 was obtained from an horizon between 60 and 120 m and sample 392 between 144 and 196 m. Their isotopic content indicates that leakage from the confined aquifer is more marked in that zone.
The flow through the blue clay has occurred before water development under steady-state conditions (cf. 2.2). If flow has taken place, the drawdown of the water level of the upper aquifer has obviously increased its amount.
128 P A Y N E et al.
FIG. 6. Location of the sampling points.
Clppm
FIG. 7. The line represents a simple mixture between sea-water and water from the upper aquifer. The points are wells located in the coastal area and fit well in the mixing line.
I A E A - A G - 1 58/9 1 2 9
I t i s n o t p o s s i b l e t o e s t i m a t e t h e a c t u a l a m o u n t o f l e a k a g e . A m o r e c a r e f u l
a n d d e t a i l e d s a m p l i n g w o u l d b e n e e d e d f o r t h a t p u r p o s e . N e v e r t h e l e s s , a s s u m i n g
t h a t s a m p l e s 3 1 1 , 3 1 5 a n d 3 1 8 a r e r e p r e s e n t a t i v e o f t h e w a t e r f r o m t h e c e n t r a l
p a r t o f t h e u p p e r a q u i f e r , i t i s c e r t a i n t h a t t h e p r o p o r t i o n o f w a t e r f r o m t h e
c o n f i n e d a q u i f e r c a n n o t b e m o r e t h a n 2 0 % i n t h a t c e n t r a l p a r t , s i n c e t h e 1 4 C
c o n t e n t o f t h e s e t h r e e s a m p l e s i s a b o u t 8 0 % a n d t h e 1 4 C c o n t e n t o f t h e d e e p
a q u i f e r i s n e a r z e r o . T h e t r i t i u m c o n t e n t o f s a m p l e s 3 1 1 , 3 1 5 a n d 3 1 8 ( l e s s
t h a n 1 T U ) e x c l u d e s t h e p r e s e n c e o f a s i g n i f i c a n t a m o u n t o f w a t e r i n f i l t r a t e d
a f t e r 1 9 5 2 a n d , t h e r e f o r e , t h e c o n t r i b u t i o n o f 1 4 C c o n t e n t s h i g h e r t h a n 1 0 0 % .
I n t h e z o n e o f t h e u p p e r a q u i f e r r e p r e s e n t e d b y w e l l P C H - 4 ( 3 9 1 , 3 9 2 )
t h e p r o p o r t i o n o f w a t e r f r o m t h e d e e p a q u i f e r s e e m s t o b e a b o u t 3 0 t o 5 0 % .
T h i s w e l l i s l o c a t e d i n a z o n e w h e r e t h e b l u e c l a y i s o n l y a f e w m e t r e s t h i c k
( F i g s 2 a n d 6) .
3 . 2 . T h e p r o b l e m o f s a l i n e i n t r u s i o n
F i g u r e 7 i s a p l o t o f c h l o r i d e v s <518O %0 o f s a m p l e s 3 2 2 - 3 4 0 , 3 4 2 - 3 4 8 ,
3 5 1 , 3 8 9 , 3 9 0 , 3 9 4 - 4 0 1 , l o c a t e d n e a r e r t h e c o a s t . T h e h y p o t h e t i c a l m i x i n g
l i n e b e t w e e n s e a - w a t e r a n d f r e s h w a t e r i s i n c l u d e d . I t i s c l e a r t h a t t h e c h l o r i d e
i n c r e a s e i s d u e t o s e a - w a t e r i n t r u s i o n . N e v e r t h e l e s s , s o m e p o i n t s s h o u l d b e
c o m m e n t e d u p o n . T h e w e l l s P C H -6 ( 3 9 5 , 3 9 6 ) , P C H - 7 ( 3 9 7 , 3 9 8 ) a n d
P C H -8 ( 3 9 9 , 4 0 0 ) s h o w t h e g r e a t e s t i n f l u e n c e o f t h e s a l i n e i n t r u s i o n ; b u t w e l l
P C H - 5 ( 3 9 4 ) l o c a t e d i n t h e s a m e a r e a a n d c l o s e r t o t h e c o a s t s h o w s o n l y a
s l i g h t i n f l u e n c e . T h i s f a c t s u p p o r t s t h e o p i n i o n t h a t s e a - w a t e r i s i n t r u d i n g i n t o
t h e z o n e b y p r e f e r e n t i a l c h a n n e l s . B o t h s a m p l e s o f w e l l P C H - 2 ( 3 8 9 , 3 9 0 ) h a v e
a v e r y s i m i l a r s t a b l e i s o t o p e c o n t e n t , b u t t h e c h e m i c a l c o m p o s i t i o n i s q u i t e
d i f f e r e n t . P r o b a b l y t h e b o t t l e 3 9 0 f o r c h e m i c a l a n a l y s i s w a s m i x e d u p . A n y w a y ,
t h e s a m p l e 3 8 9 f i t s w e l l i n a s e a - w a t e r / f r e s h - w a t e r m i x i n g l i n e ( F i g . 7 ) . F i n a l l y ,
w e l l P C H - 9 ( 4 0 1 , 4 0 2 ) i s s l i g h t l y a f f e c t e d b y s e a - w a t e r i n t r u s i o n .
4 . C O N C L U S I O N S
4 . 1 . T h e w a t e r o f t h e d e e p c o n f i n e d a q u i f e r r e p r e s e n t e d b y w e l l P H B - 1 5 ( 3 0 3 )
m o s t p r o b a b l y h a s a n a g e o f a p p r o x i m a t e l y 3 0 t h o u s a n d y e a r s .
4 . 2 . T h e s t a b l e i s o t o p e d a t a s u p p o r t t h e o c c u r r e n c e o f a f l o w f r o m t h e d e e p
c o n f i n e d a q u i f e r t o t h e u p p e r a q u i f e r b e f o r e w a t e r d e v e l o p m e n t i n t h e a r e a .
4 . 3 . I t i s n o t p o s s i b l e t o e s t i m a t e t h e a c t u a l a m o u n t o f l e a k a g e . N e v e r t h e l e s s ,
a c c o r d i n g t o t h e a v a i l a b l e 1 4 C m e a s u r e m e n t s , t h e p r o p o r t i o n o f w a t e r a r i s i n g
f r o m l e a k a g e f r o m t h e d e e p e r a q u i f e r , p u m p e d b y t h e i r r i g a t i o n w e l l s i n t h e
1 3 0 P A Y N E et al.
c e n t r a l a r e a o f t h e u p p e r a q u i f e r , w a s l e s s t h a n 20% a t t h e t i m e o f t h e
s a m p l i n g ( 1 9 7 5 ) .
4 . 4 . T h e a v a i l a b l e s t a b l e i s o t o p e d a t a s u p p o r t t h e e x i s t e n c e o f s e a - w a t e r i n t r u s i o n
i n t h e s o u t h a n d i n t h e a r e a o f K i n o B a y .
A C K N O W L E D G E M E N T S
T h e a u t h o r s a c k n o w l e d g e t h e c o - o p e r a t i o n o f t h e f o l l o w i n g d e p e n d e n c i e s
o f t h e f o r m e r W a t e r R e s o u r c e s S e c r e t a r i a t : C o m m i t t e e f o r t h e D e v e l o p m e n t
o f t h e R e s o u r c e s o f t h e C e n t r a l a n d N o r t h e r n B a s i n s o f S o n o r a , T h e G e n e r a l
M a n a g e m e n t o f S o n o r a , a n d T h e R e s i d e n c e o f G e h y d r o l o g y i n S o n o r a . T h e
a u t h o r s a p p r e c i a t e a l s o t h e c o m m e n t s o f M r . I g n a c i o S a i n z O r t i z , a n d t h e
c o m m e n t s o n t h e m a n u s c r i p t b y G i a n M a r i a Z u p p i .
R E F E R E N C E S
[1 ] SECRETA RIA DE RECURSOS HIDRAULICOS, Estudio Hidrogeológico Preliminar de los Acuíferos de la Costa de Hermosillo, Sonora, México D.F. (1 9 6 8 ).
[2] SECRETARIA DE RECURSOS HIDRAULICOS, Boletín Hidrológico N o.40 , Región Hidrológica N o.9, Sonora Sur, México D .F. (1 9 7 0 ).
[3] SECRETARIA DE RECURSOS HIDRAULICOS, Estudio Hidrogeológico Completo de los Acuíferos de la Costa de Hermosillo, Sonora, México D .F. (1 9 7 0 ).
[4] SECRETARIA DE RECURSOS HIDRAULICOS, Complemento del programa para establecer los medios de detección y control del avance de la intrusión salina en la Costa de Hermosillo, México D .F. (1 9 7 6 ).
[5] PLUMMER, N.L., JONES, B .F ., TR U ESD ELL, A.H., WATEQF - A Fortran IV version of W ATEQ, US Geol. Survey Water Resources Investigation (1 9 7 6 ) 7 6 —13.
[6] REARDON, E .J ., FR IT Z, P., Computer modelling of groundwater 13C and 14C isotope compositions, J . Hydrol. 36 (1 9 7 8 ) 2 0 1 —24.
[7] DEINES, P., LANGMUIR, D., HARMON, R.S., Stable carbon isotopes to indicate the presence or absence of a gas phase in the evolution of carbonate groundwater, Geochim. Cosmochim. Acta. 38 (1 9 7 5 ) 1 1 4 7 —64.
[8] HOEFS, J ., Stable Isotope Geochemistry, Springer Verlag ( 1973).
F I E L D I N V E S T I G A T I O N S O N G R O U N D W A T E R O R IG IN
A N D F L O W P A T T E R N S
IA E A -A G -1 5 8 /1 0
U T I L I Z A T I O N O F N A T U R A L IS O T O P E S
IN T H E S T U D Y O F S A L I N A T I O N O F
T H E W A T E R S IN T H E P A J E Ú R I V E R V A L L E Y ,
N O R T H E A S T B R A Z I L
E . S A L A T I , E . M A T S U I
C e n t r o d e E n e r g í a N u c l e a r n a
A g r i c u l t u r a ( C E N A ) ,
P i r a c i c a b a , B r a z i l
J . M . L E A L
S u p e r i n t e n d e n c i a d o D e s e n v o l v i m e n t o
d o N o r d e s t e ,
R e c i f e , B r a z i l
P . F R I T Z *
U n i v e r s i t y o f W a t e r l o o , W a t e r l o o ,
O n t a r i o , C a n a d a
Abstract
UTILIZATION OF NATURAL ISOTOPES IN THE STUDY OF SALINATION OF THE W ATERS IN THE PAJEÚ R IV ER V A LL EY , NORTHEAST BRAZIL.
Municipal and agricultural water supplies in the vast regions on the Brazilian Shield come primarily from shallow groundwater systems in weathered bedrock and minor amounts of alluvial fill. The water quality is generally poor. This salination of water supplies is a serious problem because no management schemes have been proposed to solve water shortages and to improve water quality. In this study it is shown that salination is an active process which is not related to the release of fossil sea-water trapped in the crystalline rocks since Cretaceous times, nor is it related to intensive weathering of the bedrock. Evaporation and évapotranspiration must account for it with the salt being brought primarily by precipitations. It is suggested that increased pumping of these shallow aquifers during the dry season will not result in a general improvement of water quality, thus limiting the potential of these aquifers to their present usage.
I N T R O D U C T I O N
T h e s i l i c a t e m i n e r a l s w h i c h c o m p o s e t h e b u l k o f c r y s t a l l i n e a n d m e t a m o r p h i c
r o c k s d o n o t d i s s o l v e v e r y f a s t a n d , t h e r e f o r e , s h a l l o w “ a q u i f e r s ” w i t h i n s u c h
r o c k s u s u a l l y y i e l d l i m i t e d a m o u n t s o f g o o d q u a l i t y g r o u n d w a t e r s s u i t a b l e f o r
h u m a n c o n s u m p t i o n a n d a g r i c u l t u r a l p u r p o s e s . H o w e v e r , t h r o u g h o u t t h e v a s t
* UNDP/IAEA/CENA P R O JEC T /B R A /71 /5 56.
13 3
FIG.l. Location map.
IA E A -A G -1 5 8 /1 0 135
n o r t h e a s t e r n r e g i o n o f B r a z i l s h a l l o w g r o u n d w a t e r s i n c r y s t a l l i n e o r m e t a m o r p h i c
r o c k s a r e o f t e n s o h i g h l y m i n e r a l i z e d t h a t t h e y a r e a c c e p t a b l e o n l y t o a n i m a l s .
O n l y o c c a s i o n a l l y i s s o m e b e t t e r - q u a l i t y w a t e r r e c o g n i z e d d e s p i t e t h e i n s t a l l a t i o n
o f t h o u s a n d s o f w e l l s d u r i n g t h e p a s t d e c a d e s . T h i s t a s k w a s p r i m a r i l y c a r r i e d o u t
b y t h e D i v i s i o n o f H y d r o g e o l o g y o f t h e S u p e r i n t e n d ê n c i a d o D e s e n v o l v i m e n t o
d o N o r d e s t e ( S U D E N E ) w i t h t h e o b j e c t i v e n o t o n l y t o f i n d g o o d q u a l i t y w a t e r
b u t a l s o t o s t u d y t h e c a u s e s f o r t h e s a l i n a t i o n o f t h e s e g r o u n d w a t e r s a n d t o p r o p o s e
p o s s i b l e s o l u t i o n s l e a d i n g t o a g r a d u a l i m p r o v e m e n t i n q u a l i t y o f t h e s e w a t e r
s u p p l i e s .
A r e p o r t o n t h e w a t e r q u a l i t y a n d o r i g i n o f s a l t i n t h e s e w a t e r s w a s p r e p a r e d
b y S c h o f f [ 1 ]. I n i t , h e c a m e t o t h e f o l l o w i n g c o n c l u s i o n :
“ I t i s s u g g e s t e d t h a t t h e g r o u n d w a t e r c o n t a i n s s o m e r e s i d u a l s e a w a t e r ,
i n t r o d u c e d d u r i n g a n U p p e r C r e t a c e o u s m a r i n e i n v a s i o n o f t h e l a n d , a n d t h a t t h i s
w a t e r h a s b e e n d i l u t e d b y m e t e o r i c w a t e r a n d l a r g e l y f l u s h e d o u t o f t h e r o c k s i n
a p r o c e s s t h a t i s s t i l l i n c o m p l e t e b u t i s c o n t i n u i n g . T h e p r o c e s s h a s o p e r a t e d
u n e v e n l y , i n a c c o r d w i t h d i v e r s e r a t e s o f c i r c u l a t i o n o f w a t e r i n f r a c t u r e s y s t e m s
h a v i n g a w i d e v a r i e t y o f h y d r a u l i c c h a r a c t e r i s t i c s . T h u s , t h e m i n e r a l i z e d a n d t h e
d i l u t e w a t e r s a r e i n t e r s p e r s e d h a p h a z a r d l y t h r o u g h t h e r e g i o n .
A n y a c t i o n b y m a n t h a t w o u l d a c c e l e r a t e t h e d i l u t i o n o f t h e m i n e r a l i z e d
g r o u n d w a t e r w i t h f r e s h w a t e r o r r e m o v e i t f r o m t h e r o c k s w o u l d t e n d i n t h e
s a m e d i r e c t i o n a s t h e n a t u r a l p r o c e s s , b u t m a n ’s e f f o r t s a r e u n l i k e l y t o a c c o m p l i s h
l a r g e o b s e r v a b l e i m p r o v e m e n t s i n w a t e r q u a l i t y , e x c e p t p e r h a p s l o c a l l y . P l a n n e d
p u m p i n g o f a w e l l o v e r a p e r i o d o f y e a r s m i g h t w i t h d r a w e n o u g h m i n e r a l i z e d
w a t e r f r o m a s m a l l s y s t e m o f f r a c t u r e s a n d p r o m o t e t h e i n f i l t r a t i o n o f e n o u g h
f r e s h w a t e r t o c r e a t e w o r t h w h i l e c h a n g e s i n c h e m i c a l q u a l i t y . "
O t h e r s , a s q u o t e d b y S c h o f f [ 1 ] , c o n s i d e r e d t h e d i s s o l u t i o n o f m i c a s a n d
f e l d s p a r s a s t h e p r i n c i p a l s o u r c e o f m i n e r a l i z a t i o n a n d a l l a g r e e t h a t a n i m p r o v e
m e n t i n w a t e r q u a l i t y w o u l d b e a c h i e v e d i f t h e w e l l s w e r e p u m p e d .
I n t h i s p a p e r , w e a g a i n a n a l y s e t h e p o s s i b l e r e a s o n s f o r tt h e s a l i n a t i o n o f t h e
g r o u n d w a t e r s , u s i n g e n v i r o n m e n t a l i s o t o p e s a s a b a s i c t o o l .
T h e s t u d y w a s c a r r i e d o u t w i t h U N D P - I A E A s u p p o r t i n t h e P a j e ú R i v e r B a s i n
w h i c h i s l o c a t e d i n t h e s e m i - a r i d r e g i o n o f t h e S t a t e o f P e r n a m b u c o , b e t w e e n
7 ° 3 0 ' — 9 ° 0 ' l á t i t u d e s o u t h a n d 3 7 ° 6 0 ' — 3 9 ° 0 ' l o n g i t u d e w e s t ; i t c o v e r s a b o u t
1 7 000 k m 2 , t h e m a i n c h a n n e l l e n g t h i s a b o u t 2 5 0 k m a n d d i s c h a r g e s i n t o t h e
S a o F r a n c i s c o R i v e r ( F i g . 1 ) . T h i s r e s e a r c h i s a c o n t i n u a t i o n o f t h e w o r k i n i t i a t e d
b y S a l a t i e t a l . [ 2 ] w h o s t u d i è d r e g i o n a l h y d r o g e o l o g i c a l p r o b l e m s u s i n g i s o t o p e
t e c h n i q u e s .
A s e m i - a r i d c l i m a t e ( t y p e B s h i n t h e K o p p e n c l a s s i f i c a t i o n ) i s p r e d o m i n a n t
i n t h e g r e a t e s t p a r t o f t h e P a j e û R i v e r v a l l e y w i t h h i g h t e m p e r a t u r e s , l i t t l e
n e b u l o s i t y a n d l o w r e l a t i v e h u m i d i t y . H o w e v e r , i n t h e h i g h e r h e a d - w a t e r r e g i o n s
o f t h e r i v e r s , t h e c l i m a t e r e a c h e s A w ( K o p p e n ) . P r e c i p i t a t i o n d e c r e a s e s a l o n g t h e
c o u r s e o f t h e P a j e ú R i v e r f r o m v a l u e s b e t w e e n 8 0 0 — 1 0 0 0 m m i n t h e h e a d - w a t e r s
1 3 6 S A L A T I et al.
t o l e s s t h a n 4 0 0 m m a t t h e m o u t h . R a i n f a l l o c c u r s p r i m a r i l y b e t w e e n D e c e m b e r
a n d J u n e w i t h a m a x i m u m d u r i n g F e b r u a r y - M a r c h o f e a c h y e a r .
T h e P a j e û R i v e r B a s i n i s a l m o s t e n t i r e l y u n d e r l a i n b y P r e c a m b r i a n c r y s t a l l i n e
r o c k s w h i c h c o n s i s t o f a m i x t u r e o f g n e i s s , g r a n i t e , m i g m a t i t e s , q u a r t z i t e s , m i n o r
a m p h i b o l i t e s , s o m e m a r b l e a n d s c h i s t s . M i n o r o c c u r r e n c e s o f p o s s i b l y P a l a e o z o i c
a n d M e s o z o i c s e d i m e n t a r y f o r m a t i o n s a r e r e c o g n i z e d a n d o f t e n a s s o c i a t e d i n
t e c t o n i c , g r a b e n - l i k e s t r u c t u r e s . E x t e n s i v e , b u t s h a l l o w a l l u v i a l d e p o s i t s o c c u r a l s o
a l o n g t h e r i v e r b e d s a n d a r e d e r i v e d a l m o s t e n t i r e l y f r o m t h e c r y s t a l l i n e r o c k s .
T h e t r i b u t a r i e s o f t h e P a j e ú R i v e r f l o w i n t e r m i t t e n t l y , w i t h a b r u p t c h a n g e s
a t t h e b e g i n n i n g o f t h e r a i n y s e a s o n a n d b e c o m i n g p r a c t i c a l l y d r y t w o o r t h r e e
w e e k s a f t e r t h e r a i n s a r e o v e r . T h u s , f o r e x a m p l e , t h e d i s c h a r g e f r o m t h e m a i n
t r i b u t a r y - t h e N a v i o R i v e r - v a r i e s f r o m 0 t o 2 5 0 m 3 / s o v e r a p e r i o d o f o n l y
s i x d a y s . A s a r e s u l t , t h e f l o w o f t h e P a j e ú R i v e r i s a l s o i n t e r m i t t e n t a n d i n a
m a t t e r o f a f e w d a y s i t s d i s c h a r g e c a n c h a n g e f r o m n e a r 0 t o a b o u t > 1 0 0 0 m 3/ s .
M o r e t h a n 4 0 0 w e l l s h a v e b e e n d r i l l e d i n t h e P a j e ú B a s i n , p r e f e r e n t i a l l y i n
c r y s t a l l i n e r e g i o n s w i t h l a r g e d e m o g r a p h i c d e n s i t y ; m e a n d e p t h a n d y i e l d b e i n g
3 0 m a n d 3 6 6 l t r - h _ 1 - m _ 1 , r e s p e c t i v e l y . T h e s e w e l l s h a v e s i x - o r e i g h t - i n c h
d i a m e t e r s a n d o n l y t h e i r u p p e r m o s t p a r t i s c a s e d , u s u a l l y t o l e s s t h a n 3 m d e p t h .
S a m p l e s f r o m p r e c i p i t a t i o n s , w e l l s a n d r i v e r s h a v e b e e n c o l l e c t e d f o r i s o t o p e
a n d c h e m i c a l a n a l y s e s b e t w e e n 1 9 7 4 a n d 1 9 7 7 . T h e r e s u l t s a r e d i s c u s s e d a n d a n
a t t e m p t i s m a d e t o c o m m e n t o n p r e s e n t w a t e r m a n a g e m e n t s c h e m e s . T h e g e n e r a l
c o n c l u s i o n s o b t a i n e d f r o m P a j e ú R i v e r a r e a l s o v a l i d f o r m o s t o t h e r c r y s t a l l i n e
b a s i n s o n t h e s h i e l d o f n o r t h e a s t B r a z i l .
1 . E X P E R I M E N T A L M E T H O D S
1 . 1 . F i e l d m e t h o d s
P r e c i p i t a t i o n s a m p l e s w e r e c o l l e c t e d m o s t l y a t p e r m a n e n t w e a t h e r s t a t i o n s .
M o n t h l y i n t e g r a t e d s a m p l e s w e r e o b t a i n e d b y c o l l e c t i o n u n d e r p a r a f f i n o i l i n a
f u n n e l - a n d - b u c k e t a r r a n g e m e n t .
W a t e r f r o m w e l l s w a s s a m p l e d a s c l o s e a s p o s s i b l e t o t h e p u m p w h i c h w a s
e i t h e r w i n d - o r g a s - p o w e r e d . H o w e v e r , a t s e v e r a l s t a t i o n s s a m p l e s c o u l d o n l y b e
o b t a i n e d f r o m s t o r a g e r e s e r v o i r s . T h e s e r e s e r v o i r s ( c a i x a d ’a g u a ) a r e o n l y p a r t i a l l y
c o v e r e d a n d d u r i n g t h e d r y s e a s o n s i g n i f i c a n t e v a p o r a t i o n c o u l d o c c u r . N o t
e n o u g h a t t e n t i o n h a d b e e n p a i d t o t h i s d u r i n g t h e e a r l i e s t p h a s e o f t h i s p r o j e c t
b u t w a s c o r r e c t e d a s s o o n a s i t w a s r e c o g n i z e d .
R i v e r s a m p l e s w e r e t a k e n a t s p e c i f i e d l o c a t i o n s d u r i n g e a c h s a m p l i n g t r i p .
A l l s a m p l e s w e r e s h i p p e d i m m e d i a t e l y a f t e r c o l l e c t i o n t o t h e l a b o r a t o r i e s a t
C E N A .
IA E A -A G -1 5 8 /1 0 137
YEARFIG.2. Precipitation at Afogados da lagazeira.
N o s p e c i a l s a m p l e s w e r e c o l l e c t e d f o r c h e m i c a l a n a l y s e s b u t s o m e c o n s t i t u e n t s
w e r e d e t e r m i n e d o n p o r t i o n s o f t h e i s o t o p e s a m p l e s .
1 . 2 . L a b o r a t o r y m e a s u r e m e n t s
A l l s a m p l e s w e r e a n a l y s e d f o r 180 , d e u t e r i u m a n d C l ~ a n d s o m e f o r t r i t i u m
c o n c e n t r a t i o n s .
M e a s u r e m e n t o f 1 8 0 a n d D w e r e m a d e a t C E N A , u s i n g c l a s s i c a l m e t h o d s f o r
s a m p l e p r e p a r a t i o n a n d m a s s s p e c t r o m e t r y [ 3 — 5 ] . T h e r e s u l t s a r e e x p r e s s e d i n t h e
c o n v e n t i o n a l 6% o n o t a t i o n a n d r e f e r t o t h e s t a n d a r d S M O W . R e p e a t s o n s a m p l e s
a n d s t a n d a r d s i n d i c a t e t h e o v e r a l l a n a l y t i c a l e r r o r i s b e l o w ± 0.2%o f o r 180 a n d
± 2% o f o r d e u t e r i u m d e t e r m i n a t i o n s .
T r i t i u m a n a l y s e s w e r e d o n e b y g a s c o u n t i n g o n C H 4 b y t h e l a b o r a t o r i e s o f
t h e I n t e r n a t i o n a l A t o m i c E n e r g y A g e n c y a n d b y t h e I n s t i t u t o d e P e s q u i s a s R a d i o
a c t i v a s a t B e l o H o r i z o n t e . T h e r e s u l t s a r e e x p r e s s e d i n t r i t i u m u n i t s ( T U ) .
2 . R E S U L T S A N D D I S C U S S I O N
2 . 1 . P r e c i p i t a t i o n
O n e o f t h e m a i n c h a r a c t e r i s t i c s o f t h e y e a r l y p r e c i p i t a t i o n i n t h e r e g i o n i s i t s
g r e a t v a r i a b i l i t y . V a r i a t i o n s o f u p t o a f a c t o r o f 3 h a v e b e e n o b s e r v e d f r o m o n e
y e a r t o t h e o t h e r d u r i n g t h e s h o r t s t u d y p e r i o d . A l o n g e r s e r i e s o f p r e c i p i t a t i o n
d a t a f r o m t h e s t a t i o n a t A f o g a d o s d a I n g a z e i r a i n t h e B a s i n i s s h o w n i n F i g . 2 .
1 3 8 S A L A T I e t al.
8 1е0 % 0 (SMOW)
FIG.3. 180 and deuterium in precipitation in the Pajeú River Basin. The regression line (8 D = 6.4 6 180 + 5.5j considers only precipitation events exceeding 50 mm/month (A = 0). Months with precipitation below 50 mm/month are shows as small dots (B = . ).
FIG.4. Seasonal variations of 5 180 and rainfall at the Betânia sampling stations.
IA E A -A G -1 5 8 /1 0 1 3 9
A l l 1 8 0 a n d d e u t e r i u m d a t a o b t a i n e d f r o m s t a t i o n s w i t h i n t h e P a j e ú R i v e r
B a s i n a r e s h o w n i n F i g . 3 . W e i g h t e d m o n t h l y a v e r a g e s f o r p r e c i p i t a t i o n s e x c e e d i n g
5 0 m m d u r i n g t h e m o n t h l y s a m p l i n g i n t e r v a l s a r e r e p r e s e n t e d b y o p e n c i r c l e s ,
t h o s e w i t h l e s s a r e g i v e n a s s m a l l d o t s . A l s o s h o w n a r e w e i g h t e d a v e r a g e s f o r t h e
e n t i r e s t u d y p e r i o d ( 1 9 7 4 - 1 9 7 7 ) a t t h e f i v e m a i n s a m p l i n g s t a t i o n s . T h e r e g r e s s i o n
U n e w a s o b t a i n e d b y c o n s i d e r i n g o n l y t h e m o n t h l y d a t a f r o m p e r i o d s w i t h m o r e
t h a n 5 0 m m r a i n f a l l . I t t h u s r e p r e s e n t s t h e “ l o c a l m e t e o r i c w a t e r l i n e ” f o r w h i c h
Ô D = 6 . 4 ô 1 8 0 + 5 . 5
T h e s e d a t a a l s o i n d i c a t e t h a t a d e c r e a s e i n 1 8 0 a n d d e u t e r i u m w i t h a l t i t u d e
e x i s t s , w h i c h i s p r o b a b l y t h e r e s u l t o f a c o m b i n a t i o n o f a l t i t u d e a n d c o n t i n e n t a l
e f f e c t s a n d l e a d s t o a b o u t - 0 . 6 % o c h a n g e i n 1 8 0 p e r 1 0 0 m r i s e . T h i s i s o n l y a n
a p p r o x i m a t e v a l u e , h o w e v e r , b u t i t s e x i s t e n c e i s s i g n i f i c a n t b e c a u s e , a s s h o w n
b e l o w , a s i m i l a r c h a n g e i s o b s e r v e d i n t h e g r o u n d w a t e r s .
S e a s o n a l e f f e c t s a r e v e r y p r o n o u n c e d w h e r e b y r a i n s a t t h e b e g i n n i n g a n d
f o l l o w i n g t h e r a i n s e a s o n h a v e , o n t h e a v e r a g e , h i g h e r h e a v y i s o t o p e c o n t e n t s
t h a n t h e m o r e i m p o r t a n t p r e c i p i t a t i o n s o f t h e r a i n s e a s o n . T h i s i s d e m o n s t r a t e d
i n F i g . 4 w h i c h p r e s e n t s a v e r a g e m o n t h l y p r e c i p i t a t i o n a n d 5 1 8 0 d a t a f r o m B e t â n i a
s a m p l i n g s t a t i o n s f o r t h e e n t i r e s a m p l i n g p e r i o d . I t i s i n t e r e s t i n g t o n o t e t h a t
m i n i m u m a n d m a x i m u m 6 180 v a l u e s a r e d e l a y e d w i t h r e s p e c t t o m a x i m u m a n d
m i n i m u m r a i n f a l l s , i n d i c a t i n g t h a t e x i s t i n g a m o u n t e f f e c t s a l o n e d o n o t c o n t r o l
t h e d i s t r i b u t i o n o f 1 8 0 d u r i n g t h e s e a s o n s . H o w e v e r , b e c a u s e t h e a v e r a g e a n n u a l
t e m p e r a t u r e i s a l w a y s a b o v e 2 0 ° C a n d t h e a m p l i t u d e d o e s n o t e x c e e d 5 ° C w i t h
t h e h o t t e s t m o n t h b e i n g D e c e m b e r a n d t h e c o l d e s t J u l y , t h e o b s e r v e d i s o t o p e
v a r i a t i o n s a r e a l s o n o t s i m p l y a f u n c t i o n o f t e m p e r a t u r e . A n u m b e r o f f a c t o r s
c o m b i n e t o r e s u l t i n t h e o b s e r v e d t r e n d s a n d i t i s t h u s n o t s u r p r i s i n g t o o b s e r v e
v e r y s i g n i f i c a n t d i f f e r e n c e s i n b o t h r a i n f a l l a n d i s o t o p i c c o m p o s i t i o n f r o m o n e
y e a r t o a n o t h e r .
T h e b a s i n - w i d e a v e r a g e o f 5 1 8 0 v a l u e s i s c l o s e t o — 1 . 5 % o d u r i n g t h e d r y
s e a s o n ( J u n e t o O c t o b e r ) a n d — 4 . 0 %o d u r i n g t h e r a i n y s e a s o n ( J a n u a r y t o A p r i l ) .
T h e w e i g h t e d a n n u a l a v e r a g e s f o r t h e e n t i r e b a s i n a r e
S 1 8 0 = - 2 . 9 % o a n d 5 D = - 1 2 . 0 % o .
2.2. Surface waters
2 . 2 . 1 . R i v e r s
T h e i s ó t o p i c c o m p o s i t i o n o f m a n y r i v e r s y s t e m s i s c o n t r o l l e d b y t h e i n t e r
a c t i o n s b e t w e e n d i r e c t s u r f a c e r u n o f f a n d g r o u n d w a t e r d i s c h a r g e s . T h e m o r e
s i g n i f i c a n t t h e l a t t e r t h e m o r e c o n s t a n t i s n o t o n l y t h e f l o w w i t h i n t h e r i v e r b u t
1 4 0 S A L A T I et al.
er(ppm )
1977FIG.5. The 180 and chloride 'contents and the discharge of the Pajeú River during the rainy season of 1977.
FIG. 6. The comparison of 180 and deuterium data shows that evaporation plays a significant role in the water budget of the rivers in the Pajeú River Basin. Data are for Pajeú River.
IA E A -A G -1 5 8 /1 0 141
a l s o t h e i s o t o p i c c o m p o s i t i o n o f i t s w a t e r s . I t i s i m p o r t a n t t o r e m e m b e r t h a t i n
t h i s d r a i n a g e b a s i n m o s t r i v e r s g o t o s u r f i c i a l d r y n e s s o r c o m e v e r y c l o s e . B e c a u s e
o f t h e a b s e n c e o f a s i g n i f i c a n t b a s e - f l o w c o m p o n e n t i t i s n o t s u r p r i s i n g t o n o t e
v e r y s i g n i f i c a n t s e a s o n a l v a r i a t i o n s o f t h e i r i s o t o p i c c o m p o s i t i o n s , w h i c h a p p r o x i
m a t e t h e s e a s o n a l p r e c i p i t a t i o n v a l u e s e v e n i n t h e s e e x t r e m e s . T h i s i s s h o w n
c l e a r l y f o r a n e x t e n s i v e s a m p l i n g p e r i o d d u r i n g t h e 1 9 7 7 r a i n s e a s o n a n d t h e
m o n t h s f o l l o w i n g i t ( F i g . 5 ) . T h e l o w e s t v a l u e s o c c u r a t t h e b e g i n n i n g o f M a y
w h e n t h e 5 1 8 0 = ~ l % o a p p r o a c h t h e l o w e s t p r e c i p i t a t i o n v a l u e s r e c o r d e d f o r
t h e p r e c e d i n g r a i n y s e a s o n . T h e r e a f t e r , b o t h i n c r e a s i n g e v a p o r a t i o n a n d i n c r e a s i n g
h e a v y i s o t o p e c o n t e n t s i n t h e p r e c i p i t a t i o n s c a u s e a g r a d u a l i n c r e a s e i n ô 180 a n d
ô D v a l u e s a n d ô 180 v a l u e s a s h i g h a s + 1 1 % o h a v e b e e n r e c o r d e d d u r i n g t h e d r y
s e a s o n .
E v a p o r a t i o n p l a y s a s i g n i f i c a n t r o l e i n t h i s e n r i c h m e n t a s e v i d e n c e d b y t h e
1 8 0 a n d D d a t a f r o m a l l r i v e r s a m p l e s p l o t t e d i n F i g . 6 , w h i c h s h o w t h a t m o s t o f
t h e m a r e t o t h e r i g h t o f t h e l o c a l m e t e o r i c w a t e r l i n e . A s e p a r a t i o n b e t w e e n r a i n y
s e a s o n a n d d r y s e a s o n r a i n s ( F i g . 6 ) e n h a n c e s t h i s p i c t u r e . H o w e v e r , n o t a l l c r e e k s
a n d s t r e a m s d r a i n i n g t o w a r d s t h e R i o P a j e û b e h a v e i n t h e s a m e m a n n e r , a n d
s i g n i f i c a n t d i f f e r e n c e s a r e o b s e r v e d . F o r e x a m p l e , R i a c h o T r i u n f o m a i n t a i n e d
a n a v e r a g e S 1 8 0 v a l u e c l o s e t o — 3 . 2 % o w i t h o n l y a b o u t 2 % c s e a s o n a l v a r i a t i o n s ,
w h i c h i s a s t r o n g i n d i c a t i o n t h a t i n i t s f l o w m o r e t h a n o n e o f t h e o t h e r s t r e a m s
h a s a s i g n i f i c a n t m o r e o r l e s s p e r e n n i a l b a s e - f l o w c o m p o n e n t . R i o P a j e ú , b e i n g
t h e m a i n d r a i n a g e c h a n n e l i n t h e b a s i n , i s “ a v e r a g e ” i n i t s b e h a v i o u r b u t t h e o t h e r
e x t r e m e i s t h e s m a l l C a i c a r i n h a C r e e k w h i c h h a s a n a v e r a g e 5 1 8 0 v a l u e o f a b o u t
— 0 . 8 % o w i t h s e a s o n a l v a r i a t i o n s b e t w e e n — 3 . 3 a n d + 2 . 8 % o , o r R i a c h o d o M e i o
w h o s e a v e r a g e 5 1 8 0 v a l u e i s + 0 . 3 % o w i t h v a r i a t i o n s b e t w e e n — 2 . 9 a n d + 8 . 2 % o .
T h i s v e r y v a r i a b l e b e h a v i o u r o f s t r e a m s w i t h i n t h e s a m e d r a i n a g e b a s i n i s
a s t r o n g i n d i c a t i o n t h a t n o m a j o r r e g i o n a l g r o u n d w a t e r f l o w s e x i s t i n t h i s r e g i o n .
T h e h y d r o g e o l o g y i s d o m i n a t e d b y s m a l l l o c a l g r o u n d w a t e r s y s t e m s w h i c h i n
g e n e r a l r e s p o n d q u i c k l y t o m a j o r p r e c i p i t a t i o n e v e n t s b e c a u s e o f t h e i r l i m i t e d
s t o r a g e c a p a c i t y .
T h i s a l s o h a s a p r o f o u n d i n f l u e n c e o n t h e s a l t b a l a n c e o f t h i s b a s i n . F i g u r e 5
s h o w s t h e d e p e n d e n c e o f s a l t c o n c e n t r a t i o n s i n t h e r i v e r w a t e r s o n p r e c i p i t a t i o n
e v e n t s a n d a c l e a r p a r a l l e l i s m b e t w e e n s a l i n i t y a n d i s o t o p i c c o m p o s i t i o n . T h e
l o w e s t s a l i n i t y w i t h o n l y 4 4 p p m C l - i s r e a c h e d s i m u l t a n e o u s l y w i t h t h e l o w e s t
S 1 8 0 v a l u e s o f — 6 . 7 % o . I m p o r t a n t t o n o t e i s t h a t a t m a x i m u m r i v e r d i s c h a r g e
t h e s a l i n i t y w a s h i g h e r (88 p p m ) , w h i c h i n d i c a t e s t h a t s u r f a c e a n d , p o s s i b l y m o r e
i m p o r t a n t , s u b s u r f a c e d i s c h a r g e s a i d e d i n t h e r e m o v a l o f s a l t f r o m t h e b a s i n .
2 . 2 . 2 . D a m s
A t t e m p t s h a v e b e e n m a d e t o r e t a i n s o m e o f t h e r a i n y s e a s o n r u n o f f i n d a m s ;
h o w e v e r , w i t h v a r i a b l e s u c c e s s . A r e c e n t s t u d y b y S t o l f e t a l . [ 6 ] o f a d a m w h i c h
142 S A L A T I et al.
er(ppm )
ci-MASS ( to n ) .
2 0 0
180
■ 160
■ 140
1 2 0
FIG. 7. The isotopic and chemical behaviour of the QUEBRA-Unhas dam. Note that the salt load in the dam decreases during the dry season, which indicates that a subsurface discharge does exist. A similar conclusion is found from isotope data (Stolf et al. [6]/
c o n t a i n s , b e l o w a s u r f a c e a r e a o f 1 . 0 2 k m 2 , a p p r o x i m a t e l y 3 . 2 X 1 0 6m 3 w a t e r
d e m o n s t r a t e s t h a t i t s s u r v i v a l a s a n a c c e p t a b l e w a t e r s u p p l y i s o n l y g u a r a n t e e d
i f a r e m o v a l o f t h e s a l t a c c u m u l a t i n g d u r i n g t h e d r y s e a s o n i s p o s s i b l e . S u c h i s
t h e c a s e i n t h e i n v e s t i g a t e d Q u e b r a - U n h a s d a m ( F i g . 1 ) a n d a r e s p o n s e p a t t e r n i s
s h o w n i n F i g . 7 . A c o m p a r i s o n o f 1 8 0 a n d d e u t e r i u m c o n t e n t s s h o w s t h a t i n c r e a s e s
i n s a l i n i t y a n d h e a v y i s o t o p e c o n t e n t a r e a l m o s t e x c l u s i v e l y d u e t o e v a p o r a t i o n ,
b u t t h e s e d a t a a l s o s h o w t h a t a p p r o x i m a t e l y 3 0 % o f t h e w a t e r i s l o s t t h r o u g h
s u b s u r f a c e o u t f l o w , t h u s r e m o v i n g s u f f i c i e n t s a l t t o m a i n t a i n a n e s s e n t i a l l y
s t a t i o n a r y s i t u a t i o n — t h e d a m w a s c o n s t r u c t e d i n 1 9 3 4 y e t i t s s a l i n i t y r a r e l y
e x c e e d s a f e w h u n d r e d p p m i n t o t a l d i s s o l v e d s o l i d s ( o r 1 5 0 p p m C l - ) .
M a s s b a l a n c e c a l c u l a t i o n s b a s e d o n i s o t o p e s a n d c h e m i s t r y s t r o n g l y i n d i c a t e
t h a t v i r t u a l l y a l l c h l o r i n e a r r i v i n g i n t h e d a m h a s a n a t m o s p h e r i c o r i g i n a n d k n o w i n g
t h e s i z e o f t h e d r a i n a g e b a s i n , i t c a n b e e s t i m a t e d t h a t t h e o v e r a l l a t m o s p h e r i c C l -
c o n t r i b u t i o n a m o u n t s t o a b o u t 3 . 2 X 1 0 "4 k g -2 a -1 w i t h t h e r a i n h a v i n g a b o u t
1 p p m C l - . T h i s c o m p a r e s v e r y w e l l w i t h t h e m e a s u r e m e n t s m a d e o n r a i n - w a t e r
b e t w e e n 1 9 7 4 a n d 1 9 7 7 w h e r e a r a n g e b e t w e e n 0 . 7 a n d 5 p p m C l - w a s f o u n d .
8180
%o
SM
OW
IA E A -A G -1 5 8 /1 0
1974 1975
(-2 0 0 0
• 1 0 0 0
L 0
1 9 7 4 1 9 7 5
FIG.8. Typical isotopic and chemical response pattern o f wells in the study area.
8 180 % o S M O W
FIG.9. l80 and deuterium in groundwaters.
СГ
рр
т
T A B LE I. A V ER A G E V A LU ES O F 5 180 , ÔD, ( С Г ) AND T O F W ATER SAM PLES C O LLEC T ED THROUGHOUT 19 7 4 TO 1977 FROM TH E W ELLS
Location Standard Number o f Standard N umber o f(Cl ) (ppm )
Standard N um ber o f Tritium rangeo f wells 5 i 80 % o SMOW deviation analysed 6 D%« SMOW deviation analysed deviation analysed 1 9 7 4 - 1 9 7 7
(F ig -1 ) (%») samples (%») samples (ppm ) samples (T U )
Itapetim - 2 . 6 0 .5 2 2 - 1 6 3.1 19 812 2 1 4 15
Sao Jo sé do Egito - 3 . 4 0.3 12 - 1 8 1.7 7 5 86 63 5
Riacho do Meio - 3 .1 0.5 18 - 1 6 2.3 16 7 9 0 140 10
Tuparetam a - 2 . 7 0 .6 19 - 1 4 2 .6 15 1748 3 0 0 13
Campos Novos 1 - 3 . 7 0 .5 16 - 1 8 2 .4 12 194 77 10
Campos Novos 2 - 3 . 0 0 .4 15 - 1 7 2.1 11 1867 196 10
Afogados da Ingazeira 1 - 3 . 0 0 .2 9 - 1 7 2.1 5 6 5 4 361 8
Afogados da Ingazeira 2 - 3 . 7 0 .4 9 - 1 9 1.9 5 190 4 2 3
Quixaba - 2 . 8 0 .9 14 - 1 3 4 .7 11 176 41 10
Triunfo - 3 . 2 0 .8 19 - 1 4 4 .8 13 4 0 14 13
Calumbi - 3 . 0 0 .5 11 - 1 4 2.7 8 4 3 4 2 44 11
Caiçarinha - 2 . 5 1.1 9 - 1 7 3.5 8 983 2 4 4 12
Bom Nome 1 - 3 . 4 0 .4 4 - 2 0 1.5 3 2 2 4 27 6
Bom Nome 2 - 4 . 6 0.3 9 - 2 7 2 .4 4 33 8 6
Tupanaci - 3 . 4 0 .2 9 - 1 9 6 .4 3 2 8 2 11 11
Carqueja - 1 . 6 0 .3 1 2 - 1 0 3.1 6 1838 40 2 10 4 .4 to 12 .0
Camaubeira - 3 .5 0 .9 5 5 2 4 119 7
144 S
AL
AT
Ietal.
T A B L E I. cont.
Location o f wells
(P ig-1)
5 ISWoo SMOWStandarddeviation
(%»)
Number o fanalysedsamples
5D%a SMOWStandarddeviation
(%«)
N um ber o fanalysedsamples
( С П(ppm )
Standarddeviation(ppm )
N umber o fanalysedsamples
Tritium range1 9 7 4 -1 9 7 7(TU )
Betânia + 0 .3 0 .4 23 - 1 3.7 1 0 ' 1039 174 18 2.1 to 9 .4
Poço do Pau - 3 .1 0 .4 21 - 1 8 3 .0 16 805 6 4 2 18 2 .2 to 14 .0
Lage das Pombas - 1 .5 0 .8 24 - 1 1 5.5 15 1064 4 9 3 18 5.7 to 11.3
Airi - 2 . 4 0 .6 2 0 - 1 4 4 .4 11 707 154 16 2.1 to 8 .7
V arjota - 2 . 4 0 .6 2 0 - 1 3 3 .0 13 158 65 15 2.5 to 14 .0
L/Ï
IAE
A-A
G-1S
8/10
1 4 6 S A L A T I et al.
T h e a v e r a g e 5 1 8 0 c o n t e n t o f t h e g r o u n d w a t e r c o l l e c t e d a t t h e v a r i o u s s i t e s
i s s h o w n i n F i g . 1 a n d s h o w s t h a t a s m a l l b u t s i g n i f i c a n t s p a t i a l v a r i a t i o n i s
r e c o g n i z a b l e : w e l l s l o c a t e d a t h i g h e r a l t i t u d e s a t t h e e d g e s o f t h e d r a i n a g e b a s i n
t e n d t o h a v e l o w e r 180 a n d d e u t e r i u m c o n t e n t s t h a n t h o s e i n t h e l o w e r p a r t .
I t i s n o t e w o r t h y t h a t a l l w e l l s e x h i b i t s e a s o n a l v a r i a t i o n s i n c h e m i s t r y a n d
i s o t o p i c c o m p o s i t i o n s . T y p i c a l v a r i a t i o n s f o r t h e l e a s t a n d t h e m o s t s a l i n e w e l l s
s t u d i e d i n t h i s b a s i n a r e s h o w n i n F i g . 8 . T h i s b e h a v i o u r e m p h a s i z e s t h a t t h e
d i f f e r e n t g r o u n d w a t e r s y s t e m s e x p l o i t e d h e r e a r e n o t i n t e r c o n n e c t e d b u t s u p p l i e d
l o c a l l y . F u r t h e r e v i d e n c e c o m e s f r o m a c o m p a r i s o n o f 1 8 0 a n d d e u t e r i u m d a t a
( F i g . 9 ) , w h i c h s h o w s t h a t m a n y w e l l s c o n t a i n e v a p o r a t e d w a t e r . T h i s i s e s p e c i a l l y
t r u e f o r t h o s e w e l l s w h i c h a r e n e a r r i v e r b a n k s a n d a r e p r o b a b l y t a p p i n g r i v e r -
c o n n e c t e d a q u i f e r s .
T r i t i u m a n a l y s e s h a v e b e e n m a d e o n a n u m b e r o f w e l l s i n t h e d r a i n a g e b a s i n
a n d a g a i n o n e n o t e s v e r y s i g n i f i c a n t v a r i a t i o n s ( T a b l e I ) . N o a t t e m p t s h a v e b e e n
m a d e t o i n t e r p r e t t h e s e c o n c e n t r a t i o n s i n t e r m s o f r e s i d e n c e t i m e s o r w a t e r a g e s
a l t h o u g h i t i s e v i d e n t t h a t m o s t i f n o t a l l w a t e r i n t h e s e s y s t e m s i s v e r y y o u n g .
T h i s a g r e e s w i t h t h e f i n d i n g o f G e y h a n d K r e y s i n g [ 7 ] , w h o w e r e a b l e t o d e t e r m i n e
f o r g r o u n d w a t e r i n a s i m i l a r b a s i n a p p r o x i m a t e r e s i d e n c e t i m e s b y c o m p a r i n g 1 4 C
a n d t r i t i u m d a t a , a n d f o u n d i n v i r t u a l l y a l l s y s t e m s r e s i d e n c e t i m e s t o b e m u c h l e s s
t h a n 1 0 0 y e a r s . T h e r e i s t h u s n o e v i d e n c e t o s u g g e s t t h a t f o s s i l s e a - w a t e r i s s t i l l
p r e s e n t a n d r e s p o n s i b l e f o r . t h e s a l i n a t i o n o f t h e s e g r o u n d w a t e r s .
W a t e r - l e v e l c h a n g e s i n t h e s e a q u i f e r s y s t e m s w e r e m o n i t o r e d b y S U D E N E a t
a f e w w e l l s i n s i d e t h i s d r a i n a g e b a s i n ( F i g . 1 0 ) . H o w e v e r , l i t h o l o g i c a l a n d p h y s i o
g r a p h i c s i m i l a r i t i e s s h o u l d p r e s e n t a d i r e c t c o m p a r i s o n f r o m o n e l o c a l i t y t o a n o t h e r .
T h u s , a w e l l m o n i t o r e d a t B e t â n i a s h o w s a v e r y r a p i d a n d p r o n o u n c e d r e s p o n s e t o
p r e c i p i t a t i o n e v e n t s , w h i c h i s t a k e n a s a n i n d i c a t i o n t h a t r e c h a r g e h a s o c c u r r e d .
T h i s a s s u m p t i o n i s f u r t h e r s u p p o r t e d b y s a l i n i t y v a r i a t i o n s , a l t h o u g h t h e c h e m i c a l
r e s p o n s e o f t h e s e s y s t e m s i s n o t e x a c t l y t h e s a m e a s t h e o n e o b s e r v e d f o r w a t e r -
l e v e l c h a n g e s . W i t h t h e b e g i n n i n g o f t h e r a i n y s e a s o n , w h e n w a t e r l e v e l s b e g i n t o
r i s e , t h e s a l i n i t y o f t h e g r o u n d w a t e r s s a m p l e d i n t h e w e l l s d o e s n o t d e c r e a s e b u t
s h o w s - s o m e t i m e s w i t h s o m e d e l a y — a m a r k e d i n c r e a s e . T h i s i s s h o w n i n F i g . 1 0
w h i c h c o m p a r e s t h e c h a n g e s i n s a l i n i t y f r o m a l l w e l l s ( e x p r e s s e d a s p p m C l - )
w i t h p r e c i p i t a t i o n e v e n t s a t B e t â n i a . O n l y f o l l o w i n g t h e v e r y m a j o r r a i n f a l l s
f r o m J a n u a r y t o A p r i l d o e s o n e o b s e r v e a s i g n i f i c a n t d i l u t i o n .
O n e m u s t r e m e m b e r t h a t t h e s e w e l l s h a v e v i r t u a l l y n o c a s i n g a n d t h e w a t e r
s a m p l e d w i l l n o t n e c e s s a r i l y r e f l e c t t h e a v e r a g e c h e m i c a l o r i s o t o p i c c h a r a c t e r i s t i c s
o f t h e a q u i f e r s . H o w e v e r , t h e s i m i l a r i t y o f r e s p o n s e o f m o s t w e l l s p e r m i t s a t l e a s t
a n a t t e m p t a t i n t e r p r e t a t i o n . T h e s i g n i f i c a n t s e a s o n a l v a r i a t i o n s i n i s o t o p i c
c o m p o s i t i o n a n d s a l i n i t y ( r e p r e s e n t e d b y C F a n a l y s e s ) i n d i c a t e t h a t t h e s e a q u i f e r s
a r e l a r g e l y u n c o n f i n e d a n d t h a t l o c a l r a t h e r t h a n r e g i o n a l h y d r o l o g i c a l p h e n o m e n a
2 .3 . G r o u n d w a t e r s
I A E A - A G - 1 58/10 14 7
FIG.10 A. Salinity changes in the wells o f the project area. All wells fo r which data were available are shown. The numbers refer to the following wells: 1 = Itapetim; 2 = Riacho do Meio; 3 = Tuparetama; 4 = Campos Novos 1; 5 = Triunfo; 6 = Calumbi; 7 = Tupanaci; 8 = Carqueja;9 = Faz. Poço do Pau; 10 = Lage das Pombas; 11 = Airi; 12= Varjota.
FIG .10 B. The regression line indicates that evaporation has affected the groundwaters inthe study area. Water-level changes in a shallow groundwater system in response to precipitationevents.
c o n t r o l t h e i r r e s p o n s e . T h e h i g h t r i t i u m c o n t e n t s s h o w t h a t , a t l e a s t t o t h e d e p t h
s a m p l e d , t h e r e s i d e n c e t i m e s a r e r a t h e r s h o r t . T h i s m a y h a v e b e e n e n h a n c e d b y
t h e e x p l o r a t i o n o f t h e s e a q u i f e r s a n d — a s p o i n t e d o u t a b o v e — i t w a s e v e n a r g u e d
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s e a s o n a n d t h u s i m p r o v e t h e w a t e r q u a l i t y .
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t h e w a t e r q u a l i t y d i d n o t i m p r o v e . W e s u g g e s t t h a t t h i s i s t h e r e s u l t o f t h e w a t e r
u s a g e p a t t e r n s — t h e d r i l l e d w e l l s a r e s i g n i f i c a n t l y p u m p e d d u r i n g t h e d r y s e a s o n
a n d m a y t a p s a l t w a t e r w h i c h w a s n o t r e a c h e d b e f o r e b y e a r l i e r h a n d - d u g w e l l s .
1 4 8 S A L A T I et al.
T h u s , a n i n c r e a s e d a m o u n t o f w a t e r a n d s a l t a r e b r o u g h t t o t h e s u r f a c e . H o w e v e r ,
b e c a u s e n o w a t e r l e a v e s t h e l o c a l a r e a d u r i n g t h e d r y s e a s o n , a l l s a l t s w i l l s t a y
b e h i n d . T h e y a r e h i g h l y s o l u b l e a n d w i l l b e m o b i l i z e d w i t h t h e o n s e t o f t h e r a i n y
s e a s o n . A t t h a t t i m e i n f i l t r a t i o n o c c u r s a n d t h e s a l t s a r e s i m p l y f l u s h e d b a c k i n t o
t h e s u b s u r f a c e , c a u s i n g a t e m p o r a r y i n c r e a s e i n s a l i n i t y .
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a l s o b e c o m e a c t i v e . T h i s s h o u l d b e t h e t i m e w h e n t h e s a l t s a r e f l u s h e d f r o m t h e
b a s i n b u t , b e c a u s e l i t t l e o r n o b a s e f l o w i s a d d e d t o t h e s t r e a m s , t h e s a l t w a t e r
i n t h e d e e p e r a q u i f e r s i s n o w e s s e n t i a l l y i m m o b i l e . W e l l s a r e n o w l i t t l e u s e d o r
w i l l d r a w f r e s h w a t e r m i x e d w i t h s o m e s a l t w a t e r . T h e l o w s a l i n i t y o f t h e P a j e ú
R i v e r d u r i n g m a x i m u m d i s c h a r g e (88 p p m C F v s a n a v e r a g e o f c l o s e t o 7 0 0 p p m ’
C l - f o r t h e w e l l w a t e r s ) ( T a b l e I ) a l s o a t t e s t s t o t h e a b s e n c e o f a s i g n i f i c a n t b a s e -
f l o w c o m p o n e n t .
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T h e d a t a a c c u m u l a t e d i n t h e c o u r s e o f t h e s e i n v e s t i g a t i o n s [ 6 , 8 ] a n d i n f o r
m a t i o n g a t h e r e d a n d s u m m a r i z e d e l s e w h e r e [ 1 , 9 — 1 2 ] l e a d t o t h e c o n c l u s i o n t h a t
c h l o r i d e a n d m o s t o t h e r d i s s o l v e d c o n s t i t u e n t s o f t h e s e g r o u n d w a t e r s d o n o t
o r i g i n a t e f r o m t h e w e a t h e r i n g o f r o c k s a n d d i s s o l u t i o n o f m i n e r a l s . I n s t e a d t h e y
c a n b e e x p l a i n e d l a r g e l y t h r o u g h a t m o s p h e r i c c o n t r i b u t i o n s a n d m o d i f i c a t i o n s
o f t h i s a t m o s p h e r i c l o a d t h r o u g h s u b s u r f a c e r o c k - w a t e r i n t e r a c t i o n s .
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f o s s i l s e a - w a t e r i s p r e s e n t a n d t h a t t h e r e s i d e n c e t i m e s o f w a t e r i n t h e s e s h a l l o w
s y s t e m s a r e r e l a t i v e l y s h o r t . T h e a q u i f e r s a r e l a r g e l y u n c o n f i n e d a n d r e s p o n d
r a p i d l y t o a t m o s p h e r i c i n p u t s .
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s e a s a l t , e s p e c i a l l y i f o n e e v o k e s l o c a l r e c i r c u l a t i o n o f a c c u m u l a t e d s a l t s .
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é v a p o t r a n s p i r a t i o n t h e n s u c h m e c h a n i s m s c o u l d a l s o a c c o u n t f o r t h e h i g h s a l i n i t i e s
s i m p l y t h r o u g h a p r o g r e s s i v e e n r i c h m e n t t h r o u g h w a t e r l o s s . N o s i g n i f i c a n t e x p o r t
o f p l a n t m a t e r i a l o c c u r s f r o m t h e s e l o c a l b a s i n s a n d , t h e r e f o r e , e v e n s a l t a c c u m u
l a t e d w i t h i n p l a n t m a t t e r , a n d t h u s t e m p o r a r i l y w i t h d r a w n f r o m t h e a q u e o u s
r e s e r v o i r , w i l l a l s o e v e n t u a l l y r e t u r n t o t h e s u b s u r f a c e .
T h i s h a s s e r i o u s c o n s e q u e n c e s b e c a u s e , i f o n e a c c e p t s a c o n t i n u o u s i n p u t
t h r o u g h p r e c i p i t a t i o n s a s b é i n g t h e d o m i n a n t r e a s o n f o r t h e p r e s e n c e o f s a l t i n
t h e s e b a s i n s , t h e n a n y f l u s h i n g t h r o u g h p u m p i n g a n d w a t e r - q u a l i t y i m p r o v e m e n t
t h r o u g h e n h a n c e d r e c h a r g e b e c o m e s m u c h m o r e d i f f i c u l t t h a n i f o n e h a d t o d e a l
w i t h a f i n i t e s a l t r e s e r v o i r . F o r t h i s r e a s o n a c o m p a r i s o n o f a t m o s p h e r i c i n p u t
a n d s a l t d i s c h a r g e t h r o u g h t h e P a j e ú R i v e r b e c o m e s v e r y i n t e r e s t i n g . S u c h a s a l t
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b u d g e t i s u n f o r t u n a t e l y n o t a n e a s y t a s k — t h e g r e a t v a r i a b i l i t y o f t h e a m o u n t o f
a n n u a l p r e c i p i t a t i o n s c a n c a u s e i n o n e y e a r t h e l o w a r e a s o f t h e v a l l e y t o b e
f l o o d e d a n d w i l l i n a n o t h e r y e a r h a r d l y s a t u r a t e t h e s o i l . T h i s n e c e s s i t a t e s l o n g
m e a s u r i n g p e r i o d s o f b o t h d i s c h a r g e a n d s a l t l o a d b o t h o f w h i c h a r e a v a i l a b l e
o n l y f o r 1 9 7 6 a n d 1 9 7 7 .
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f r o m t h e b a s i n a n d t h a t t h e s u r f a c e r u n o f f t h r o u g h t h e P a j e ú R i v e r a c c o u n t s f o r
t h e t o t a l s a l t r e m o v a l f r o m t h e b a s i n . I n t h e l i g h t o f t h e d a t a p r e s e n t e d a b o v e t h i s
a p p e a r s t o b e a r e a s o n a b l e a s s u m p t i o n . U n d e r t h e s e c o n d i t i o n s t h e s a l t b u d g e t
o f t h e b a s i n i s d e s c r i b e d b y
P C p = R C r
w e r e P a n d R a r e p r e c i p i t a t i o n a n d r i v e r o u t f l o w a n d C p a n d C r t h e c o r r e s p o n d i n g
s a l t c o n c e n t r a t i o n s . C p v a r i e s b e t w e e n 0 . 7 a n d 3 m g / l t r a n d a v e r a g e s a b o u t
1 . 9 m g / l t r , w h e r e a s d u r i n g t h e 1 9 7 7 r u n o f f s e a s o n C r v a r i e d b e t w e e n 3 5 a n d
1 4 5 m g / l t r a v e r a g i n g 8 4 m g / l t r . T h u s , t h e r a t i o o f s a l t c o n t e n t s f o r 1 9 7 7 i s
C p / C r = 0 . 0 2
U n d e r t h e a b o v e c o n d i t i o n s t h i s w o u l d h a v e t o b e e q u a l t o R / P w h i c h i n d e e d f o r
1 9 7 6 w a s ~ 0 . 0 2 a n d f o r 1 9 7 7 w a s ~ 0 . 0 5 . T h e s i m i l a r i t y o f t h e s a l t c o n t e n t a n d
w a t e r - m a s s r a t i o s i s a n i n d i c a t i o n t h a t t h i s b a s i n i s e s s e n t i a l l y a t a s t e a d y s t a t e w i t h
r e s p e c t t o i t s s a l t b u d g e t .
T h e t i m e r e q u i r e d t o r e a c h t h i s s t a t e w i l l d e p e n d o n t h e g r o u n d w a t e r r e s e r v o i r s
a n d t h e h y d r o d y n a m i c s o f t h e b a s i n . I t i s d i f f i c u l t t o e s t i m a t e t h i s a l t h o u g h i t i s
f a i r l y s a f e t o a s s u m e t h a t i t w o u l d n o t t a k e l o n g , e s p e c i a l l y s i n c e v i r t u a l l y a l l
g r o u n d w a t e r i s f o u n d i n v e r y s h a l l o w s y s t e m s .
C O N C L U S I O N S
S a l i n e g r o u n d w a t e r i n c r y s t a l l i n e r o c k s i s n o t u n u s u a l a n d c a n b e g e n e r a t e d
b y a n u m b e r o f p r o c e s s e s . I t s p r e s e n c e i s , h o w e v e r , o f m a j o r c o n c e r n i f i t i s f o u n d
i n s h a l l o w a q u i f e r s i n a r i d o r s e m i - a r i d e n v i r o n m e n t s . I t i s o f t e n t h e o n l y s o u r c e
o f w a t e r f o r t h e p o p u l a t i o n l i v i n g i n t h e s e a r e a s a n d a t t e m p t s h a v e t o b e m a d e
t o u n d e r s t a n d t h e r e a s o n s f o r t h e s a l i n a t i o n o f t h e s e g r o u n d w a t e r s a n d t o p r o p o s e
r e m e d i e s b a s e d o n h y d r o g e o l o g i c a l , g e o c h e m i c a l , i s o t o p i c a n d g e o p h y s i c a l c o n
s i d e r a t i o n s .
I n t h i s s t u d y t h e P a j e ú R i v e r B a s i n i n N E B r a z i l w a s i n v e s t i g a t e d w i t h i s o t o p i c
a n d g e o c h e m i c a l m e t h o d s . C o m b i n e d w i t h i n f o r m a t i o n o b t a i n e d d u r i n g p r e v i e w
i n v e s t i g a t i o n s i t c a n b e s h o w n t h a t v i r t u a l l y a l l g r o ú n d w a t e r s a r e f o u n d i n s h a l l o w
1 5 0 S A L A T I et al.
a q u i f e r s w i t h i n w e a t h e r e d o r f r a c t u r e d c r y s t a l l i n e b e d r o c k o r t h e s h a l l o w a l l u v i a l
f i l l s d e r i v e d f r o m i t . T h e i r w a t e r s u p p l i e s d e p e n d o n l o c a l r e c h a r g e e v e n t s w h i c h
m a y o c c u r i n t h e f o r m o f p r e c i p i t a t i o n o r r i v e r r e c h a r g e . T h e r e i s n o e v i d e n c e
t h a t r e g i o n a l f l o w s y s t e m s a r e o f i m p o r t a n c e a n d e v e n i f t h e y e x i s t t h e y c a n n o t
y i e l d s i g n i f i c a n t a m o u n t s o f w a t e r b e c a u s e i t w o u l d h a v e t o b e t r a n s m i t t e d
p r i m a r i l y t h r o u g h f r a c t u r e s y s t e m s i n t h e r o c k m a s s .
T h e o r i g i n o f t h e s a l t i n t h e s e g r o u n d w a t e r s i s m o s t p r o b a b l y a t m o s p h e r i c .
T h e r e f o r e , t h e s a l t r e s e r v o i r i s v i r t u a l l y u n l i m i t e d a n d s c h e m e s d e s i g n e d t o
i m p r o v e t h e g r o u n d w a t e r q u a l i t y w i l l h a v e t o t a k e t h i s i n t o a c c o u n t . M o s t p r e v i o u s
i n v e s t i g a t o r s h a v e a s s u m e d t h a t t h i s r e s e r v o i r w a s l i m i t e d a n d t h a t p u m p i n g w o u l d
r e m o v e t h e s a l t f r o m t h e g r o u n d w a t e r a n d l e a d t o t h e r e p l a c e m e n t s o f t h e s a l t y
g r o u n d w a t e r b y f r e s h w a t e r . W e d o n o t e n t i r e l y a g r e e w i t h t h i s c o n c e p t .
T h e s e a s o n a l c h a n g e s o f s a l i n i t y i n t h e p u m p e d w e l l w a t e r i n d i c a t e t h a t ,
w i t h t h e o n s e t o f t h e r a i n y s e a s o n , s a l t s t h a t h a v e a c c u m u l a t e d a t t h e s u r f a c e
o w i n g t o a t m o s p h e r i c f a l l o u t , i r r i g a t i o n w i t h m o r e o r l e s s s a l t y g r o u n d w a t e r a n d
d o m e s t i c p r a c t i c e s w i l l b e f l u s h e d i n t o t h e s u b s u r f a c e r a t h e r t h a n f l u s h e d f r o m
t h e b a s i n . D u r i n g t h e l a t t e r p a r t o f t h e r a i n y s e a s o n s u r f a c e r u n o f f d o e s o c c u r
a n d m i n o r a m o u n t s o f s a l t w i l l b e r e m o v e d b u t m o s t s e e m s t o r e m a i n p r a c t i c a l l y
i m m o b i l e i n t h e s u b s u r f a c e . T o a c h i e v e a s a l t b a l a n c e w i t h a n e t l o s s f r o m t h e
b a s i n , o n e w o u l d h a v e t o p u m p s a l t w a t e r d u r i n g t h e r a i n y s e a s o n i n t o t h e s u r f a c e
d r a i n a g e s y s t e m s a n d t h u s e n s u r e i t s d i s c h a r g e .
T h i s c o n c e p t h a s n o t b e e n v e r i f i e d , a n d d e t a i l e d h y d r o g e o l o g i c a l a n d g e o
c h e m i c a l s t u d i e s w o u l d b e r e q u i r e d i n o r d e r t o d o s o . T h e r e m o t e n e s s o f t h e a r e a
h a s s o f a r p r e v e n t e d t h e e x e c u t i o n o f s u c h a p r o g r a m m e . F u r t h e r m o r e , i t m a y b e
i m p r a c t i c a l t o e x e c u t e s u c h p u m p i n g s c h e m e s o n a l a r g e s c a l e a n d , i f s o , t h e u l t i m a t e
c o n c l u s i o n w o u l d i n e v i t a b l y b e t h a t a n y . f u r t h e r m o b i l i z a t i o n o f d e e p e r g r o u n d
w a t e r s t h r o u g h a d d i t i o n a l w e l l i n s t a l l a t i o n s w i l l p r o b a b l y n o t i m p r o v e t h e s i t u a t i o n
b u t l e a d t o a n e v e n g r e a t e r s a l t l o a d i n g i n t h e b a s i n , o r a t l e a s t a m o r e e v e n s a l t
d i s t r i b u t i o n w h i c h c o u l d m e a n a s l i g h t a m e l i o r a t i o n o f t h e w a t e r q u a l i t y o f t h e
d e e p e r g r o u n d w a t e r a n d a g r a d u a l d e t e r i o r a t i o n o f t h e m o r e s h a l l o w r e s o u r c e s .
A C K N O W L E D G E M E N T S
P a r t s o f t h i s p r o j e c t h a v e b e e n c a r r i e d o u t w i t h t h e f i n a n c i a l a s s i s t a n c e o f
t h e I n t e r n a t i o n a l A t o m i c E n e r g y A g e n c y . W e w i s h t o e x p r e s s o u r s i n c e r e t h a n k s
f o r t h i s h e l p a n d g r a t e f u l l y a c k n o w l e d g e t h e c o n s t r u c t i v e i n p u t f r o m
D r . B . R . P a y n e a n d t h e s t a f f o f t h e S e c t i o n f o r I s o t o p e H y d r o l o g y . T h a n k s a r e a l s o
d u e t o D r . J . G a t w h o v i s i t e d B r a z i l a t t h e s a m e t i m e P . F . w a s t h e r e u n d e r a n
I A E A t e c h n i c a l a s s i s t a n c e p r o g r a m m e a n d w i t h w h o m m a n y a s p e c t s u f t h i s
p r o j e c t w e r e d i s c u s s e d . D r a f t i n g w a s d o n e b y M r s . M . M a z i a r z , D e p a r t m e n t o f
E a r t h S c i e n c e , U n i v e r s i t y o f W a t e r l o o .
IA E A -A G -1 5 8 /1 0 151
R E F E R E N C E S
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[3] EPSTEIN, S., M AYEDA, T ., Variations of 180 content of waters from natural sources, Geochim. Cosmochim. Acta 4 (1 9 5 3 ) 2 1 3 —24.
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[5] FRIEDMAN, I., Deuterium content of natural waters and other substances, Geochim. Cosmochim. Acta 4 (1 9 5 3 ) 8 9 —107.
[6] STOLF, R., LEA L, J.M ., FR IT Z , P., SALATI, E ., “Water budget of a dam in the semi-arid region of the northeast of Brazil based on oxygen-18 and chlorine contents,” Isotopes in Lake Studies (Proc. Advisory Group Meeting Vienna, 1977), IAEA, Vienna (1 9 7 9 ) 57.
[7 ] G EYH, H.A., KREYSING, K., Sobre a idade das aguas subterráneas no polígono das secas do nordeste Brasileiro, Rev. Bras. Geosci. 3 (1 9 7 3 ) 5 3 - 5 9 .
[8] MATSUI, E ., Origem e dinamica de salinaz acad da aqua do Nordeste Brasileiro, Ph.D. thesis, Univ. of Sao Paolo, Piracicaba, S.P. May 1978.
[9 ] LEA L, J.M ., Estudo Geológico e Hidrogeológico da Bacia Hidrográfica do Rio Pajeú, SUDENE, Recife (1 9 6 6 ) 23 pp.
[1 0 ] CRUZ, W .B., MELO, F .A .F ., Estudo Geoquímico preliminar das águas subterráneas do Nordeste do Brasil, Série: Brasil, SUDENE, Hidrogeol. 19 (1 9 6 8 ) 147.
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[1 2 ] REBOUCAS, A.C., Le problème de l’eau dans la zone semi-aride du Brésil, DSc. Thèse présentée à l’Université Louis Pasteur de Strasbourg (1 9 7 3 ) 291 pp.
IA E A -A G -1 5 8 /1 1
IS O T O P E I N V E S T I G A T I O N S A S A
T O O L F O R R E G I O N A L
H Y D R O G E O L O G I C A L S T U D I E S IN
T H E L I B Y A N A R A B J A M A H I R I Y A
D . S R D O t , A d e l a S L I E P Í E V I C ,
В . O B E L I C , N a d a H O R V A T I N Ö I C
R u d e r B o S k o v i c I n s t i t u t e ,
Z a g r e b , C r o a t i a , Y u g o s l a v i a
H . M O S E R , W . S T I C H L E R
I n s t i t u t f ü r R a d i o h y d r o m e t r i e d e r
G e s e l l s c h a f t f ü r S t r a h l e n - u n d
U m w e l t f o r s c h u n g M b H , M u n i c h ,
F e d e r a l R e p u b l i c o f G e r m a n y
Abstract
ISOTOPE INVESTIGATIONS AS A TOOL FO R REGIONAL HYDROGEOLOGICAL STUDIES IN THE LIBYAN ARAB JAM AHIRIYA.
14 C-, SH and stable isotope analyses were done on Libyan waters from wells in the Wadi Sawfajjin-Wadi Zamzam-Al Jufrah area during a complex hydrogeological survey. Water from different aquifers can be differentiated by their isotope content. The results generally show I4C ages older than 12 0 0 0 years with increasing groundwater age going towards the east and southeast. The hydrogeological concept of a continuous flow from the Palaeozoic aquifer through the Chicla aquifer towards the Mediterranean Sea is confirmed by the I4C measurements. This means that the Socna aquifer is recharged from the Palaeozoic aquifer, and the Eocene aquifer from the Chicla aquifer. The stable isotope content sorts the waters o f the investigated aquifers into two groups. The relatively high 2H- and 180-contents of the Upper Cretaceous aquifers and the Chicla aquifer indicate a possible recharge during pluvial times from Jebel Nefusa. Low 2H- and 180-contents o f the Palaeozoic, Chicla, Eocene and, in particular, the Socna aquifer, confirm the recharge o f the Socna aquifer from deeper aquifers and show that the Palaeozoic and Chicla aquifers belong to an interconnected underground system.
I N T R O D U C T I O N
T h e C o n s u l t i n g C o m p a n y , “ E n e r g o p r o j e k t ” , B e l g r a d e , c a r r i e d o u t a n e x t e n s i v e
s t u d y f o r t h e L i b y a n W a t e r A u t h o r i t i e s i n t h e W a d i S a w f a j j i n - W a d i Z a m z a m - A l
J u f r a h a r e a ( 3 2 ° 4 5 ' t o 2 9 ° 0 0 ' n o r t h , 1 2 ° 0 0 ' t o 1 6 ° 1 5 ' e a s t ) ( F i g . l ) t o d e t e r m i n e
t h e m o s t p r o m i s i n g a r e a s f o r g r o u n d w a t e r d e v e l o p m e n t w i t h i n t h e r e g i o n , t o
e v a l u a t e e x i s t i n g g r o u n d w a t e r p o t e n t i a l s a n d c o n d i t i o n s f o r t h e i r e x p l o i t a t i o n ,
1 5 3
1 5 4 S R D O C et al.
FIG .l. Map o f the area under investigation and o f sampling points with sample Nos (see Table II].
a n d t o i n i t i a t e s o m e p r o j e c t s o f g r o u n d w a t e r d e v e l o p m e n t . F o r t h e s e p u r p o s e s ,
a c o m p l e x h y d r o g e o l o g i c a l s u r v e y w a s c a r r i e d o u t i n c l u d i n g h y d r o g e o l o g i c a l
m a p p i n g , a g e o p h y s i c a l s u r v e y , e x p l o r a t o r y d r i l l i n g , t e s t p u m p i n g , w a t e r - l e v e l
o b s e r v a t i o n s , a n d c h e m i c a l a n d i s o t o p i c a n a l y s e s o f w a t e r . A l l r e s u l t s a r e t o b e
f o u n d i n t h e F i n a l R e p o r t [ 1 ] o f E n e r g o p r o j e k t . T h i s p a p e r s u m m a r i z e s t h e r e s u l t s
o b t a i n e d f r o m t h e i s o t o p e m e a s u r e m e n t s o n w a t e r s a m p l e s , c o l l e c t e d i n t h e a r e a s
o f i n v e s t i g a t i o n .
IA E A -A G -1 5 8 /1 1 155
T A B L E I . L I T H O S T R A T I G R A P H I C C H A R A C T E R I S T I C S O F S E L E C T E D
A Q U I F E R S I N T H E W A D I S A W F A J J I N - W A D I - Z A M Z A M - A L J U F R A H A R E A
System Series Form ation Aquifer unit Lithology
QuaternaryHolocene
Pleistocene
Oligocene Limestone
Tertiary
Eocene Limestone
Paleocene Bechima Floskulina Dolomite
? ? Zman Socna Chalky dol.
Upper
Mizdah MazuaTigrinna
Limestone Dol. limestone
CretaceousNefusa Gharyan
Ain TobiLimestone Limestone dol.
Lower
Chicla Formation Sandstone
Kabaw Form ation Sandstone
JurassicTriassic
Palaeozoic Cambrian-Ordovician
Sandstone
1 . H Y D R O G E O L O G Y
T h e s t u d y a r e a r e p r e s e n t s t h e n o r t h e a s t e r n e x t e n s i o n o f t h e H a m a d a a l
H a m r a b a s i n , w h o s e e x t r e m e e a s t e r n p a r t i n c l u d e s t h e H u n - G r a b e n . T h e g e o l o g i c a l
s t r u c t u r e i s a p l a t f o r m w i t h a s l i g h t l y d i s t i n g u i s h a b l e s y n c l i n e , t h e a x i s o f w h i c h
d i p s n o r t h e a s t w a r d s . T h e H u n - G r a b e n i s c a u s e d b y a f a u l t s y s t e m r u n n i n g i n a
S E - N W d i r e c t i o n . T h e l i t h o s t r a t i g r a p h i c c h a r a c t e r i s t i c s o f t h e s e l e c t e d a q u i f e r s
a r e g i v e n i n T a b l e I .
T h e f o l l o w i n g m a i n a q u i f e r s w e r e i d e n t i f i e d a n d c h a r a c t e r i z e d i n t h e s t u d y
a r e a b y E n e r g o p r o j e k t e x p e r t s [ 1 ] ( s e e a l s o R e f s [ 2 , 3 ] ) .
1 5 6 S R D O C e t al.
T h e O l i g o c e n e a q u i f e r , l i m i t e d t o t h e H u n - G r a b e n , i s t a p p e d b y n u m e r o u s
w e l l s n e a r H u n a n d W a d d a n . H o w e v e r , i t s c a p a c i t y i s l i m i t e d a n d d e e p e r
a q u i f e r s y i e l d l a r g e r a m o u n t s o f w a t e r o f b e t t e r q u a l i t y .
T h e E o c e n e a q u i f e r a l s o o c c u r s i n t h e H u n - G r a b e n , t o t h e N E o f i t . C o m p r i s i n g
l i m e s t o n e b e d s , t h e E o c e n e i s a c o n f i n e d a r t e s i a n a q u i f e r . A h i g h a r t e s i a n h e a d
o c c u r s i n t h e S E p a r t o f t h e H u n - G r a b e n . S o m e w e l l s t a p p i n g t h e a q u i f e r i n W a d i
Z a m z a m , W a d i B a y y A l K a b i r a n d A b u N u j a y m a r e u s e d f o r a g r i c u l t u r e .
T h e P a l a e o c e n e a q u i f e r c o n t a i n s p e r c h e d w a t e r t a b l e s t h r o u g h o u t t h e m o s t o f
t h e s t u d y a r e a i n c l u d i n g t h e s o u t h e r n a n d s o u t h w e s t e r n p a r t s . I n s i d e t h e H u n -
G r a b e n a n d N E o f i t , t h e a q u i f e r i s c o n f i n e d a n d i s a r t e s i a n . O n l y i n t h e w a d i s
r u n n i n g i n a S W - N E d i r e c t i o n b e t w e e n W a d i S a w f a j j i n a n d H u n - G r a b e n d o e s t h e r e
o c c u r a w a t e r t a b l e . T h e a q u i f e r c o n s i s t s o f m o r e h y d r a u l i c a l l y s e p a r a t e a q u i f e r s
o w i n g t o i t s l i t h o l o g i c a l c h a n g e s , t e c t o n i c s t r u c t u r e a n d t o p o g r a p h i c f a c t o r s . I t
y i e l d s o n l y s m a l l a m o u n t s o f w a t e r t o t h e w e l l s a n d d o e s n o t r e p r e s e n t w a t e r
r e s o u r c e s f o r a g r i c u l t u r a l d e v e l o p m e n t .
T h e S o c n a a q u i f e r , s i t u a t e d i n t h e S o c n a a r e a , i s o n e o f t h e m o s t i m p o r t a n t
a q u i f e r s i n t h e s t u d y a r e a . A r t e s i a n g r o u n d w a t e r o c c u r s i n l i m e s t o n e b e d s c o n f i n e d
b y t h e s c a l e w i t h a h e a d u p t o 6 0 m . C o n s e q u e n t l y , d r i l l e d w e l l s y i e l d m o r e t h a n
200 l t r / s , a l t h o u g h t r a n s m i s s i v i t y o f t h e a q u i f e r i s m o d e r a t e .
T h e a q u i f e r s w i t h i n t h e M i z d a h F o r m a t i o n o c c u r i n a l l t h e l i t h o s t r a t i g r a p h i c
u n i t s i n a l t e r n a t i o n w i t h w a t e r - b e a r i n g a n d w a t e r - c o n f i n i n g b e d s a n d d i s c o n t i n u i t i e s
c a u s e d b y f a u l t i n g a n d w a d i e r o s i o n . M o s t o f t h e e x i s t i n g w e l l s t a p t h e T i g r i n n a
a q u i f e r . S t i l l , i t s e e m s t h a t t h e M a z u z a a q u i f e r y i e l d s l a r g e r a m o u n t s o f w a t e r .
T h e a q u i f e r s a r e l i m i t e d t o t h e n o r t h e r n p a r t o f t h e s t u d y a r e a ; t h e r e f o r e , t a p p i n g
w e l l s a r e s i t u a t e d o n t h e M i z d a h a n d B a n i W a l i d s h e e t s . F r o m w e s t t o e a s t , t h e
M i z d a h a q u i f e r s c h a n g e t h e h y d r a u l i c c h a r a c t e r i s t i c s , c o n t a i n i n g a p e r c h e d w a t e r
t a b l e w i t h i n t h e M i z d a h a n d B a n i W a l i d h i g h l a n d s , a f r e e w a t e r t a b l e i n s o m e o f
t h e l e f t t r i b u t a r i e s o f W a d i S a w f a j j i n , a n d a r t e s i a n w a t e r f u r t h e r t o t h e e a s t .
T h e i r p r a c t i c a l i m p o r t a n c e i s l i m i t e d .
T h e G h a r y a n a q u i f e r , i n i t s e x t e n s i o n , c o i n c i d e s w i t h t h e M i z d a h f o r m a t i o n
a q u i f e r s , i . e . i t i s l i m i t e d t o t h e n o r t h e r n p a r t o f t h e s t u d y a r e a . I t s t r a n s m i s s i v i t y
i s l o w ( d o w n t o 1 0 -6 m 2/ s ) . I t s h y d r a u l i c h e a d a l s o i n c r e a s e s f r o m w e s t t o e a s t .
L i k e t h e M i z d a h F o r m a t i o n i t i s o n l y o f l i m i t e d p r a c t i c a l i m p o r t a n c e .
T h e A i n T o b i l i m e s t o n e a n d d o l o m i t e b e d s a p p e a r t o b e a p o t e n t i a l a q u i f e r ,
b u t i t c a n n o t b e c o n s i d e r e d a s o n e o f g r e a t e x t e n t . I t i s m o s t l y u n d e r t h e p i e z o
m e t r i c s u r f a c e a n d o v e r l i e s t h e C h i c l a - K a b a w a q u i f e r . T h e r e f o r e , i t i s i n c l u d e d i n
IA E A -A G -1 5 8 /1 1 1 57
t h e d e e p g r o u n d w a t e r r e s e r v o i r c o n s t i t u t e d b y t h e P a l a e o z o i c - K a b a w - C h i c l a s a n d
s t o n e b e d s .
T h e C h i c l a - K a b a w a q u i f e r u n d e r l i e s a l m o s t t h e e n t i r e s t u d y a r e a , b u t a t g r e a t
d e p t h s . I t r e a c h e s l a t . 2 9 ° N i n a s o u t h e r l y d i r e c t i o n . O n l y i n t h e e x t r e m e N W
d o t h e s e f o r m a t i o n s l i e a b o v e t h e r e g i o n a l w a t e r l e v e l . T r a n s m i s s i v i t y i s r e l a t i v e l y
h i g h ( 10~3 t o 1 С Г 2 m 2/ s ) , s p e c i f i c c a p a c i t y o f w e l l s i s f r o m 1 — 5 I t r s _ 1- m _1 ;
o p t i m a l d i s c h a r g e r a t e s a r e 3 0 l t r / s . F o r m o s t o f t h e a r e a , C h i c l a - K a b a w i s t h e
m a i n a q u i f e r .
T h e P a l a e o c o i c a q u i f e r u n d e r l i e s t h e e n t i r e s t u d y a r e a . I t i s v e r y d e e p a n d
t h e r e f o r e d r i l l i n g o f w e l l s w o u l d n o t b e j u s t i f i e d e x c e p t i n t h e s o u t h . T h e C h i c l a -
K a b a w a n d P a l a e o z o i c a q u i f e r s a p p e a r t o b e i n l a t e r a l c o n t a c t a n d c o n s t i t u t e
t h e s a m e g r o u n d w a t e r r e s e r v o i r . T h e r e f o r e , i t s e e m s t o b e r e a s o n a b l e t o d e v e l o p
t h e s h a l l o w a q u i f e r f i r s t . D i r e c t g r o u n d w a t e r d e v e l o p m e n t o f t h e a q u i f e r i s
p r o v i d e d i n t h e a r e a s o u t h o f Q a s r A s h S h u w a y r i f , i n t h e w a d i s t o w a r d s t h e H u n -
G r a b e n . T h e P a l a e o z o i c a q u i f e r i s o n e o f t h e m o s t i m p o r t a n t g r o u n d w a t e r s o u r c e s
i n t h e s t u d y a r e a .
2 . I S O T O P E S T U D I E S
F o r t y - o n e w a t e r s a m p l e s , t a k e n f r o m d i f f e r e n t a q u i f e r s w e r e a n a l y s e d f o r
t h e i r 2H a n d 180 c o n t e n t s i n t h e I n s t i t u t f ü r R a d i o h y d r o m e t r i e i n M u n i c h ; a n d
m o s t o f t h e m f o r t h e i r 14C , 13C , a n d 3H c o n t e n t s i n t h e R u d e r B o s k o v i c I n s t i t u t e
i n Z a g r e b ( T a b l e I I ) .
T h e e v a l u a t i o n o f 1 4C a g e s , a s g i v e n b e l o w , w a s d o n e b y m e a n s o f t h e 13C
c o r r e c t i o n Д = % m o d e r n 14C — ( 2 5 13C + 5 0 ) ( 1 + % 0 m o d e m —1 0 0 0
5 13C v a l u e s , i n g e n e r a l , d o n o t d i s p l a y a n y l a r g e r d e v i a t i o n s a n d r a n g e f r o m
— 2 . 3 t o — 7 . 5 % o . P a r t i a l l y c h a r a c t e r i s t i c v a l u e s o f 13C v a l u e s i n i n d i v i d u a l a q u i f e r s
c a n b e s u g g e s t e d . N o o n e w a t e r s a m p l e w a s f o u n d t o b e y o u n g e r t h a n
« 12 000 y e a r s , w h i l e i n t h e c a s e o f v e r y o l d w a t e r s t h e l i m i t i n g f a c t o r w a s t h e
d e t e c t i o n l i m i t o f 14C a c t i v i t y , i . e . « 3 0 0 0 0 t o 4 0 0 0 0 y e a r s .
T h e 5 2 H - 5 180 r e l a t i o n ( F i g . 2 ) e n a b l e s w a t e r s t o b e c l a s s i f i e d i n t o t w o g r o u p s .
G r o u p I i n c l u d e s t h e v a l u e s 5 180 f r o m — 5 . 8 t o — 7 . 5 % o a n d 5 2H f r o m — 3 5 t o — 5 0 % o .
G r o u p I I i n c l u d e s c o n s i d e r a b l y l o w e r v a l u e s : 6 180 f r o m —8 t o — 1 1 % o , a n d S D
f r o m — 6 0 t o - 8 0 % c . I n a d d i t i o n , t h e g r o u p s d i f f e r f r o m e a c h o t h e r r e g a r d i n g
d e u t e r i u m e x c e s s . G r o u p I i s d i s p e r s e d a r o u n d t h e M e t e o r i c W a t e r L i n e
5 2H = 8 5 180 + 1 0 , w h e r e a s g r o u p I I i s s i t u a t e d o n t h e l i n e 6 2 H = 8 6 180 + 4 .
T h e s a m p l e s o f g r o u p I I a r e c h a r a c t e r i z e d b y a c o m p a r a t i v e l y h i g h a p p a r e n t
14C a g e ( F i g . 3 ) . T h u s , t h e i n d i v i d u a l a q u i f e r s c a n b e c h a r a c t e r i z e d b y t h e i r
i s o t o p i c c h a r a c t e r i s t i c s a s f o l l o w s :
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T A B L E I I . I S O T O P I C C O M P O S I T I O N O F S E L E C T E D G R O U N D W A T E R S A M P L E S
F R O M T H E L I B Y A N A R A B J A M A H I R I Y A
ampleNo.
Typ, L o c a t i o n an d/ or Name o f w e l l Geogr N l a t
Coord. E lo n g
WaterDepth
(m)
A q u i f e r 14c l )
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á I3c'>{7 .o)
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S ' V >(Zo)
1 DuW, B i r G a r a n i a , U adi Zamzam 3 1°o8 I 5 ° o5 ' 4 7 . 4 E o ce n e - - - 0 - 5 5 . 3 - 7 . 3 6
2 DrW, W S - lo , U adi Zamzam 3 1 ° l o 15 ° o 6 1 1 7 2 . 0 E o ce n e 3 . 8 + 0 .4 26 З00 •+ 9oo - 3 . 7 7 3 . 8 + 1 4 - 6 8.4 - 8 . 9 9
3 DrW, W S-2o, Wadi Zamzam 3 1 °1 4 1 5 ° o 8 ' 1 4 o ,o E o ce n e - - - 2 . 3 7 0 - 6 8 .0 - 8 . 7 9
4 DrW, W S-21 , Wadi Zamzam 31 ° 17 1 5 ° 1 1 ' 7 5 . 0 E o c e n e 4 . 8 + 0 . 5 24 З00 + 800 - 3 . 3 7 0 - 6 9 . 4 - 9.08
5 DrW, WS-1, Wadi K a b i r 3 o °5 2 15 °1 9* 1 3 o .o E o ce n e 2 . 3 + 0 . 5 3o 2oo +15oo - 2 . 1 7 1 4 . 6 + 1 7 - 7 3 . 5 - 9 . 7 3
6 AW, Hun Old W e l l , Hun 2 9 °o 5 1 5 ° 5 5 ' 4 4 5 . 0 E o ce n e 1 . 8 + o .54) 32 О О 1 + CO 0 0 - 6 . 17 0 - 6 3 . 7 - 8 . 2 9
7 DrW, Abu Nujaym-I 3 o °3 5 1 5 ° 2 7 ' 4 5 o . o E o ce n e ? < 0 .6 > 4o 000 - 3 . 8 8 2 . 3 + 0 9 - 7 4 . 2 - 9 . 5 9
8 DrW, Abu Nujaym-2 3 o °3 5 15 ° 2 7 * 4 5 o . o E o c e n e < 0 .6 > 4o 000 - 2 . 3 9 3 . 8 + 0 8 - 7 4 . 6 - 9 . 7 7
9 DuW, B i r a l M a l l a h , Wadi Zurzur 3 1 °2 5 1 4 ° 3 7 ’ 1 7 . 5 P a l e o c e n e - - - 7 . 1 + 1 3 - 3 6 . 7 - 5 .8 1
lo DrW, W S -3 / I , Q.A. S h a r q u i y a n 3 o °2 5 1 3 ° 3 7 ’ З о . о P a l e o c e n e - - - 1 . 6 + 1 6 - 5 9 . 0 - 7 . 3 4
I 1 DrW, S o c n a - 4 2 9 °o 4 1 5 ° 5 o ' 1 8 o .o S o c n a 1 .4 + 0 . 5 31 loo +2 2 oo - 4 . 7 7 0 - 7 7 . 3 - I 0 . 2 I
12 DrW, S o c n a - 2 2 9 °o 4 1 5 ° 5 o ' 2o 2 .0 Socn a 2 . 0 + 0 . 5 31 З00 +17oo - 4 . 2 9 2 . 7 + 1 4 - 7 7 . 9 - 1 0 . 2 6
13 DrW, S o c n a-1 1 2 9 °o 4 1 5 ° 5 o ' 1 5 o .o S oc n a < 0 .6 ">4o 000 - 4 . 18 0 - 7 6 . 9 - I 0 . 4 5
14 DrW, F e r j a n J - 3 T 2 8 ° 5 5 I 5 ° 3 8 ' 3 3 2 . 0 Socn a 2 . 3 + 0 . 5 3o 2oo +15oo - 4 . 7 7 2 . 5 + 1 3 - 7 9 . 8 - I 0 . 5 1
15 DrW, Nesma 2 3 l ° o 2 1 3 ° 2 5 1 7 o . o4)T i g r i n n a 2 4 . 7 ♦ 0 .6 1 1 15o + 19o - 5 . 4 7 3 . 8 + 1 0 - З 6 . 0 - 6 . 2 5
16 DrW, Nesma 2 3 l ° o 2 1 3 ° 2 5 ' 80.0 T i g r i n n a 6 . 5 + 0 . У 21 9oo + 56o - 1 . 4 9 4 . 0 + 1 2 - 3 9 . 0 - 6 . 3 5
17 DuW, Mizdah 3 1 ° 2 6 i v13 oo 1 4 .2 T i g r i n n a - - - 1 4 . 7 + 1 4 - 3 9 . 4 - 6 . 3 4
18 DrW, Wadi Mimun 3 I ° 3 4 ' 1 4 ° 2 3 ' 9 5 . 0 T i g r i n n a 6 . 9 + o . ^ > 21 5oo + 5oo - 3 . 7 8 - - 6 . 2o
SRDO
C et
al.
SampleNo.
Type, location and/or name of well Geogr. coord. N lat E long
Waterdepth(m)
Aquifer »4C0% mod. age3)
(years)
5 I3C°(%o)
з„ 0(TU)
i :Hy(%»)
6 ‘*0 2)(W)
19 DrW, Wadi S a w f a j j i n 3 1 ° 3 4 I 4 ° 2 3 ' 5 o o . o Mizdah 2 3 . 0 + 0 . 6 1 1 7oo + 2oo - 6 .8 6 0 - 4 1 . 2 _ 5 . 7 6
2o DrW, Wadi S a w f a j j i n 3 I ° 3 4 I 4 ° 2 3 ' 5 o o . o Gharyan - - - - - 6 1 . 7 - 8 . 2 4
21 DrW, Ras T a b a l 3 l °o 2 13 ° 2 5 * 1 7 4 . 0 Gharyan? .P*
1 +
О £> 5oo + 25o - 3 . 4 8 0 - 5 o . 6 - 6 . 8 3
22 DrW, B an i W a l id 3 1°45 14 ° o 4 ' Зоо.о Gharyan 1 2 . 8 *_ 0 . 5 16 5oo + З00 - 3 . 7 8 0 - 3 8 . 9 - 6 . 1 8
23 DrW, B i r Sanam , Wadi S a w f a j j i n 3 J ° 2 1 1 3 ° 4 1 1 4 5 o . o Gharyan 2 . 9 ♦ 0 . 5 4 ’ 28 З00 + J2oo - 1 . I 9 4 ) 3 . 8 + . 1 - 4 7 . 2 - 7 . 15
24 DrW, Mizdah 1 31 °2 7 13 ° o o ' 1 2 8 . Ô Gharyan - - - 3 . 7 8 0 - 4 1 . 9 - 6 . 5 1
25 DrW, Khartum 6 3 1° 4 7 1 2 ° 1 о ' 1 8 o .o Gharyan 9 . 4 + o . 5 4 ) 19 lo o + 4oo - 4 . 7 7 2 . 5 + .6 - 4 4 . 2 - 6 . 3 3
26 DrW, WS-16 3 1 °3 8 1 3 ° o o ' 4 1 7 . 0 Ain T o b i - - - - - 4 8 .0 - 6 .8 0
27 AW, Z Z -1 , Wadi Zamzam 3 l ° o 9 1 5 ° 0 3 ’ 10 00 .0 C h i c l a 1 .7 + 0 . 5 32 5oo +19oo - 5 . 4 7 2 . 3 + . 5 - 6 8 . 2 - 9 . 1 6
28 AW, Z Z -2 , Wadi Zamzam 3 l ° l o 1 5 ° o 5 ' 10 00 .0 C h i c l a 2 . 2 + 0 . 3 3o 7oo + l 2 o o - 5 . 4 2 1 . 8 + , 24 > - 6 8 .0 - 9 . 1 6
29 DrW, New W e l l Nura 3 I ° 4 7 I 3 ° 5 3 ' 9 7 5 . 0 C h i c l a < 0 . 6 > 4o 000 - 6 .8 6 0 - 4 5 . 3 - 6 . 6 9
3o AW, WS-2 3 o °58 1 4 ° 3 5 ' l o l o . o C h i c l a 1 .2 + 0 . 3 35 З00 + I 600 - 4 . 6 5 0 - 6 8 . 7 - 9 . 4 3
31 DrW, WS-4 3 o°24 13 ° 3 6 ' 8 0 1 .0 C h i c l a < 0 . 6 > 4o 000 - 5 . 5 4 0
,74)
- 6 7 . 4 - 8 . 9 4
32 DrW, WS- 6 °3o oo 14 ° 1 6 ' 6 9 4 . 0 C h i c l a - - - 1 . 9 + - 7 o . l - 9 . 5 5
33 DrW, WS-14M, Mizdah 31°2 7 13 ° o o ' 7 7 2 . 0 C h i c l a - - - 5 . 1 7 4 . 1 + .7 - 4 7 . 1 - 7 . 2 7
34 DrW, WS-I4M, Mizdah 3 1° 2 7 1 3 ° o o ' 7 7 2 . 0 Kabaw 1 1 .6 + 0 . 4 17 25o + 27o - 7 . 4 6 - - 4 6 . 3 - 7 .0 8
35 DrW, WS- 8 2 9°o2 1 4 ° 1 8 ' 4 6 o . o P a l e o z o i c 5 . 8 + o . 3 4 ) 22 7oo + 4oo - 3 . 4 o 3 . 2 ♦ .3 - 6 9 . 0 - 9 . 5 9
36 DrW, No.- 1 , Wadi Zimam 2 9 °2 o 1 5 ° 2 2 ' 5 1 9 , 0 P a l e o z o i c 4 . 5 + o . 3 4 ) 24 9oo + 600 - 3 . 3 5 0 - 7 2 . 7 - 9 . 8 8
DrW — Drilled Well; DuW = Dug Well; AW — Artesian Well; Well data, water depth and aquifer nomination see Ref.[ 1 ].
Measured at the Ruder Bo5kovi6 Institute, Zagreb, Yugoslavia.* Measured at the Institut fur Radiohydrometrie der Gesellschaft fur Strahlen- und Umweitforschung mbH, Neuherberg, Fed. Rep. of Germany.* F o r age ca lcu lation see Sect. 2.^ N o t re liable measurement.
IAEA
-AG
-158/11 1
59
1 6 0 S R D O C et al.
62Н(%.)
FIG .2. 5 2H-¿ 180 relation o f water samples taken from different aquifers. E = Eocene;PI = Paleocene; So = Socna; Mz = Mizdah; Ti = Tigrinna; Gr = Gharyan; AT = Ain Tobi; Ch = Chicla; Pz = Palaeozoic.
E o c e n e a q u i f e r : T h e c o n c o r d a n t q u a n t i t i e s o f 2 H a n d 180 i n d i c a t e t h a t t h e
o r i g i n o f w a t e r s a m p l e s N o s 5 , 7 a n d 8 i s t h e s a m e . T h e w a t e r s b e l o w t o g r o u p I I ,
t h e 14C a g e s ( 3 0 0 0 0 t o > 4 0 0 0 0 y e a r s ) s u p p o r t t h i s s t a t e m e n t . M o s t o f t h e
r e m a i n i n g w a t e r s a m p l e s f r o m t h i s a q u i f e r ( N o s 2 , 3 , 4 , 6 ) a l s o b e l o n g t o g r o u p I I .
T h e i r 1 4 C a g e s o f 2 4 0 0 0 t o 2 6 0 0 0 y e a r s m a y i n d i c a t e a s m a l l b u t d e f i n i t e
r e c h a r g e f r o m W a d i Z a m z a m . S a m p l e N o . l i s t h e o n l y o n e w h i c h c o u l d b e c o n
s i d e r e d a s a m i x t u r e o f g r o u p s I a n d I I . 1 4 C w a s n o t m e a s u r e d , u n f o r t u n a t e l y .
T h e d i f f e r e n c e i n a g e o f w a t e r s a m p l e N o . 5 , t a k e n i n W a d i A l K a b i r ( 3 0 0 0 0 y e a r s ) ,
a n d t h a t t a k e n f r o m A b u N u j a y m N o s 7 a n d 8 ( > 4 0 0 0 0 y e a r s ) , p r o b a b l y i n d i c a t e s
a p o s s i b l e i n t e r f e r e n c e o f r e c e n t w a t e r s i n t h e W a d i A l K a b i r a r e a .
A c e r t a i n c o n f i r m a t i o n m a y b e g i v e n b y t h e r e l a t i v e l y h i g h 3H v a l u e o f N o . 5 ,
a l t h o u g h s o m e d o u b t s e x i s t i n g e n e r a l c o n c e r n i n g t h e 3H v a l u e s ( s e e S e c t i o n 3 ) .
P a l a e o c e n e a q u i f e r : O n l y t w o w a t e r s a m p l e s ( N o s 9 a n d 1 0 ) w e r e m e a s u r e d .
A s i t w a s c o n s i d e r e d t h a t r e c e n t w a t e r s w e r e i n q u e s t i o n , 1 4 C m e a s u r e m e n t s w e r e
n o t m a d e . T h i s a s s u m p t i o n c a n b e a c c e p t e d a c c o r d i n g t o t h e 2H , 3H a n d 1 8 0
c o n t e n t s f o r s a m p l e N o . 9 . S a m p l e N o . 1 0 , h o w e v e r , c o n t a i n s a t b e s t c o n s i d e r a b l e
p a r t s o f o l d w a t e r b e l o n g i n g t o g r o u p I I .
I A E A - A G - 1 58/11 161
FIG.3. 62H-14C age relation o f water samples taken from different aquifers (for abbreviations see caption ofF ig .2). For calculation o f 14C age from 14C content see Sect. 2.
S o c n a a q u i f e r : A l l w a t e r s a m p l e s ( N o s 1 1 , 1 2 , 1 3 , 1 4 ) t a k e n f r o m t h i s
a q u i f e r b e l o n g t o g r o u p I I , c h a r a c t e r i z e d b y v e r y l o w 5 2H a n d 5 l 8 0 v a l u e s a n d a
1 4 C a g e o f a b o u t 3 0 0 0 0 y e a r s . I t s e e m s t h a t n o a d m i x t u r e s o r o t h e r h y d r a u l i c
e f f e c t s i n f l u e n c e d t h e w a t e r o f t h i s c l o s e d a q u i f e r ( s e e a l s o R e f . [ 2 ] ) .
M i z d a h F o r m a t i o n a q u i f e r s : T h e w a t e r s a m p l e s ( N o s 1 5 , 1 6 , 1 7 , 1 8 a n d 1 9 )
b e l o n g t o g r o u p I . T h e 1 4 C m e a s u r e m e n t s g a v e n o r e l i a b l e r e s u l t s a n d t h e r e f o r e
w e r e n o t c o n s i d e r e d . T h e 3H r e s u l t s i n d i c a t e t h a t m o s t s a m p l e s c o n t a i n p o r t i o n s
o f r a i n - w a t e r y o u n g e r t h a n 20 y e a r s a n d t h u s c o n f i r m t h e h y d r o g e o l o g i c a l
a s s u m p t i o n o f a p e r c h e d a q u i f e r .
G h a r y a n a q u i f e r : A s f a r a s t h e S 2 H a n d 5 1 8 0 v a l u e s a r e c o n c e r n e d , w a t e r
s a m p l e s N o s 2 1 , 2 2 , 2 3 , 2 4 , 2 5 , 3 7 a n d 3 8 b e l o n g t o g r o u p I w i t h a 1 4 C a g e o f
a b o u t 1 5 0 0 0 y e a r s . T h e p r e s e n c e o f a s m a l l p e r c e n t a g e o f r a i n - w a t e r y o u n g e r
t h a n 2 0 y e a r s i n w a t e r s a m p l e N o . 2 3 ( a n d 2 5 ) c a n n o t b e e x c l u d e d . W a t e r s a m p l e
N o . 2 0 , a t t r i b u t e d t o t h e G h a r y a n a q u i f e r , b e l o n g s m o r e t o g r o u p I I a n d m a y
o r i g i n a t e f r o m a n o t h e r a q u i f e r .
A i n T o b i a q u i f e r : T h e o n l y s a m p l e f r o m t h i s a q u i f e r ( N o . 2 6 ) b e l o n g s t o
g r o u p I a s f a r a s t h e 5 2 H a n d S 1 8 0 v a l u e s a r e c o n c e r n e d a n d c a n n o t t h e r e f o r e b e
c o n n e c t e d w i t h w a t e r f r o m t h e C h i c l a - K a b a w - P a l a e o z o i c a q u i f e r ( s e e b e l o w ) .
3 H a n d 1 4 C m e a s u r e m e n t s w e r e n o t m a d e .
16 2 S R D O C et al.
C h i c l a - K a b a w - a q u i f e r : T h e w a t e r s a m p l e s t a k e n f r o m t h i s a q u i f e r c a n b e
d i v i d e d i n t o g r o u p s I a n d I I w i t h r e g a r d t o t h e i r 6 2H a n d 5 1 8 0 v a l u e s . S a m p l e
N o s 2 7 , 2 8 , 3 0 , 3 1 a n d 3 2 , t a k e n a t t h e e a s t e r n a n d s o u t h w e s t e r n p a r t s o f t h e
s t u d y a r e a , b e l o n g t o g r o u p I I w h e r e a s s a m p l e N o s 2 9 , 3 3 a n d 3 4 t a k e n a t t h e
n o r t h w e s t e r n p a r t o f t h e s t u d y a r e a , b e l o n g t o g r o u p I . T h e 1 4 C a g e s m o s t l y
o c c u r a s f o l l o w s — G r o u p I I s a m p l e s a r e a s s o c i a t e d w i t h 1 4 C a g e s b e t w e e n 3 0 0 0 0
a n d 4 0 0 0 0 y e a r s , w h e r e a s g r o u p I s a m p l e N o . 3 4 s h o w s a 1 4 C a g e o f 1 7 0 0 0 y e a r s
w i t h a 3H c o n t e n t o f 4 T U . T h e o n l y u n e x p l a i n e d e x c e p t i o n , s a m p l e N o . 2 9
b e l o n g i n g t o g r o u p I , w i t h r e g a r d t o 5 2 H a n d § 180 v a l u e s , g i v e s a 1 4 C a g e o f
4 0 0 0 0 y e a r s a n d n o 3 H c o n t e n t . T h e r e f o r e , i t s e e m s t h a t i n t h e n o r t h w e s t e r n
p a r t o f t h e a q u i f e r , l y i n g a b o v e t h e r e g i o n a l w a t e r l e v e l , a c e r t a i n r e c h a r g e t a k e s
p l a c e l o n g a f t e r t h e f o r m e r r e c h a r g e o f t h e g r o u p I I w a t e r s .
P a l a e o z o i c a q u i f e r : S a m p l e s 3 5 a n d 3 6 b o t h c o n t a i n s i m i l a r q u a n t i t i e s o f 2 H
a n d 180 , c o r r e s p o n d i n g t o g r o u p I I . 1 4C a g e s f r o m 2 2 0 0 0 t o 2 4 0 0 0 y e a r s a r e n o t
v e r y r e l i a b l e .
3 . D I S C U S S I O N O F I S O T O P I C D A T A A N A L Y S I S
T r i t i u m a n a l y s e s : A c c o r d i n g t o t h e t r i t i u m c o n t e n t i n g r o u n d w a t e r s t h e r e
a r e o n l y f e w c a s e s i n t h e w h o l e i n v e s t i g a t e d a r e a w h e r e t h e t r i t i u m c o n c e n t r a t i o n
i n w a t e r c o u l d i n d i c a t e c o n t r i b u t i o n f r o m r e c e n t p r e c i p i t a t i o n , s u c h a s s a m p l e
N o . 5 t a p p e d f r o m t h e E o c e n e a q u i f e r , s a m p l e N o . 1 7 t a p p e d f r o m t h e T i g r i n n a
a q u i f e r , a n d s a m p l e N o . 2 3 t a p p e d f r o m t h e G h a r y a n a q u i f e r . N e v e r t h e l e s s , i t
m u s t b e t a k e n i n t o c o n s i d e r a t i o n t h a t c o n t a m i n a t i o n d u r i n g s a m p l i n g o r e x c h a n g e
w i t h a t m o s p h e r i c w a t e r v a p o u r p o s s i b l y i n c r e a s e d t h e 3H c o n t e n t o f s o m e o f t h e
w a t e r s a m p l e s . H o w e v e r , i n m o s t c a s e s t r i t i u m c o n c e n t r a t i o n i n d i c a t e s e i t h e r a n
i n s i g n i f i c a n t c o n t r i b u t i o n f r o m p r e c i p i t a t i o n o r a t o t a l a b s e n c e o f p r e c i p i t a t i o n
w a t e r i n g r o u n d w a t e r i n t h e i n v e s t i g a t e d a r e a , i n c l u d i n g t h e r e l a t i v e l y e l e v a t e d
r e g i o n s o f J e b e l N e f u s a .
T h i s c o n c l u s i o n , b a s e d o n t r i t i u m a n a l y s e s , e x p l a i n s t h e o b s e r v e d f a c t t h a t
t h e g r o u n d - w a t e r l e v e l d o e s n o t s h o w f l u c t u a t i o n s r e l a t e d t o t h e a m o u n t o f
p r e c i p i t a t i o n w a t e r . I t c o u l d b e s t a t e d t h a t t h e c o n t r i b u t i o n o f p r e c i p i t a t i o n
w a t e r t o t h e u n d e r g r o u n d w a t e r i s n e g l i g i b l e a n d i f i t d o e s o c c u r a t a l l i t h a s a
v e r y l i m i t e d r a n g e n e a r w a d i s d u r i n g t h e r a i n y p e r i o d .
I 4C - a n a l y s e s : A l t h o u g h t h e r a d i o c a r b o n “ a g e ” o f u n d e r g r o u n d w a t e r s c o u l d
b e a l t e r e d b y m a n y g e o c h e m i c a l p r o c e s s e s , t h u s g i v i n g e r r o n e o u s a n d t h e r e f o r e
m i s l e a d i n g r e s u l t s , a u s e f u l s e t o f i n f o r m a t i o n c a n b e d e r i v e d f r o m 1 4 C m e a s u r e m e n t s
o f d i s s o l v e d h y d r o c a r b o n a t e s .
IA E A - A G - 1 58/11 1 6 3
G e n e r a l l y , t h e m e a s u r e d a g e o f L i b y a n g r o u n d w a t e r s i s v e r y o l d , r a n g i n g
f r o m 12 000 y e a r s t o o r a b o v e t h e l i m i t s s e t b y t h e r a d i o c a r b o n m e t h o d , w h i c h i s
^ 4 0 0 0 0 y e a r s . G r o u n d w a t e r s o f t h e n o r t h w e s t e r n m o u n t a i n o u s r e g i o n J e b e l
N e f u s a , f r o m C r e t a c e o u s a q u i f e r s i n c l u d i n g t h e C h i c l a a q u i f e r ( L o w e r C r e t a c e o u s ) ,
a r e r e l a t i v e l y y o u n g e r , w h i c h i n d i c a t e s c o n t r i b u t i o n s f r o m p r e c i p i t a t i o n u p t o
r e c e n t t i m e s , a l t h o u g h v e r y l i m i t e d . T h e r e f o r e , i t m u s t . b e c o n s i d e r e d t h a t , b e n e a t h
t h e m a i n f l o w i n t h e P a l a e o z o i c f r o m s o u t h t o n o r t h , a s e c o n d f l o w c o m p o n e n t
e x i s t s i n t h e C r e t a c e o u s a q u i f e r s f r o m w e s t t o e a s t . I n d e e d , g o i n g e a s t w a r d s a n d
s o u t h e a s t w a r d s t h e g r o u n d w a t e r a g e i n c r e a s e s , w h i c h c a n b e e x p l a i n e d b y t h e l o n g e r
d i s t a n c e f r o m r e g i o n s w h e r e p r e c i p i t a t i o n o c c u r s a n d t h e v e r y l o w f l o w i n g r a t e
o f u n d e r g r o u n d w a t e r .
I t s h o u l d b e e m p h a s i z e d t h a t t h e u n d e r g r o u n d w a t e r i n t h e i n v e s t i g a t e d r e g i o n
i s r e l a t i v e l y o l d . T h i s i s t r u e n o t o n l y f o r d e e p a r t e s i a n w e l l s w h i c h b e l o n g t o t h e
P a l a e o z o i c a n d C h i c l a a q u i f e r s , b u t a l s o f o r t h e r e l a t i v e l y s h a l l o w S o c n a a n d E o c e n e
a q u i f e r s . T h e S o c n a a q u i f e r i s l o c a t e d i n t h e s o u t h e a s t r e g i o n , w h e r e a s t h e E o c e n e
a q u i f e r i s n e a r t h e H u n - G r a b e n a n d i s s h a l l o w .
T h e e x p e c t e d v e r y o l d a g e o f u n d e r g r o u n d w a t e r f r o m t h e P a l a e o z o i c a n d
C h i c l a ( L o w e r C r e t a c e o u s ) a q u i f e r s , l o c a t e d u n d e r U p p e r C r e t a c e o u s n o n - p o r o u s
l a y e r s , i s c o n f i r m e d b y 1 4 C m e a s u r e m e n t s . B u t i t s h o u l d b e e m p h a s i z e d t h a t w a t e r
f r o m t h e C h i c l a a q u i f e r i s o l d e r t h a n w a t e r f r o m t h e P a l a e o z o i c a q u i f e r , w h i c h i s
i n a g r e e m e n t w i t h t h e c o n c e p t o f a c o n t i n u o u s f l o w f r o m t h e P a l a e o z o i c a q u i f e r
t h r o u g h t h e C h i c l a a q u i f e r t o w a r d s t h e M e d i t e r r a n e a n s e a .
A s a r e s u l t o f 1 4 C c a r b o n m e a s u r e m e n t s t o g e t h e r w i t h o t h e r s t a b l e i s o t o p e
a n d c h e m i c a l a n a l y s e s , i t h a s b e e n e s t a b l i s h e d t h a t t h e S o c n a a q u i f e r i s r e c h a r g e d
f r o m t h e P a l a e o z o i c a q u i f e r t h r o u g h H u n G r a b e n a s o p p o s e d t o s o m e p r e v i o u s
s p e c u l a t i o n s w h i c h c o n s i d e r e d t h e p r e c i p i t a t i o n f r o m t h e J e b e l S o d a h i g h l a n d s
t o b e t h e m a i n s o u r c e o f w a t e r r e c h a r g i n g t h e S o c n a a q u i f e r .
T h e s a m e r e a s o n i n g i s v a l i d f o r t h e E o c e n e a q u i f e r , w h i c h i s r e c h a r g e d f r o m
t h e C h i c l a a q u i f e r .
S t a b l e I s o t o p e s : S e v e r a l v e r y i n t e r e s t i n g a n d i m p o r t a n t r e s u l t s w e r e o b t a i n e d
b y m e a s u r i n g t h e r e l a t i v e c o n c e n t r a t i o n o f s t a b l e i s o t o p e s 2 H a n d 180 . I t w a s
p o s s i b l e t o s o r t s a m p l e s i n t o t w o g r o u p s a c c o r d i n g t o t h e d i a g r a m s h o w n o n F i g . 2 .
G r o u p I o f t h e s a m p l e s b e l o n g s t o t h e U p p e r C r e t a c e o u s a q u i f e r s a n d t h e C h i c l a
a q u i f e r f r o m t h e J e b e l N e f u s a h i g h l a n d r e g i o n . T h e y h a v e t h e h i g h e s t 2 H a n d
180 c o n t e n t s , p r e s u m a b l y o w i n g t o t h e i n f l u e n c e o f t h e M e d i t e r r a n e a n c l i m a t e .
T h i s f a c t a l s o i n d i c a t e s a p o s s i b l e r e c h a r g e d u r i n g p l u v i a l t i m e s f r o m t h e J e b e l
N e f u s a h i g h l a n d s . I t c a n n o t b e e x c l u d e d t h a t u n d e r g r o u n d w a t e r t a p p e d f r o m
t h e C h i c l a a q u i f e r i s c o m p o s e d o f P a l a e o z o i c w a t e r f l o w i n g f r o m t h e s o u t h a n d o f
a s m a l l f r a c t i o n o f p r e c i p i t a t i o n w a t e r f r o m t h e J e b e l N e f u s a h i g h l a n d s w i t h
r e l a t i v e l y h i g h 5 - v a l u e s [ 2 ] .
T h e f i r s t p a r t o f g r o u p I I , w i t h t h e l o w e s t s t a b l e i s o t o p e c o n t e n t , w a s
c o n n e c t e d w i t h t h e S o c n a a q u i f e r . L o w 2 H a n d 180 c o n t e n t s i n d i c a t e a l o w
1 6 4 S R D O Í et al.
t e m p e r a t u r e e f f e c t , a l t h o u g h i t i s n o t c l e a r w h e t h e r i t w a s a n e f f e c t o f a l t i t u d e , o r
a s o - c a l l e d c o n t i n e n t a l e f f e c t , o r a l o w e r i n g o f t h e t e m p e r a t u r e d u r i n g t h e
P l e i s t o c e n e . I n a n y c a s e , s t a b l e i s o t o p e d a t a s u p p o r t t h e a s s u m p t i o n t h a t t h e
S o c n a a q u i f e r i s r e c h a r g e d f r o m a d e e p e r a q u i f e r . T h e r e c h a r g e t o S o c n a f r o m t h e
P a l e o z o i c a q u i f e r h a s a l s o b e e n f o u n d b y m e a s u r e m e n t s i n o t h e r w e l l s o f t h e
r e g i o n s o u t h o f t h e a r e a u n d e r i n v e s t i g a t i o n [ 2 ] , a n d i s c o n f i r m e d b y s i m i l a r
13C v a l u e s o f b o t h a q u i f e r s . T h i s c o n c l u s i o n h a s a v e r y i m p o r t a n t p r a c t i c a l c o n
s e q u e n c e s i n c e i t i m p l i e s t h a t a n i n t e n s i v e e x p l o i t a t i o n o f t h e P a l a e o z o i c a q u i f e r
w o u l d b e p o s s i b l e .
T h e s e c o n d p a r t o f g r o u p I I b e l o n g s t o t h e P a l a e o z o i c , C h i c l a a n d E o c e n e
a q u i f e r s . T h e 2H a n d 180 c o n t e n t s a r e h i g h e r w h e n c o m p a r e d w i t h t h e f i r s t p a r t ,
i n d i c a t i n g a h i g h e r c o n d e n s a t i o n t e m p e r a t u r e . T h e f a c t t h a t t h e s t a b l e i s o t o p e
c o n t e n t o f w a t e r s a m p l e s f r o m t h e P a l a e o z o i c a n d C h i c l a a q u i f e r s a r e v e r y c l o s e
c o n f i r m s t h e a s s u m p t i o n , b a s e d o n p i e z o m e t r i c o b s e r v a t i o n s , t h a t b o t h a q u i f e r s
a r e i n d e e d p a r t o f a h u g e i n t e r c o n n e c t e d u n d e r g r o u n d s y s t e m . T h e a s s u m p t i o n
o n t h e r e c h a r g e o f t h e E o c e n e a q u i f e r f r o m t h e C h i c l a a q u i f e r i s b a s e d o n l y o n
d a t a f r o m i s o t o p i c a n a l y s e s .
R E F E R E N C E S
[1] ENERGOPROJECT BEOGRAD, Regional Hydrogeological Study: Wadi Sawfajjin- Wadi Zamzam-Al Jufrah, Final Rep. to the Secretariat of Dams and Water Resources,Socialist People’s Libyan Arab Jamahiriya (1977).
[2] SALEM, O., VISSER, J.H ., DRAY, M., GONFIANTINI, R„ “Flow patterns o f groundwater in the western Libyan Arab Jamahiriya — Evaluations from isotopic data”, IAEA-AG-158/13, these Proceedings.
[3] GOUDARZI, G.H., Geology and mineral resources of Libya: A reconnaissance, US Geol. Survey Profess. Paper 660, Washington, D.C. (1970) 104 pp.
I A E A - A G - 1 58 /12
G R O U N D W A T E R F L O W P A T T E R N S IN
T H E W E S T E R N L I B Y A N A R A B J A M A H I R I Y A
E V A L U A T E D F R O M IS O T O P IC D A T A
O . S A L E M
S e c r e t a r i a t o f D a m s a n d W a t e r R e s o u r c e s ,
T r i p o l i
J . H . V I S S E R
U n i t e d N a t i o n s F o o d a n d A g r i c u l t u r e
O r g a n i z a t i o n ,
L a n d a n d W a t e r I n v e s t i g a t i o n P r o j e c t ,
T r i p o l i ,
L i b y a n A r a b J a m a h i r i y a
M . D R A Y , R . G O N F I A N T I N I
I n t e r n a t i o n a l A t o m i c E n e r g y A g e n c y ,
V i e n n a
A b stract
GROUNDWATER FLOW PATTERNS IN THE W ESTERN LIBYAN ARAB JAM AHIRIYA EVALUATED FROM ISOTOPIC DATA.
Stable isotope analyses were used to provide evidence of the main features of regional groundwater flow patterns and to show interconnections between aquifers in the western Libyan Arab Jamahiriya. The main flow is from south to north in the Palaeozoic aquifer, but a component deriving from a recharge area in the west is present in the Wadi Ash Shati Valley and increases significantly to the north of Jebel Gargaf. This groundwater, which originated in the west, is also the major recharge source of the important Kicla aquifer which in turn recharges the overlying Upper Cretaceous formations in the area of Misratah. More to the north, the Kicla aquifer is also recharged by precipitation over the Jebel Nefusa. In the north, the Tawurgha spring is fed mainly by the Miocene aquifer and not by the Upper Cretaceous ones.
I N T R O D U C T I O N
G e n e r a l l y s p e a k i n g , t h e r e g i o n a l f l o w d i r e c t i o n o f d e e p g r o u n d w a t e r i n t h e
L i b y a n A r a b J a m a h i r i y a i s f r o m s o u t h t o n o r t h ; t h a t i s , f r o m t h e S a h a r a D e s e r t t o
t h e M e d i t e r r a n e a n S e a . H o w e v e r , t h e d e t a i l e d c o n f i g u r a t i o n o f g r o u n d w a t e r f l o w
p a t t e r n s , t h e a r e a s a n d p e r i o d s o f r e c h a r g e , a n d t h e i n t e r c o n n e c t i o n s b e t w e e n
a q u i f e r s , a r e s t i l l t o a l a r g e e x t e n t u n k n o w n o r n o t s u f f i c i e n t l y p r o v e d .
A n a t t e m p t t o t h r o w s o m e l i g h t o n t h e s u b j e c t b y u s i n g t h e v a r i a t i o n s o f
e n v i r o n m e n t a l i s o t o p e c o n c e n t r a t i o n s i n g r o u n d w a t e r h a s b e e n m a d e a s a p a r t o f
165
1 6 6 S A L E M et al.
a c u r r e n t i n v e s t i g a t i o n , c a r r i e d o u t j o i n t l y b y t h e S e c r e t a r i a t o f D a m s a n d W a t e r
R e s o u r c e s o f t h e L i b y a n A r a b J a m a h i r i y a , b y t h e F o o d a n d A g r i c u l t u r a l
O r g a n i z a t i o n o f t h e U n i t e d N a t i o n s , a n d b y t h e I n t e r n a t i o n a l A t o m i c E n e r g y A g e n c y ,
w i t h i n t h e f r a m e w o r k o f t h e L a n d a n d W a t e r I n v e s t i g a t i o n P r o j e c t ( T F - 9 1 8 4 )
e x e c u t e d b y F A O o n b e h a l f o f t h e L i b y a n G o v e r n m e n t . O n l y p a r t o f t h e r e s u l t s
o b t a i n e d a r e r e p o r t e d h e r e — t h o s e d e a l i n g w i t h d e e p g r o u n d w a t e r i n w e s t e r n
L i b y a . T h e s t u d y , i n i t i a t e d i n l a t e 1 9 7 6 , i s s t i l l i n p r o g r e s s , b u t w e a r e c o n v i n c e d
t h a t t h e m a i n c o n c l u s i o n s a l r e a d y r e a c h e d w i l l r e m a i n b a s i c a l l y u n c h a n g e d a n d
v a l i d w h e n t h e w o r k i s c o m p l e t e d .
T h e a r e a u n d e r i n v e s t i g a t i o n i s i n t h e w e s t o f t h e c o u n t r y , f r o m t h e F e z z a n t o
t h e M e d i t e r r a n e a n S e a ( F i g . 1 ) . T h e b o u n d a r i e s o f t h e a r e a a r e a p p r o x i m a t e l y t h e
2 6 t h p a r a l l e l o n t h e s o u t h , t h e 1 3 t h m e r i d i a n o n t h e w e s t , t h e 1 6 t h m e r i d i a n o n
t h e e a s t , a n d t h e c o a s t o n t h e n o r t h . T h i s v e r y l a r g e a r e a o f a b o u t 1 8 0 0 0 0 k m 2 i s
e s s e n t i a l l y f l a t , w i t h o n l y f e w m o u n t a i n r a n g e s o f l i m i t e d e x t e n t a n d h e i g h t , t h e
m o s t i m p o r t a n t o f w h i c h a r e t h e J e b e l N e f u s a h i n t h e n o r t h a n d t h e J e b e l S a w d a
i n t h e c e n t r a l p a r t . T h e m a j o r p a r t o f t h e a r e a c o n s i s t s o f d e s e r t , e i t h e r o f
c l a s s i c a l s a n d - d u n e s ( U b a r i S a n d S e a , M u r z u k S a n d S e a ) o r o f s t o n e y d e s e r t
( H a m a d a h e l H a m r a ) .
T h e r a i n f a l l i s a b o u t 2 5 0 m m / y e a r i n t h e c o a s t a l z o n e , b u t r a p i d l y d r o p s
t o w a r d s t h e i n t e r i o r t o v a l u e s o f l e s s t h a n 5 0 m m / y e a r . U n d e r t h e s e c l i m a t i c
c o n d i t i o n s , t h e r e i s , o f c o u r s e , n o p e r m a n e n t s u r f a c e f r e s h - w a t e r b o d y a v a i l a b l e ,
a n d a n y a g r i c u l t u r a l a c t i v i t y i n t h e m a j o r w a d i v a l l e y s m u s t r e l y e n t i r e l y o n
g r o u n d w a t e r r e s o u r c e s f o r i r r i g a t i o n .
H e r e w e s u m m a r i z e t h e h y d r o g e o l o g i c a l s i t u a t i o n o f t h e a r e a . M o s t o f t h e
i n f o r m a t i o n c o n t a i n e d i n t h i s b r i e f o u t l i n e i s t a k e n f r o m G o u d a r z i [ 1 ] , P a l l a s [ 2 ] ,
a n d f r o m d i s c u s s i o n s w i t h t h e h y d r o g e o l o g i s t s o f t h e F A O t e a m i n L i b y a , e s p e c i a l l y
P . P a l l a s , G . K r u s e m a n , a n d M . T a k a w h o s e c o n t r i b u t i o n s a r e g l a d l y a c k n o w l e d g e d .
F i g u r e 2 , s h o w i n g a s c h e m a t i z e d s o u t h - n o r t h g e o l o g i c a l c r o s s - s e c t i o n , c a n b e
u s e d t o i l l u s t r a t e t h e m a j o r h y d r o g e o l o g i c a l f e a t u r e s o f t h e a r e a . I n t h e s o u t h ,
i n t h e M u r z u k d e p r e s s i o n a n d i n t h e W a d i A j a l z o n e , t h e l a r g e s t g r o u n d w a t e r
r e s e r v o i r i s p r o b a b l y t h e P a l a e o z o i c S a n d s t o n e ( C a m b r o - O r d o v i c i a n ) . T h i s f o r m a t i o n ,
c o n f i n e d a n d m o s t l y a r t e s i a n , i s , h o w e v e r , g e n e r a l l y t o o d e e p f o r e x p l o i t a t i o n .
T h e r e f o r e , t h e w a t e r - b e a r i n g a q u i f e r m o s t l y e x p l o i t e d i n t h i s a r e a i s t h e N u b i a n -
P o s t t a s s i l i a n S a n d s t o n e ( U p p e r P r i m a r y t o L o w e r C r e t a c e o u s ) , a l s o o f t e n c o n f i n e d
a n d a r t e s i a n , s e p a r a t e d f r o m t h e C a m b r o - O r d o v i c i a n b y m a r i n e a n d c o n t i n e n t a l
d e p o s i t s , t h e t h i c k n e s s o f w h i c h i s a b o u t 1 0 0 0 m . T h e m a r i n e l a y e r s ( L o w e r
S i l u r i a n , M i d d l e a n d L a t e D e v o n i a n , L o w e r C a r b o n i f e r o u s ) a r e e s s e n t i a l l y r e p r e s e n t e d
b y s h a l e s , s o m e t i m e s w i t h s o m e f i n e - g r a i n s a n d s t o n e i n t e r c a l a t i o n s . T h e c o n t i n e n t a l
d e p o s i t s ( L o w e r D e v o n i a n a n d U p p e r C a r b o n i f e r o u s ) s h o w s a n d s t o n e s a n d c l a s t i c
f a c i e s a l w a y s a s s o c i a t e d w i t h s a n d y c l a y s .
M o r e t h e n o r t h , t h e P a l a e o z o i c F o r m a t i o n f o r m s a l a r g e a n t i c l i n a l a r c h ,
o r i e n t e d w e s t - e a s t , t h e c r e s t o f w h i c h o u t c r o p s i n t h e J e b e l G a r g a f ( a l s o c a l l e d J e b e l
F e z z a n ) a t a b o u t 6 0 0 m a . s . l . , i m m e d i a t e l y t o t h e n o r t h o f t h e W a d i A s h S h a t i V a l l e y .
I A E A - A G - 1 58 /12 1 6 7
I n t h e l a t t e r , t h e P a l a e o z o i c a q u i f e r i s v e r y n e a r t h e s u r f a c e , s t i l l b e i n g a r t e s i a n , a n d
i s l a r g e l y e x p l o i t e d f o r i r r i g a t i o n p u r p o s e s . A c c o r d i n g t o P . P a l l a s , t h e J e b e l G a r g a f
z o n e c o u l d b e a n i m p o r t a n t r e c h a r g e a r e a f o r t h e d i f f e r e n t g r o u n d w a t e r u n i t s
f l o w i n g n o r t h w a r d s .
O n t h e n o r t h e r n f l a n k o f t h e J e b e l G a r g a f , t h e P a l a e o z o i c d e e p e n s a g a i n a n d i s
e x p l o i t e d o n l y i n t h e z o n e o f t h e A l J u f r a h , a f t e r w h i c h t h e d e p t h b e c o m e s
p r o h i b i t i v e f o r c u r r e n t w a t e r d r i l l i n g s . B e t w e e n t h e J e b e l G a r g a f a n d t h e A l J u f r a h ,
t h e w h o l e s e d i m e n t a r y s e r i e s i s p i e r c e d b y i n t r u s i v e b a s a l t w h i c h h a s b r o k e n
o u t t o f o r m t h e J e b e l S a w d a .
I n t h e z o n e o f t h e A l J u f r a h , t h e P a l a e o z o i c i s d i r e c t l y o v e r l a i n b y t h e L o w e r
M e s o z o i c s a n d s t o n e s , m a i n l y b e l o n g i n g t o t h e K i c l a F o r m a t i o n o f L o w e r C r e t a c e o u s
EXPLANAT IO N
Q uate rnary :
Lower Eocene-
Pa leocene:
I3 sa n d ,s it t s ,c la y s
Нт^7~Н W addan Form ation^ lim estones and gyp sum r = — =] , ГТ7ТТТП i- i J u ra s s ic + T r ia s +F ^ J m o r l s P iV / i/ i lim estones Upper P r im ary :
Upper C re taceous: 1Socna facies: F o s s il ife ro u s lim e ston esan d m arls
Lower Cretaceous: | . | K ic la - s a n d sto n e s and c la y sor N ub ian s a n d s to n e s
S h a le s a n d c la y sor
S a n d s and sa n d s to n e s
C a m b ro -o rd o v ic ia n s a n d s to n e s
[Т Т Т Т lim estones and m arls
Lower Prim ary :
C rystalline bedrock! *+ l
E x tru s iv e r o c k s : B a sa lt ic la v a s
FIG.2. Geological cross-sections (indicated in Fig.lj
IA E A - A G - 1 5 8 /1 2 1 6 9
a g e , w h i c h i s p r o b a b l y t h e m o s t i m p o r t a n t a q u i f e r o f t h e w h o l e n o r t h e r n p a r t o f
t h e i n v e s t i g a t e d a r e a . I t i s b e l i e v e d t h a t t h e g r o u n d w a t e r c o m i n g f r o m t h e s o u t h i s
d i s c h a r g i n g f r o m t h e P a l a e o z o i c i n t o t h e K i c l a , f o r w h i c h i t w o u l d a c t u a l l y c o n s t i t u t e
t h e m a j o r s o u r c e o f r e c h a r g e . I n t h e a r e a o f A l J u f r a h , a n o t h e r f o r m a t i o n c u r r e n t l y
b e i n g e x p l o i t e d i s t h e U p p e r C r e t a c e o u s l i m e s t o n e e x p l o i t e d i n S o c n a a r e a ( a r t e s i a n )
w h i c h a l s o w o u l d b e r e c h a r g e d b y t h e P a l a e o z o i c .
O t h e r i m p o r t a n t a q u i f e r s e x p l o i t e d i n t h e n o r t h a r e t h o s e o f t h e U p p e r
C r e t a c e o u s a g e - A i n T o b i , G h a r i a n ( a l s o c a l l e d N a l u t ) , a n d M i z d a h , c o n s i s t i n g o f
l i m e s t o n e a n d / o r d o l o m i t i c l i m e s t o n e . A l l t h e s e f o r m a t i o n s a r e s u p p o s e d t o d e r i v e
m o s t o f t h e i r w a t e r f r o m t h e K i c l a a q u i f e r , f r o m w h i c h t h e y w o u l d n o t b e s e p a r a t e d
b y a n y c o n t i n u o u s i m p e r m e a b l e b a r r i e r . O n e o f t h e m a j o r d i s c h a r g e p o i n t s o f a l l
t h e s e a q u i f e r s i s b e l i e v e d t o b e t h e i m p o r t a n t s p r i n g o f T a w u r g h a n e a r t h e
M e d i t e r r a n e a n c o a s t , a b o u t 4 0 k m s o u t h o f M i s r a t a h .
I n a w e s t - e a s t c r o s s - s e c t i o n , a n i m p o r t a n t g e o l o g i c a l f e a t u r e i s t h e l a r g e H u n
G r a b e n , s o u t h - n o r t h o r i e n t e d , ( a p p r o x i m a t e l y a l o n g t h e 1 6 t h m e r i d i a n ) s t a r t i n g
i n t h e z o n e o f A l J u f r a h ( F i g . 2 ) . H e r e , o n e o f t h e m a j o r h y d r o g e o l o g i c a l p r o b l e m s
i s t o v e r i f y w h e t h e r o r n o t t h e P a l a e o z o i c a q u i f e r i s d i s c h a r g i n g a c r o s s t h e f a u l t
s y s t e m i n t o t h e W a d d a n f o r m a t i o n ( E o c e n e ) i n t h e G r a b e n .
1 . S A M P L I N G A N D A N A L Y S E S
S e l e c t e d i s o t o p i c d a t a o n d e e p g r o u n d w a t e r s a m p l e s a r e r e p o r t e d i n T a b l e I .
T h e p r e s e n t w o r k i s b a s e d m a i n l y o n s t a b l e i s o t o p e v a r i a t i o n s i n g r o u n d w a t e r ;
h o w e v e r , 14C a n d 13C d a t a , w h e n a v a i l a b l e , h a v e a l s o b e e n r e p o r t e d . T r i t i u m d a t a
a r e n o t i n c l u d e d i n t h e t a b l e b e c a u s e t h e t r i t i u m c o n t e n t , m e a s u r e d i n p a r t o f t h e
s a m p l e s , w a s a l w a y s n e g l i g i b l e a s s h o u l d b e e x p e c t e d f o r w a t e r f r o m d e e p ,
c o n f i n e d a q u i f e r s .
T h e g r e a t m a j o r i t y o f t h e i s o t o p i c r e s u l t s u s e d i n t h i s p a p e r w e r e o b t a i n e d a t
t h e I A E A I s o t o p e H y d r o l o g y L a b o r a t o r y , b u t a f e w a d d i t i o n a l r e s u l t s h a v e b e e n
t a k e n f r o m o t h e r s o u r c e s . T h e d a t a d e s i g n a t e d b y t h e L e t t e r “ K ” i n T a b l e I w e r e
d e r i v e d f r o m R e f . [ 3 ] a n d w e r e o b t a i n e d a t t h e I n s t i t u t f ü r U m w e l t p h y s i k o f
t h e U n i v e r s i t y o f H e i d e l b e r g , F e d e r a l R e p u b l i c o f G e r m a n y , o n s a m p l e s c o l l e c t e d
i n 1 9 7 2 — 7 3 . A m o n g t h e d a t a r e p o r t e d i n R e f . [ 3 ] o n l y t h o s e o b t a i n e d o n s a m p l e s
d e r i v e d f r o m a r t e s i a n w e l l s h a v e b e e n u s e d a n d o t h e r d a t a , w h i c h c o u l d h a v e b e e n
a f f e c t e d b y c o n t a m i n a t i o n w i t h w a t e r f r o m t h e s h a l l o w a q u i f e r , w e r e d i s c a r d e d .
T h e d a t a d e s i g n a t e d b y t h e l e t t e r “ E ” w e r e o b t a i n e d f r o m s a m p l e s c o l l e c t e d
i n 1 9 7 5 b y E n e r g o p r o j e k t , a Y u g o s l a v i a n c o n s u l t i n g f i r m . T h e s t a b l e i s o t o p e
m e a s u r e m e n t s w e r e p e r f o r m e d a t t h e I n s t i t u t f ü r R a d i o h y d r o m e t r i e i n N e u h e r b e r g ,
F e d e r a l R e p u b l i c o f G e r m a n y , w h i l e t h e 1 4 C a n a l y s e s w e r e c a r r i e d o u t a t t h e
R u d e r B o s k o v i c I n s t i t u t e , Z a g r e b , Y u g o s l a v i a . A l l t h e i s o t o p i c d a t a f r o m t h i s s o u r c e
a r e d i s c u s s e d i n a n o t h e r p a p e r i n t h e s e p r o c e e d i n g s [ 4 ] .
T A B L E I . I S O T O P I C C O M P O S I T I O N O F S E L E C T E D G R O U N D W A T E R S A M P L E S
F R O M T H E L I B Y A N A R A B J A M A H I R I Y A
Code Location and/or name Depth (m) Aquifer Sl80 6D 14 C% 8,3C MC age (years BP)
MURZUK-WADI AJALK-62 Murzuk 150-700 Mesozoic -10.4 -74.1 2.5 ± 0.3 -8 .5 24 900 ± 3300T-5 Traghen, Experimental Farm 700 Mesozoic -10.76 -81.4K-51 Traghen 150-700 Mesozoic -10.4 -80.1 2.7 ± 0.3 -11.8 27 000 ± 2900T-4 Um El Araneb, Well No.3 273-553 Permo-Trias -10.84 -82.5K-48 Zawilah 150-700 Mesozoic -11.1 -83.7 2.0 ±0.3 28 100 ±4700“T-14 Ubari, Agrie. Project, Well No.l 424 Upper Trias -11.28 -81.5T-15 Ubari, Agrie. Project, Well No. 5 8 189 Lower Cretaceous -10.90 -80.7K-26 Ezzighen
WADI ASH SHATI
200 Mesozoic -76.7 22.2 ± 0.7 8200 ± 4100a
T-32 Edri, Piezometer No.8 600 Palaeozoic -10,66 -77.2A-12 Edri, Well 1-A 606 Palaeozoic -10.75 -76.7 0.0 ± 0.4 -13.34 >30 000T-33 Timisan 250 Palaeozoic -10.78 -78.2K-19 Wensrick 20-40 Palaeozoic -10.6 -75.8 0.8 ±0.3 -13.8 > 30 000T-27 Gotta 40 Palaeozoic -11.00 -80.6T-36 Al Gorda 250 Palaeozoic -11.18 -82.4K-13 Agar 60-100 Palaeozoic -10.9 -78.2 1.8 ± 0.3 -11.5 30 100 ± 3600
T-25 Agar, Artesian Spring Palaeozoic -11.12 -82.2K-14 Al Maharuga 60-100 Palaeozoic -78.9 0.2 ±0.1 -11.3 > 30 000
SALEM
et al.
T A B L E I . ( c o n t . )
Code Location and/or name Depth (m) Aquifer
T-37 Ain NossT-22 Brak, Artesian SpringK-8 Brak
T-38 Eshkeda, Agrie. Project, Well No.5A-11 Bir Al Gamajal, Well 1-177K-l 96 Km E of Brak
AL JUFRAHE-33 Well WS-8E-34 Wadi Zimam, Well No. 1T-39 Al Jufrah, Agrie. Project, Well J-l 8T-l 17 Al JufrahA-10 Al Jufrah, Well J-l 8AK-5 SocnaA-9 Socna, Well S-l
ASH SHUWAYREF-WADI ZAMZAMT-43 Ash Shuwayref, Well No.6
T-40 Wadi Qirzah, Well WS-2T-48 Well B-l - 39T-41 Wadi Zamzam, WeU ZZ-13
T-42 Wadi Zamzam, WeU ZZ-8
200
60-100
340
200
60-100
460
519380-700
193400
300
650
1000
1000-14001000
1000
PalaeozoicPalaeozoicPalaeozoicPalaeozoic
Palaeozoic
Palaeozoic
PalaeozoicPalaeozoic
PalaeozoicUpper CretaceousUpper CretaceousMesozoicUpper Cretaceous
KiclaKiclaKicla
KiclaKicla
SibO 8D 14C% 5n C 14C age (years BP)
-10.93 -79.8-11.02 -80.8-10.9 -81.7 <0.5 -11.2 >30 000-11.36 -82.0 0.4 + 0.4 -11.51 >25 900-11.32 -82.6 0.0 ± 0.4 -10.79 > 30 000
-80.9 0.8 + 0.3 -11.2 >30 000
-9.58 -69.0 (5.8 ± 0.3)b -3 .40 (10 400 ±-9.88 -72.7 (4 .5± 0 .3)b -3 .35 (12 300 ±
-10.67 -79.4 0.3 + 0.4 -5.55 >16 500-10.53 -77.6 0.6 ± 0.4 -4.96 >20*500-10.34 -75.5 0.0 ± 0.3 -4.66 >27 500-10.2 -79.6 0.8 ± 0.3 -5 .6 >23 700-10.51 -76.6 0.0+ 0.5 -5 .39 > 24 900
-9.68 -70.7 0.2 + 0.4 -5.96 >9600-9 .50 -69.5 0.0 ± 0.4 -4.80 >25 400-9.21 -68.1 0.2 + 0.4 -6.85 >10 700-9.42 -68.9 0.2 + 0.4 -6.23 >9900-9.37 -67.8
IAEA
-AG
-158/12
-Jго
T A B L E I . ( c o n t . )
Code Location and/or name Depth (m) Aquifer s l8o SD 14 C% S13C 14 С age (years BP)
IEBEL NEFUSAHT-50 Wadi Marmuta 471 Kicla -7.13 -45.9 2.4 ± 1.4 -8.55 >15 300
T-44 Wadi Faysal, Well No.3 505 Kicla -6 .82 -43.8 3.2 ± 0.5 -8.31 22 700 ± 3800
T-46 Wadi Faysal, Well 1-A 120 Upper Cretaceous -6 .20 -41.0 18.4 ± 1.1 -5 .06 4100 + 4100
T-45 Beni Walid, Well Nora-1 975 Kicla -6 .94 -45.7 -7.25
TAWURGHAT-100 Tawurgha, Well P-22 Gharian -9.49 -68.4 -6 .72T-102 Tawurgha, Well P-18 Mizdah -9 .39 -67.8 0.8 ± 0.5 -3.08 >13 800
T-103 TawuTgha 25 Miocene -7.92 -58.2 18.6 ± 1.9 -4 .30 2700± 4900
T-101 Tawurgha Spring -8.21 -60.4 2.8 ± 0.8 -3.38 16 400 ± 7400T-104 Tumminah, Agrie. Project Mizdah -9.55 -68.3 1.0 ± 0.5 -5.29 >19 500
a Values calculated assuming for the total dissolved carbonate specied the value 5 13C = 10 ± 2. b 14C measurements are from Ref. [4], but are believed to be unreliable by the authors.
SALEM
et al.
I A E A - A G - 1 58 /12 1 7 3
T h e d a t a d e s i g n a t e d b y t h e l e t t e r “ A ” w e r e o b t a i n e d a t t h e I A E A o n s a m p l e s
c o l l e c t e d i n 1 9 7 8 b y D r . A . B e l l o o f A q u a t e r , a n I t a l i a n c o n s u l t i n g f i r m o p e r a t i n g
i n t h e L i b y a n A r a b J a m a h i r i y a . A l l o t h e r d a t a , d e s i g n a t e d b y t h e l e t t e r “ T ” ,
r e f e r t o s a m p l e s c o l l e c t e d i n t h e p e r i o d 1 9 7 6 — 7 8 w i t h i n t h e f r a m e w o r k o f t h e L a n d
a n d W a t e r I n v e s t i g a t i o n P r o j e c t e x e c u t e d b y F A O a n d a n a l y s e d a t t h e I A E A .
T h e i s o t o p i c d a t a a r e e x p r e s s e d i n t h e u s u a l w a y s , i . e . 5 ° / 0 0 d i f f e r e n c e f r o m t h e
m e a n c o n t e n t o f o c e a n w a t e r f o r 1 8 0 a n d D a n d f r o m t h e P D B r e f e r e n c e f o r 13C .
E r r o r s o f s t a b l e i s o t o p e d e t e r m i n a t i o n s a r e a b o u t 0 . 1 °/00 f o r 5 1 8 0 , 1 % 0 f ° r ^ D ,
a n d 0 . 5 % o f ° r 5 13C , t h e l a s t b e i n g m a i n l y d u e t o t h e s a m p l i n g t e c h n i q u e .
T h e 1 4 C c o n t e n t i s e x p r e s s e d i n p e r c e n t o f “ m o d e r n ” , i . e . o f t h e 1 4 C c o n t e n t o f
p l a n t s i n 1 8 9 0 ( b e f o r e a n y i m p o r t a n t i n j e c t i o n i n t o t h e a t m o s p h e r e o f 14C - f r e e
C 0 2 f r o m f o s s i l f u e l c o m b u s t i o n a n d b e f o r e t h e 1 4 C r e l e a s e f r o m t h e r m o n u c l e a r
e x p l o s i o n s i n r e c e n t y e a r s ) . E r r o r s o f m e a s u r e m e n t ( l a ) a r e q u o t e d f o r e a c h
1 4 C v a l u e .
T h e 1 4 C c o n t e n t i s , i n g e n e r a l , v e r y l o w — o f t e n b e l o w t h e d e t e c t i o n l i m i t .
I n a f e w c a s e s , h o w e v e r , t h e 1 4 C i s w e l l a b o v e t h e d e t e c t i o n l i m i t . T h e a g e o f
g r o u n d w a t e r i s e v a l u a t e d f r o m t h e 1 4 C a n d 1 3 C d a t a b y u s i n g t h e f o r m u l a s :
t ( y e a r s ) = 8 2 6 7 I n ( C 0/ C )
1 0 0 ( 6 — ôc )Co = « * Л X О + 2e/1000)SG - 6C +e
w h e r e С i s t h e 1 4 C c o n c e n t r a t i o n i n t h e s a m p l e a n d C 0 i s t h e “ i n i t i a l ” o n e , c o r r e c t e d
f o r c h a n g e s d u e t o i n t e r a c t i o n w i t h t h e a q u i f e r m a t r i x ; 5 i s t h e 1 3 C c o n t e n t o f t h e
c a r b o n a t e s p e c i e s d i s s o l v e d i n t h e s a m p l e ( w h i c h i s a s s u m e d t o b e m a i n l y
b i c a r b o n a t e ) ; ö c i s t h a t o f t h e a q u i f e r c a r b o n a t e ; 6G i s t h a t o f t h e s o i l C 0 2 a t t h e
t i m e o f r e c h a r g e ; a n d e i s t h e f r a c t i o n a t i o n f a c t o r b e t w e e n b i c a r b o n a t e a n d C 0 2 .
T h e v a l u e s a d o p t e d f o r t h e c a l c u l a t i o n a r e : S c = 0 ± l ° / 0 0 , 6G = — 2 5 ± 1 °/0 0 ,
e = 8 ± 0.5°/oo- T h e a g e e r r o r ( 2 a ) g i v e n i n T a b l e I t a k e s i n t o a c c o u n t t h e
u n c e r t a i n t y o f a l l t h e s e t e r m s a n d , o f c o u r s e , t h e e r r o r a s s o c i a t e d w i t h t h e 1 4 C a n d
1 3 C d e t e r m i n a t i o n s . H o w e v e r , w h e n t h e 1 4 C c o n t e n t i s b e l o w 1 % o f m o d e r n , o r i s
e q u a l t o o r l e s s t h a n t w i c e i t s s t a n d a r d d e v i a t i o n , a m i n i m a l a g e i s g i v e n , f o r w h i c h
a v a l u e o f 30 000 y e a r s i s u s e d w h e n t h e c a l c u l a t i o n p r o d u c e s a h i g h e r f i g u r e -
a b o v e 30 000 y e a r s , i n f a c t , t h e a g e r e s o l u t i o n o f t h e m e t h o d b e c o m e s t o o p o o r .
T h e r e a s o n s f o r i s o t o p e v a r i a t i o n s i n n a t u r a l w a t e r s a n d t h e g e n e r a l p r i n c i p l e s
o f i s o t o p e s t u d i e s i n g r o u n d w a t e r a r e a s s u m e d t o b e f a m i l i a r t o t h e r e a d e r , w h o
o t h e r w i s e m a y f i n d t h i s t o p i c d i s c u s s e d i n R e f s [ 5 , 6 ] .
2 . I N T E R P R E T A T I O N O F T H E I S O T O P I C R E S U L T S
I m p o r t a n t v a r i a t i o n s a r e s h o w n b y t h e o x y g e n a n d h y d r o g e n s t a b l e i s o t o p e s ,
w h i c h c a n b e u s e d t o f o r m u l a t e h y p o t h e s e s o n t h e o r i g i n o f d e e p g r o u n d w a t e r a n d
1 7 4 S A L E M et al.
FIG.3. Geographical distribution o f 8 1S0 data in groundwater. Letters designate the aquifers:P = Palaeozoic, PT = Permo-Trias, M=Mesozoic, UT = Upper Trias, LC = Lower Cretaceous,K = Kicla (Lower Cretaceous), UC = Upper Cretaceous (Ain Tobi, Gharian, Mizdah), W= Waddan (Eocene), M = Miocene.
o n i t s m a i n r e g i o n a l f l o w p a t t e r n s , a n d t o i n v e s t i g a t e i n t e r c o n n e c t i o n s b e t w e e n
a q u i f e r s . F o r t h e s e p u r p o s e s , s t a b l e i s o t o p e s a p p e a r t o b e a l m o s t i d e a l e n v i r o n m e n t a l
t r a c e r s b e c a u s e t h e y a r e c o n s e r v a t i v e , w h i l e t h e c h e m i c a l c o m p o s i t i o n o f g r o u n d w a t e r
i s n o t , b u t d e p e n d s o n t h e a q u i f e r l i t h o l o g y a n d o n t h e d u r a t i o n o f i n t e r a c t i o n
b e t w e e n w a t e r a n d r o c k .
T h e g e o g r a p h i c a l d i s t r i b u t i o n o f s t a b l e i s o t o p e v a l u e s i s s h o w n i n F i g . 3 .
I n t h e m o s t s o u t h e r n a r e a , t h a t o f t h e M u r z u k d e p r e s s i o n a n d o f t h e W a d i A j a l V a l l e y ,
I A E A - A G - 1 58/12 175
DISTANCE FROM EDRI ( k m )
FIG .4. Variation of the stable isotope composition of groundwater of the Paleozoic aquifer in the Wadi Ash Shati Valley. Distances are calculated from Edri to an easterly direction (samples are listed in the same order in Table I).
t h e s t a b l e i s o t o p e c o m p o s i t i o n o f t h e g r o u n d w a t e r d e r i v e d f r o m t h e M e s o z o i c
a q u i f e r v a r i e s w i t h i n a n a r r o w r a n g e . T h e m e a n v a l u e s a r e : S 1 8 0 = — 1 0 . 8 1 °/00 a n d
5 D = — 8 0 . 1 °/0 0 , a n d t h e s t a n d a r d d e v i a t i o n s a r e 0 . 3 3 ° / o o a n d 3 . 2 ° / 0 0 , r e s p e c t i v e l y .
T h e 1 4 C i s l o w b u t n o t n e g l i g i b l e - t h r e e s a m p l e s f r o m t h e M u r z u k d e p r e s s i o n
a n a l y s e d b y K l i t z s c h e t a l . [ 2 ] g a v e s i m i l a r r e s u l t s w i t h a m e a n v a l u e 2 . 4 % o f m o d e m .
S u r p r i s i n g l y h i g h ( 2 2 . 2 % o f m o d e r n ) i s t h e 1 4 C c o n t e n t o f s a m p l e K - 2 6 , t h e m o s t
e a s t e r n s i t e s a m p l e d i n t h e W a d i A j a l a r e a , p r o b a b l y i n d i c a t i n g t h e v i c i n i t y o f a
r e c h a r g e a r e a ( n o t n e c e s s a r i l y a c t i v e n o w ) .
I n t h e W a d i A s h S h a t i V a l l e y , o n t h e n o r t h e r n s i d e o f t h e U b a r i S a n d S e a ,
t h e 1 4 C c o n t e n t o f g r o u n d w a t e r i s s u e d b y t h e P a l a e o z o i c a q u i f e r d r o p s t o n e g l i g i b l e
v a l u e s . T h e m e a n s t a b l e i s o t o p e r a t i o s a r e : 5 1 8 0 = — 1 0 . 9 6 7 o o a n d 5 D = — 7 9 . 7 °/00 w i t h s t a n d a r d d e v i a t i o n s o f 0 . 2 °/00 a n d 2 . 3 ° / 0 0 , r e s p e c t i v e l y . T h e s e v a l u e s a r e
p r a c t i c a l l y i d e n t i c a l t o t h o s e o b s e r v e d i n t h e M u r z u k - W a d i A j a l a r e a , a n d s u p p o r t
t h e h y p o t h e s i s t h a t t h e g r o u n d w a t e r h a s t h e s a m e o r i g i n i n t h e t w o a r e a s . I n o t h e r
w o r d s , t h e w a t e r i s s u e d b y t h e M e s o z o i c a q u i f e r i n t h e M u r z u k - W a d i A j a l z o n e w o u l d
d e r i v e f r o m t h e d e e p e r P a l a e o z o i c a q u i f e r b y u p w a r d s l e a k i n g t h r o u g h t h e
i n t e r m e d i a t e c l a y d e p o s i t s ( w i t h s a n d y l e n s e s ) , o r t h e M e s o z o i c a n d t h e P a l a e o z o i c
a q u i f e r s w o u l d h a v e b e e n r e c h a r g e d u n d e r s i m i l a r c o n d i t i o n s , b o t h i n t e r m s o f
r e c h a r g e a r e a c h a r a c t e r i s t i c s a n d o f c l i m a t i c c o n d i t i o n s . A c c o r d i n g t o P a l l a s , t h e
f i r s t o f t h e s e h y p o t h e s e s d o e s n o t s e e m a c c e p t a b l e , o w i n g t o t h e l a r g e t h i c k n e s s
( s e v e r a l h u n d r e d s o f m e t r e s ) o f t h e i m p e r m e a b l e d e p o s i t s s e p a r a t i n g t h e t w o a q u i f e r s .
A c l o s e r e x a m i n a t i o n o f s t a b l e i s o t o p e d a t a i n t h e W a d i A s h S h a t i V a l l e y
r e v e a l s t h a t a s m a l l b u t r e g u l a r v a r i a t i o n o c c u r s f r o m w e s t t o e a s t , a s s h o w n i n F i g . 4 .
1 7 6 S A L E M et al.
I n t h i s f i g u r e , o n l y d a t a o b t a i n e d a t t h e I A E A ( s a m p l e s T a n d A ) w e r e u s e d s o a s t o
e l i m i n a t e a n y s p r e a d o f v a l u e s d e r i v e d f r o m a n i m p e r f e c t i n t e r c a l i b r a t i o n o f
l a b o r a t o r i e s w h i c h c o u l d m a s k t h e s m a l l v a r i a t i o n o c c u r r i n g . A p a r a l l e l w e s t - e a s t
v a r i a t i o n i s a l s o s h o w n b y t h e 1 3 C v a l u e s a n d b y t h e c h e m i c a l c o m p o s i t i o n o f
g r o u n d w a t e r , c h a n g i n g b o t h i n s a l t c o n c e n t r a t i o n a n d i n a n i o n s p e r c e n t
d i s t r i b u t i o n . B o t h t h e c h e m i c a l a n d t h e i s o t o p i c t r e n d s i n d i c a t e a m i x i n g o f t w o
d i f f e r e n t w a t e r s ; o r , i n o t h e r w o r d s , a n i n f l o w i n t o t h e P a l a e o z o i c a q u i f e r o f a n
i s o t o p i c a l l y h e a v i e r w a t e r f r o m t h e w e s t w h i c h m i x e s w i t h t h e w a t e r c o m i n g f r o m
t h e s o u t h .
I n t h e a r e a n o r t h o f J e b e l G a r g a f a n d i n A l J u f r a h t h e s t a b l e i s o t o p e c o m p o s i t i o n
o f w a t e r o f t h e P a l a e o z o i c a q u i f e r s h o w s a m u c h l a r g e r v a r i a t i o n f r o m w e s t t o e a s t ,
i . e . f r o m t h e v a l u e s o f 5 1 8 0 = — 9 . 5 8 ° / 0 0 a n d 5 D = — 6 9 . 0 ° / o o o f s a m p l e E - 3 3 ( w e l l
W S - 8 ) t o t h o s e o f 5 1 8 0 = - 1 0 . 6 7 ° / o o a n d 5 D = - 7 9 . 4 °/00 o f s a m p l e T - 3 9 ( w e l l J - 1 8
o f t h e A l F u g r a h A g r i c u l t u r a l P r o j e c t ) , p a s s i n g t h r o u g h t h e i n t e r m e d i a t e v a l u e s o f
E - 3 4 ( w e l l N o . 1 a t W a d i Z i m a m ) . T h i s a g a i n i n d i c a t e s t h e i n f l o w o f a n i s o t o p i c a l l y
h e a v i e r w a t e r f r o m t h e w e s t , w h i c h h e r e w o u l d b e m u c h m o r e i m p o r t a n t t h a n i n t h e
W a d i A s h S h a t i a r e a 1 . O n t h e o t h e r h a n d , t h e m o r e n e g a t i v e б - v a l u e s o f s a m p l e
T - 3 9 , s i m i l a r t o t h o s e o f t h e P a l a e o z o i c i n t h e W a d i A s h S h a t i a r e a , w o u l d c o n f i r m
t h e a r r i v a l o f w a t e r f r o m t h e s o u t h . T h e 1 4 C a g e , i n c r e a s i n g f r o m w e s t t o e a s t ,
w o u l d a l s o b e i n a g r e e m e n t w i t h t h i s h y p o t h e s i s , b u t u n f o r t u n a t e l y 14 С
m e a s u r e m e n t s o f s a m p l e s E - 3 3 a n d E - 3 4 a r e u n r e l i a b l e [ 4 ] . C o n c e r n i n g 13C , t h e
a b r u p t c h a n g e i n v a l u e s w i t h r e s p e c t t o t h o s e f o u n d i n m o r e s o u t h e r n a r e a s s h o u l d
b e n o t e d . T h i s i s n o t i n c o n t r a d i c t i o n , h o w e v e r , w i t h a n o r i g i n o f w a t e r f r o m t h e
s o u t h b e c a u s e t h e 1 3 C ( a n d t h e 14C ) i s s t r o n g l y a f f e c t e d b y d i s s o l u t i o n -
p r é c i p i t a t i o n p r o c e s s e s o f c a r b o n a t e — t h e i n c r e a s e o f c a r b o n a t e c o n t e n t o b s e r v e d
i n t h e A l J u f r a h w e l l s , w i t h r e s p e c t t o t h e m o r e s o u t h e r n a r e a s , w o u l d t h e r e f o r e
f u l l y e x p l a i n t h e 1 3 C c h a n g e .
I n t h e A l J u f r a h a r e a , t h e w a t e r i n t h e M e s o z o i c a q u i f e r h a s t h e s a m e s t a b l e
i s o t o p e c o m p o s i t i o n a s t h a t o f t h e P a l a e o z o i c , c o n f i r m i n g t h a t t h e l a t t e r i s r e c h a r g i n g
t h e a q u i f e r s a b o v e . T h i s r e c h a r g e s e e m s , h o w e v e r , o f l i m i t e d i m p o r t a n c e b e c a u s e
m u c h l e s s n e g a t i v e ¿ ¡ - v a l u e s a r e o b s e r v e d i n t h e M e s o z o i c a q u i f e r s i n o t h e r p l a c e s .
B y c o n t r a s t , t h e P a l a e o z o i c a q u i f e r d o e s n o t s e e m t o s u p p l y w a t e r l a t e r a l l y t o t h e
W a d d a n f o r m a t i o n i n t h e H u n G r a b e n , a t l e a s t i n t h e A l J u f r a h a r e a . I n f a c t , t h e
i s o t o p i c c o m p o s i t i o n o f w a t e r f r o m t h e W a d d a n f o r m a t i o n i s q u i t e d i f f e r e n t , a s
o b s e r v e d i n t w o w e l l s s a m p l e d b y A q u a t e r a t H u n ( I C - 2 1 a n d I C - 2 2 ) , g i v i n g v a l u e s o f
— 8 . 4 °/00 f o r 1 8 0 a n d o f — 6 3 ° / 0 0 f o r d e u t e r i u m .
N o r t h o f A l J u f r a h , f i v e w e l l s e x p l o i t i n g t h e K i c l a a q u i f e r ( L o w e r C r e t a c e o u s )
w e r e s a m p l e d i n t h e a r e a b e t w e e n A s h S h u w a y r e f a n d t h e W a d i Z a m z a m V a l l e y .
1 P. Pallas suggests that this water could derive from recharge on the outcrops of the Palaeozoic in the Jebel Gargaf. However, the distribution of stable isotope values tends to indicate an origin from the west. It is hoped that future samples might shed light on this question.
I A E A - A G - 1 58 /12 1 7 7
T h e y d o n o t c o n t a i n 1 4 C i n a n y a p p r e c i a b l e a m o u n t a n d h a v e a v e r y u n i f o r m s t a b l e
i s o t o p e c o m p o s i t i o n w i t h m e a n v a l u e s o f S 1 8 0 = — 9 . 4 4 °/00 a n d 5 D = — 6 9 . 0 o/oo
( 0 . 1 8 a n d 1 . 2 °/00 a r e t h e r e s p e c t i v e s t a n d a r d d e v i a t i o n s ) . T h e s e v a l u e s r e p r e s e n t t h e
i s o t o p i c c o m p o s i t i o n o f t h e g r o u n d w a t e r c o m p o n e n t a r r i v i n g f r o m t h e w e s t , p e r h a p s
f r o m t h e H a m a d a a l H a m r a , w h i c h h a s b e e n a l r e a d y o b s e r v e d i n t h e t w o w e l l s ,
E - 3 3 a n d E - 3 4 , w e s t o f A l J u f r a h . I t m i g h t b e i n t e r e s t i n g t o n o t e t h a t s i m i l a r v a l u e s
o f s t a b l e i s o t o p i c c o m p o s i t i o n o f g r o u n d w a t e r a r e a l s o f o u n d i n t h e a r e a o f G h a d a m e s
o n t h e w e s t s i d e o f t h e H a m a d a a l H a m r a , n e a r t h e p o i n t w h e r e t h e A l g e r i a n ,
T u n i s i a n a n d L i b y a n b o r d e r s m e e t .
S t a b l e i s o t o p e § - v a l u e s , t h o s e s o f a r r e p o r t e d , f o r t h e R e z z a n a q u i f e r s a n d t h e
K i c l a f o r m a t i o n , a r e d e f i n i t e l y m u c h m o r e n e g a t i v e t h a n t h o s e o b s e r v e d i n a f e w
s h a l l o w g r o u n d w a t e r s a m p l e s o f c e r t a i n r e c e n t o r i g i n . S i m i l a r l o w h e a v y i s o t o p e
c o n t e n t s h a v e b e e n o b s e r v e d i n m a n y o t h e r d e e p w e l l s a n d s p r i n g s i n t h e S a h a r a
[ 3 , 7 - 1 2 ] a n d a p p e a r t o b e c h a r a c t e r i s t i c o f o l d g r o u n d w a t e r w h i c h w a s r e c h a r g e d
u n d e r m o r e h u m i d c l i m a t i c p e r i o d s t h a t o c c u r r e d d u r i n g t h e Q u a t e r n a r y .
I n t h e J e b e l N e f u s a h a r e a , t h e w a t e r f r o m t h e K i c l a a q u i f e r e x h i b i t s ô - v a l u e s
w h i c h a r e d e f i n i t e l y m o r e p o s i t i v e t h a n t h o s e e n c o u n t e r e d u p t o n o w i n t h e d e e p
a q u i f e r s . T h e m e a n v a l u e s o f t h e s e s a m p l e s a r e , i n f a c t , 5 1 8 0 = — 6 . 9 6 ° / 0 0 a n d
S D = — 4 5 . 1 % 0 w i t h a v e r y l i m i t e d s c a t t e r . T h e s e v a l u e s a r e m o s t p r o b a b l y
c h a r a c t e r i s t i c o f g r o u n d w a t e r l o c a l l y r e c h a r g e d . T h e 1 4 C c o n t e n t , a l t h o u g h l o w ,
i s n o t n e g l i g i b l e — u p t o 3 . 2 % o f m o d e m i n t h e K i c l a w a t e r , c o r r e s p o n d i n g t o a n
a g e o f a b o u t 2 0 0 0 0 y e a r s , a n d u p t o 1 8 . 4 % i n t h e U p p e r C r e t a c e o u s w h i c h w o u l d
i n d i c a t e a n a g e r a n g i n g f r o m R e c e n t t o 8 0 0 0 y e a r s .
I n t h e a r e a o f T a w u r g h a , n e a r t h e c o a s t , t w o d i f f e r e n t g r o u p s o f s t a b l e i s o t o p e
v a l u e s a r e f o u n d . I n t h e G h a r i a n a n d M i z d a h a q u i f e r s ( s a m p l e s T - 1 0 0 , 1 0 2 a n d 1 0 4 )
t h e m e a n 5 v a l u e s ( 5 1 8 0 = — 9 . 4 8 ° / 0 0 , 5 D = — 6 8 . 2 °/0 0 ) a r e p r a c t i c a l l y i d e n t i c a l t o
t h o s e o f t h e K i c l a a q u i f e r o f t h e a r e a A s h S h u w a y r e f - W a d i Z a m z a m ; t h i s w o u l d
i n d i c a t e t h a t t h e K i c l a , t h e G h a r i a n a n d t h e M i z d a h a q u i f e r s a r e a l l i n t e r c o n n e c t e d ,
a n d t h a t w a t e r f l o w s f r o m t h e f i r s t i n t o t h e o t h e r s . I n t h e M i o c e n e a q u i f e r
( s a m p l e T - 1 0 3 ) t h e h e a v y i s o t o p e c o n t e n t i s d e f i n i t e l y h i g h e r : S l s O = — 1 . 9 2 ° / 0 0 a n d
S D = — 5 8 . 2 ° / 0 0 , w h i c h m i g h t r e s u l t f r o m a m i x i n g o f w a t e r d e r i v e d f r o m t h e
u n d e r l y i n g U p p e r C r e t a c e o u s a q u i f e r s w i t h w a t e r r e c h a r g e d b y r a i n f a l l w e s t o f
T a w u r g h a . T h e l a t t e r c o m p o n e n t s h o u l d h a v e 5 - v a l u e s o f a b o u t — 5 °/00 f o r 1 8 0 a n d
o f — 3 0 ° / o o f o r d e u t e r i u m : a s a c o m p a r i s o n , t h e a v e r a g e i s o t o p i c c o m p o s i t i o n o f
r a i n f a l l a t T u n i s - C a r t h a g e ( a n I A E A - W M O n e t w o r k s t a t i o n i n T u n i s i a ) h a s b e e n
ô 1 8 0 = — 4 . 6 °/00 a n d 5 D = — 2 6 ° / 0 0 o v e r a p e r i o d o f a b o u t f i v e y e a r s ; a n d t h a t o f
w a t e r i n t h e Q u a t e r n a r y a q u i f e r a t M u t r a d o n t h e M e d i t e r r a n e a n c o a s t i s
5 180 = — 5 . 0 3 ° / o o a n d S D = — 2 8 . 6 ° / 0 0 . A s s u m i n g t h e s e l a s t v a l u e s , t h e c o n t r i b u t i o n
o f p r e c i p i t a t i o n t o t h e r e c h a r g e o f t h e M i o c e n e a q u i f e r w o u l d b e a b o u t 3 0 % a s
o p p o s e d t o 7 0 % d e r i v e d f r o m t h e K i c l a f o r m a t i o n .
T h e i s o t o p i c j p m p o s i t i o n o f w a t e r f r o m t h e T a w u r g h a s p r i n g s ( 5 1 8 0 = — 8 . 2 l 7 0 0 ,
S D = — 6 0 . 4 ° / o o ) i s d e f i n i t e l y s i m i l a r t o t h a t o f t h e M i o c e n e a q u i f e r f r o m w h i c h i t
17 8 S A L E M et al.
m a i n l y d e r i v e s . T h e d i r e c t c o n t r i b u t i o n o f t h e U p p e r C r e t a c e o u s a q u i f e r s t o t h e
s p r i n g d i s c h a r g e w o u l d b e m i n o r , a n d a t t h e t i m e o f t h i s s a m p l i n g w a s a b o u t
2 0 ± 1 5 % o f t h e t o t a l a s c o m p u t e d f r o m 1 8 0 a n d D c o n t e n t . T h e c h e m i c a l
c o m p o s i t i o n o f t h e s p r i n g a l s o c o m p a r e s w e l l w i t h t h a t o f t h e M i o c e n e a q u i f e r .
T h e 1 4 C c o n t e n t a p p a r e n t l y c o n t r a d i c t s t h i s c o n c l u s i o n b e i n g o n l y 2 . 8 % o f
m o d e r n a s c o m p a r e d w i t h 1 8 . 6 % o f s a m p l e T - 1 0 3 . H o w e v e r , t h e l a t t e r i s d e r i v e d
f r o m a w e l l o n l y 2 5 m d e e p a n d , t h e r e f o r e , i t r e p r e s e n t s o n l y t h e w a t e r i n t h e u p p e r
p a r t o f t h e M i o c e n e a q u i f e r w i t h p r o b a b l y a h i g h e r 1 4 C c o n t e n t t h a n t h e b u l k o f
w a t e r o f t h e w h o l e a q u i f e r . T h i s w o u l d e x p l a i n w h y t h e 1 4 C c o n t e n t o f t h e s p r i n g
i s m u c h l e s s t h a n t h a t o b s e r v e d i n s a m p l e T - 1 0 3 .
3 . C O N C L U S I O N S
W e n o w t r y t o s u m m a r i z e t h e i n f o r m a t i o n g a i n e d w i t h t h e e n v i r o n m e n t a l
i s o t o p e s o n t h e f l o w p a t t e r n s o f d e e p g r o u n d w a t e r . T h e w a t e r i n t h e P a l a e o z o i c
a q u i f e r i n F e z z a n f l o w s n o r t h w a r d s , a n d i t i s f o u n d u n t i l t h e a r e a o f A l J u f r a h w h e r e
i t d i s c h a r g e s a l s o i n t h e M e s o z o i c a q u i f e r . T h e K i c l a a q u i f e r i s n o t s i g n i f i c a n t l y
r e c h a r g e d b y t h e P a l a e o z o i c , w i t h t h e e x c e p t i o n o f t h e A l J u f r a h a r e a , b u t i t r e c e i v e s
i t s w a t e r f r o m t h e w e s t , p o s s i b l y f r o m t h e H a m a d a A l H a m r a .
T h e w a t e r i n t h e K i c l a t h e n f l o w s t o t h e n o r t h a n d i s f o u n d t o f l o w i n t o t h e
U p p e r - C r e t a c e o u s a q u i f e r i n t h e a r e a o f M i s r a t a h . T h e T a w u r g h a S p r i n g , d e r i v i n g
m o s t o f i t s w a t e r f r o m t h e M i o c e n e , d o e s n o t a p p e a r t o b e a m a j o r d i r e c t d i s c h a r g e
p o i n t o f t h e U p p e r C r e t a c e o u s a q u i f e r s . T h e s e a q u i f e r s , h o w e v e r , m i g h t d i s c h a r g e
f i r s t i n t h e M i o c e n e , t h e w a t e r o f w h i c h w o u l d i n p a r t d e r i v e a l s o f r o m i n f i l t r a t i o n
o r l o c a l r a i n f a l l .
T h e K i c l a a q u i f e r i s a l s o r e c h a r g e d i n t h e a r e a o f t h e J e b e l N e f u s a , a s i n d i c a t e d
b y t h e d i f f e r e n t i s o t o p i c g r o u n d w a t e r c o m p o s i t i o n .
A C K N O W L E D G E M E N T S
T h a n k s a r e d u e t o P . P a l l a s a n d G . K r u s e m a n , w h o r e a d t h e m a n u s c r i p t a n d
s u g g e s t e d m o d i f i c a t i o n s . A l s o t h e u s e f u l d i s c u s s i o n s w i t h M . T a k a , w h o g u i d e d t w o
o f u s t o a f i e l d t r i p i n t h e F e z z a n , a r e g l a d l y a c k n o w l e d g e d . A . E l D i e b a n d
A . M ’ S a t a r h e l p e d i n s a m p l i n g i n t h e F e z z a n . T h a n k s a r e a l s o , d u e t o t h e s t a f f o f
t h e I s o t o p e H y d r o l o g y L a b o r a t o r y o f I A E A , w h o c a r r i e d o u t t h e i s o t o p e a n a l y s e s .
I A E A - A G - 1 58/12 1 7 9
R E F E R E N C E S
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[6] BR A D LEY, E., BROWN, R.M., GONFIANTINI, R., PAYNE, B .R ., PRZEW LOCKI, K„ SAUZAY, G., YEN , C.K., YU R T SEV ER , Y ., “Nuclear techniques in groundwater hydrology, groundwater studies” , An International Guide for Research and Practice, UNESCO, Paris (1 9 7 2 ) Chap. 10.
[7] DEGENS, E.T., Geochemische Untersuchungen von Wässern aus der Ägyptischen Sahara, Geol. Rundsch. 5 2 ( 1 9 6 2 ) 6 2 5 - 3 9 .
[8] CONRAD, G., FONTES, J.-Ch., “Hydrologie isotopique du Sahara Nord-Occidental, Isotope Hydrology 1970 (Proc. Symp. Vienna, 1970), IAEA, Vienna (1 9 7 0 ).
[9 ] GAT, J ., Comments on the Stable Isotope Method in Regional Groundwater Investigations, Water Resour. Res. 7 (1 9 7 1 ) 9 8 0 —93.
[10] GONFIANTINI, R., CONRAD, G., FONTES, J.-Ch„ SAUZAY, G., PAYN E, B.R ., “Etude isotopique de la nappe du continental intercalaire et de ses relations avec les autres nappes du Sahara septentrional,” Isotope Techniques in Groundwater Hydrology 19 7 4 (Proc.Symp. Vienna 1 974) 1, IAEA, Vienna (1 9 7 4 ) 2 2 7 - 4 1 .
[1 1 ] SONNTAG, C., KLITSCH, E., LOEHNERT, P., MÜNNICH, K.O., E L SHAZLY, E.M., KALINKE, C., THORWEIHE, U , W EISTR O FFER , K., SWAILEM, F.M ., “ Paleoclimatic information from deuterium and oxygen-18 in carbon-14-dated north Saharian groundwaters: Groundwater formation in the past,” Isotope Hydrology 1978 (Proc. Symp. Neuherberg (1 9 7 8 ) 2, IAEA, Vienna (1 9 7 9 ) 5 6 9 - 8 1 .
[1 2 ] EDMUNDS, W.M., WRIGHT, E.P., Groundwater recharge and paleoclimate in the Sirte and Kufra Basins, Libya, J. Ну drol. 4 0 ( 1 9 7 9 ) 2 1 5 —41.
IA E A - A G - 1 58/13
R E C H A R G E O F G R O U N D W A T E R S IN
A R I D A R E A S : C A S E O F T H E
D J E F F A R A P L A I N IN T R I P O L I T A N I A ,
L I B Y A N A R A B J A M A H I R I Y A
M . A L L E M M O Z
G r o u p e m e n t d ’ E t u d e s e t d e
R é a l i s a t i o n s d e s S o c i é t é s
d ’ A m é n a g e m e n t R é g i o n a l ,
N î m e s
P h . O L I V E *
C e n t r e d e R e c h e r c h e s G é o d y n a m i q u e s ,
T h o n o n - l e s - B a i n s ,
F r a n c e
Abstract
RECHARGE OF GROUNDWATERS IN ARID A REAS: CASE OF THE D JE FFA R A PLAIN IN TRIPOLITANIA, LIBYAN ARAB JAM AHIRIYA.
By means of soil water contents and variations of piezometric levels and tritium concentration, the recharge of groundwaters is determined. If direct infiltration of precipitation is negligible, the major recharge mechanism is the infiltration through the wadi beds during floods.
T w o q u i t e d i f f e r e n t m o r p h o l o g i c a l a s p e c t s c h a r a c t e r i z e T r i p o l i t a n i a : t o t h e
s o u t h , J e b e l N e f u s a a n d t o t h e n o r t h t h e D j e f f a r a p l a i n ( F i g . l ) . J e b e l N e f u s a
a p p e a r s a s a m o u n t a i n a r e a c u l m i n a t i n g a t 9 0 0 m n e a r G h a r y a n a n d i s d e e p l y
c l e a v e d b y r i v e r s o r w a d i s . T h e D j e f f a r a p l a i n o f f e r s a v e r y l e v e l s u r f a c e , t h e
g r a d i e n t o f w h i c h d o e s n o t e x c e e d 1 % . T h e m i n o r b e d s o f w a d i s a r e d i s t i n c t l y
o u t l i n e d .
1 . M A I N H Y D R O C L I M A T O L O G I C A L O U T L I N E S
O n F i g . 2 a r e s h o w n t h e a v e r a g e m o n t h l y v a l u e s o f p r e c i p i t a t i o n i n m i l l i m e t r e s ,
m o i s t u r e i n p e r c e n t , a i r t e m p e r a t u r e i n c e n t r i g r a d e a n d p o t e n t i a l é v a p o t r a n s p i r a t i o n
( T u r c ’s f o r m u l a ) a t t h e A l A z i z i y a h ( 1 2 5 m ) a n d G h a r y a n ( 7 2 5 m ) s t a t i o n s .
I t s h o u l d b e n o t e d t h a t r a i n b u r s t s o c c u r i n a u t u m n ( O c t o b e r , N o v e m b e r ,
D e c e m b e r ) a n d i n w i n t e r ( J a n u a r y , F e b r u a r y , M a r c h ) . T h e i r d i s t r i b u t i o n i n t i m e
* Present address: Direction de l’Hydraulique, Divisions des Ressources en Eau,B.P.A 4 8 , Fez, Morocco
181
1 8 2 A L L E M M O Z and O L I V E
FIG .l. The two different morphological aspects o f Tripolitania.
o r s p a c e v a r i e s g r e a t l y a n d t h e i n f l u e n c e o f r e l i e f i s v e r y c l e a r . T h e d a i l y v a r i a t i o n s
o f a i r m o i s t u r e a r e s u c h t h a t t h e d e w a n d t h e m o r n i n g f o g a r e f a r f r o m n e g l i g i b l e .
F i n a l l y , l e t u s m e n t i o n t h e s u d d e n n e s s a n d v i o l e n c e o f f l o o d s w h i c h , s i n c e
t h e y a r e i n d i r e c t r e l a t i o n t o p r e c i p i t a t i o n , o c c u r i n a u t u m n a n d i n w i n t e r . T h e
m a i n p a r t o f t h e f l o w i s o v e r i n a f e w h o u r s . N o w a d i i s p e r e n n i a l i n T r i p o l i t a n i a .
2 . E V A L U A T I O N O F I N F I L T R A T I O N
T h r e e k i n d s o f d a t a h a v e b e e n u s e d t o m e a s u r e i n f i l t r a t i o n i n t h i s a r e a : t h e
e v o l u t i o n o f m o i s t u r e m e a s u r e d w i t h a n e u t r o n p r o b e i n t h e u n s a t u r a t e d z o n e , t h e
v a r i a t i o n s o f p i e z o m e t r i c l e v e l s , a n d t h e t h e r m o n u c l e a r t r i t i u m c o n t e n t o f g r o u n d
w a t e r s .
2 . 1 . N e u t r o n p r o b e
A s y s t e m a t i c i n v e s t i g a t i o n o f 1 0 s i t e s f o r o v e r o n e y e a r a l l o w e d u s t o e v a l u a t e
t h e r e a l é v a p o t r a n s p i r a t i o n i n t h e a r e a c o n s i d e r e d [ 1 ] . S i t e 3 , c o v e r e d w i t h
80
60
¿0
20
0
60
50
40
30
24
20
16
12
8
160
120
80
40
!. >
I A E A - A G - 1 58/13 1 8 3
Precipitations (mm) GHARYAN (1924 -1975)
Zcnd^AL AZIZIYAH - /(1920 -1 9 7 5 )
f c
Humidity (Vo) . GHARYAN ( 1 92 6 -1 9 75 ) " ond( AL AZIZIYAH _
'(1926 - 1975)
Temperature ( ”C) GHARYAN (1924 -1975)
and AL AZIZIYAH *-"^(1913 - 1975)
Potentiol évapotranspiration (mm) GHARYAN (1959 -1975)
S O N D J F M A M J J A
verage monthly precipitation values (in mm).
1 8 4 A L L E M M O Z and O L I V E
H u m i d i t y ( % )0 5 10 15
v e g e t a t i o n ( e u c a l y p t u s ) a n d l o c a t e d a s s h o w n b y F i g . l , a l l o w s t h e r e a l é v a p o
t r a n s p i r a t i o n t o b e m e a s u r e d . I t g o e s t h r o u g h m u d d y s a n d , w i t h m o r e c l a y a t
1 . 9 0 m . O n 9 M a r c h 1 9 7 4 , t h e w a t e r s t o r e d i n t h e 0 — 1 2 0 c m s e c t i o n a m o u n t e d
t o 88 m m ( F i g . 3 ) . O n 1 2 — 1 3 M a r c h , r a i n s o f 5 8 m m r e s u l t e d i n a n i n c r e a s e o f
t h e s t o r e d w a t e r w h i c h w a s 1 3 7 m m h i g h o n 1 7 M a r c h . T h i s w a t e r w a s s u b j e c t e d
t o é v a p o t r a n s p i r a t i o n a n d o n 2 1 A p r i l t h e s t o r e d w a t e r a g a i n f e l l t o 8 5 m m . S o
t h e a v e r a g e r e a l é v a p o t r a n s p i r a t i o n c a l c u l a t e d d u r i n g t h e s e 3 5 d a y s ( f r o m 1 7 M a r c h
t o 2 1 A p r i l ) w a s 1 . 5 m m / d .
I n f a c t t h e r e a l é v a p o t r a n s p i r a t i o n ( E T R ) v a r i e s g r e a t l y a c c o r d i n g t o t h e
v a r i o u s s i t e s ( e f f e c t o f v e g e t a t i o n a n d o f t h e p r o f i l e t e x t u r e ) a n d t o t h e s e a s o n .
F r o m N o v e m b e r t o F e b r u a r y ( r a i n y m o n t h s a n d l o w t e m p e r a t u r e ) t h e E T R v a r i e s
f r o m 0 . 8 t o 1 . 3 m m d a i l y . I n M a r c h a n d A p r i l ( b e g i n n i n g o f t h e d r y s e a s o n ) , i t
r i s e s f r o m 1 . 5 t o 2 . 5 m m d a i l y . T h e n , d u r i n g s u m m e r , i t i s a l m o s t r e d u c e d t o
n o t h i n g ( l e s s t h a n 0 . 5 m m / d ) s i n c e w a t e r r e s e r v e s a v a i l a b l e i n t h e s o i l a r e e x h a u s t e d .
T h e s a m e c a n b e s a i d f o r a u t u m n a n d w i n t e r m o n t h s i f i t d o e s n o t r a i n . I t s h o u l d
b e n o t e d t h a t t h e E T R n e v e r h a s a n y c o n s t a n t v a l u e — i t v a r i e s g r e a t l y i n r e l a t i o n
t o t h e s a t u r a t i o n d e g r e e o f t h e s o i l ’s f i r s t d e c i m e t r e s .
S i t e 7 s h o w s s t r a t i f i e d a n d c l a y - s i l t l a y e r s n e a r 1 a n d 2 m . O n
3 S e p t e m b e r 1 9 7 4 , t h e s t o r e d w a t e r f o r t h e 0 — 5 4 0 c m p o r t i o n a m o u n t e d t o
5 0 0 m m ( F i g . 4 ) . O w i n g t o t h e f l o o d o f 2 0 O c t o b e r , i t w a s s u b m e r g e d b y 1 . 1 4 0 m m
w a t e r , w h i c h d i s a p p e a r e d i n s i x d a y s , a n d t h e f i r s t m e a s u r e m e n t r e c o r d e d a f t e r
Humididy (%)
IA E A -A G -1 58/13 185
t h i s f l o o d o n 3 N o v e m b e r i n d i c a t e d 9 0 0 m m s t o r e d w a t e r , i . e . a n i n c r e a s e o f 4 0 0 m m
i n t h e r e s e r v e s . I t m u s t b e a d m i t t e d t h a t b e t w e e n 3 S e p t e m b e r a n d 2 0 O c t o b e r t h e
E T R w a s a t z e r o ( d r y s o i l ) , a n d b e t w e e n 2 0 O c t o b e r a n d 3 N o v e m b e r i t w a s
n e g l i g i b l e w i t h r e s p e c t t o t h e w a t e r s h e e t w h i c h w a s r e s o l v i n g . A s a m a t t e r o f f a c t ,
o u t o f t h e 1 1 4 0 m m , 4 0 0 m m s e r v e t o m o i s t e n t h e g r o u n d a g a i n ; t h e r e f o r e , 7 0 0 m m
w o u l d r e m a i n t h a t h a v e n e c e s s a r i l y i n f i l t r a t e d .
T h i s s i t u a t i o n i s a n o u t s t a n d i n g i l l u s t r a t i o n o f t h e d u a l m e c h a n i s m o f
i n f i l t r a t i o n . T h e f i l l i n g o f t h e b i g c o n d u i t s ( m a c r o p o r o s i t y ) i s r e s p o n s i b l e f o r a
s p e e d y t r a n s m i s s i o n o f p a r t o f t h e w a t e r ( 7 0 0 m m ) t o l o w e r l e v e l s , t h e n t h e f i l l i n g
o f f i n e c o n d u i t s ( m i c r o p o r o s i t y ) a l l o w s a s l o w i n c r e a s e i n t h e m o i s t u r e r a t e ( f r o m
5 0 0 t o 9 0 0 m m ) . F i n a l l y , i f t h e p r o f i l e o f 1 6 N o v e m b e r i s c o m p a r e d w i t h t h a t
o f 3 N o v e m b e r , w h e n t h e m o i s t e n i n g f r o n t h a d r e a c h e d 4 2 0 c m , i t i s n o t e d t h a t o n
1 6 N o v e m b e r t h e f r o n t h a d g o n e b e y o n d t h e l o w e s t a l t i t u d e ( 5 4 0 c m ) a n d t h a t t h e
p r o f i l e b e g a n t o d r y o u t a b o v e 4 2 0 c m .
S i t e 1 0 , c o m p o s e d o f a l t e r n a t i n g l a y e r s o f s a n d ( 1 0 % o f m o i s t u r e ) a n d c l a y
s i l t ( 2 0 t o 3 0 % o f m o i s t u r e ) , b r i n g s o u t l a t e r a l i n f l o w s . F i r s t l e t u s c o n s i d e r t h e
0 — 2 5 0 c m s e c t i o n ( F i g . 5 ) . O n 9 N o v e m b e r 1 9 7 4 , t h e w a t e r s t o c k w a s 5 3 5 m m
h i g h , i t f e l l t o 4 4 0 m m o n 2 0 F e b r u a r y 1 9 7 5 a f t e r p r e c i p i t a t i o n o f 8 0 m m i n
D e c e m b e r a n d J a n u a r y . D u r i n g t h i s p e r i o d , t h e E T R c o u l d b e e s t i m a t e d a t a b o u t
1 0 0 m m . T h e r e f o r e , 7 5 m m w e r e , i n f i l t r a t e d . F o r t h e 2 5 0 — 4 9 0 c m s e c t i o n , t h e
1 8 6 A L L E M M O Z and O L I V E
Humidity (%)
FIG.5. Site 10: Stored water measurements in relation to precipitation and infiltration from November 1974 to February 1975.
w a t e r s t o c k r o s e f r o m 1 9 0 m m o n 9 N o v e m b e r t o 3 6 0 m m o n 2 0 F e b r u a r y , i . e .
a n i n c r e a s e o f 1 7 0 m m , w h i c h c o u l d o n n o a c c o u n t b e e x p l a i n e d b y i n f i l t r a t i o n
i n t o t h e s u p e r f i c i a l a r e a , b e c a u s e t h e E T R s h o u l d t h e n h a v e b e e n a t z e r o d u r i n g
t h e s e t h r e e w i n t e r m o n t h s . T h e o n l y p o s s i b l e e x p l a n a t i o n f o r t h i s i n c r e a s e o f t h e
w a t e r s t o c k i n t h e 2 5 0 — 4 9 0 c m a r e a i s t o b e s o u g h t i n a l a t e r a l w a t e r i n f l o w
i s s u i n g f r o m t h e w a d i f l o w i n g c l o s e b y . T h i s f l o w i s j u s t b e l o w t h e c l a y s i l t l e v e l
l o c a t e d a t 2 0 0 c m . P r o f i l e s d e f i n e d b e t w e e n 9 N o v e m b e r a n d 2 0 F e b r u a r y
i l l u s t r a t e d t h i s p h e n o m e n o n . I t s h o u l d b e n o t e d t h a t t h i s w a t e r w a s p r e s e r v e d
f r o m t h e E T R b y t h e o v e r l y i n g c l a y l a y e r a n d w a s c o n v e y e d t o t h e g r o u n d w a t e r
t h r o u g h s a n d d r a i n i n g .
T h u s , i f t h e p r e c i p i t a t i o n s a r e t a k e n b a c k c o m p l e t e l y b y t h e E T R ( s i t e 3 ) ,
t h e y n e v e r t h e l e s s c o n t r i b u t e t o m a i n t a i n i n g i n t h e s o i l a w a t e r s t o c k w h i c h i s u s e d
b y v e g e t a t i o n d u r i n g t h e s i x m o n t h s w e t s e a s o n . T h e g r o u n d w a t e r r e c h a r g e o c c u r s
a f t e r t h e f l o o d s i n t h e w a d i b e d s , i n c l o s e d d i p s c o n s t i t u t i n g t h e t e r m i n a t i o n o f
w a t e r d i s t r i b u t i o n a r e a s ( s i t e 7 ) , a n d i n f l o o d - s l a c k e n i n g a r e a s ( s i t e 1 0 ) . I n t h e s e
s e l e c t a r e a s i n f i l t r a t i o n i s m a i n l y d u e t o m a c r o p o r o s i t y .
2 . 2 . P i e z o m e t r i c v a r i a t i o n s
F i r s t l e t u s c o n s i d e r t h e s e a s o n a l v a r i a t i o n s . T h e P Z H 3 p i e z o m e t e r i s s i t u a t e d
i n a n o v e r f l o w a r e a o f t h e A l H i r a w a d i . I n t h i s a r e a , p u m p f l o w s a r e l o w . I t w a s
IA E A - A G - 1 58/13 1 87
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P iezom eter PZH .3
i i____:_I_uFloods
I I________I I______ L _______ I________I________I------------ L .
- 9 6 2 ~Hi
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- 9 6 -6 У-96 .8 'S £- 9 7 S
J J A S O N D | J F M A M J J
1 9 7 4 1 9 7 5
FIG. 6. Piezometric variations in the overflow area o f the Al Hira wadi.
n o t e d t h a t t h e f l o o d o f 2 0 O c t o b e r 1 9 7 4 ( l a g t i m e o f 2 0 y e a r s ) e n a b l e d t h e s t a t i c
l e v e l t o r i s e a g a i n b y 0 . 4 m ( F i g . 6 ) . T h e i m p a c t z o n e o f t h e r e c h a r g e o f t h e A l H a r i
w a d i w a s e s t i m a t e d a t 3 0 0 X 1 0 6 m 2 . T h e g r o u n d w a t e r a v e r a g e u p w e l l i n g , f u r t h e r
t o t h e O c t o b e r f l o o d , a m o u n t e d t o 0 . 1 5 m i n f o r m a t i o n s w h e r e t h e s t o r a g e c o e f f i
c i e n t , c a l c u l a t e d a f t e r p u m p i n g t e s t s , v a r i e d f r o m 5 t o 1 0 % . S o t h e i n f i l t r a t e d
v o l u m e w a s b e t w e e n 2 . 2 5 a n d 4 . 5 X 1 0 6 m 3 . C o n s i d e r i n g t h a t t h e v o l u m e o f t h e
O c t o b e r f l o o d w a s 1 2 X 1 0 6 m 3 ( A l H i r a w a d i a n d i t s a f f l u e n t s ) , t h e i n f i l t r a t i o n
r a t e m a y b e e v a l u a t e d a t 3 0 ± 1 0 % .
O n a p l u r i a n n u a l s c a l e ( F i g . 7 ) i t a p p e a r s t h a t p u m p i n g s e n t a i l a d e c r e a s e i n
t h e p i e z o m e t r i c l e v e l w h i c h i n c r e a s e s m o r e a n d m o r e r a p i d l y . A s u d d e n d i s c o n t i
n u i t y w a s n o t e d i n 1 9 6 4 . U n d o u b t e d l y t h i s w a s d u e t o t h e r e c h a r g e r e s u l t i n g f r o m
t h e e x c e p t i o n a l f l o o d s o f 1 9 6 4 .
W h a t e v e r t h e t i m e - s c a l e m a y b e , i t a p p e a r s t h a t t h e f l o o d s a l o n e e n s u r e t h e
r e c h a r g e o f g r o u n d w a t e r s . W i t h a n a v e r a g e a n n u a l i n f l o w e v a l u a t e d a t 1 2 X 1 0 6 m 3 a n d a n i n f i l t r a t i o n r a t e o f 3 0 % , t h e a v e r a g e e f f i c i e n t i n f i l t r a t i o n t o b e e x p e c t e d i s
a b o u t 3 X 1 0 6 m 3 a y e a r . P u m p i n g s i n t h i s a r e a , a m o u n t i n g a t p r e s e n t t o
2 5 X 1 0 6 m 3 a n n u a l l y , c o r r e s p o n d t o s e v e n t i m e s a s m u c h a s t h e n a t u r a l r e c h a r g e .
T h i s r e s u l t s i n t h e m o s t a p p a r e n t d r o p i n t h e p i e z o m e t r i c l e v e l s , a s r e c o r d e d o n
F i g . 7 .
2 . 3 . T r i t i u m c o n t e n t s
W i t h r e g a r d t o t h e t r i t i u m c o n t e n t s m e a s u r e d i n t h e G i b r a l t a r a n d A l e x a n d r i a
p r e c i p i t a t i o n s [ 2 ] , a n d w h a t i s k n o w n a b o u t f a l l o u t s i n E u r o p e , i t i s p o s s i b l e t o
t r a c e b a c k t o t h e a v e r a g e a n n u a l v a l u e s o f t h e t r i t i u m c o n t e n t s i n t h e r a i n s o f t h i s
p a r t o f N o r t h A f r i c a ( F i g . 8) .
conc
entr
otio
n (T
U)
1 8 8 A L L E M M O Z and O L I V E
FIG. 7. Decrease in the piezometric level owing to the pumping o f water.
UT
FIG.8. Average annual values o f tritium contents in the rains o f the North African region under study.
IA E A -A G -1 5 8 /1 3 1 8 9
б 1во (•/..)
FIG.9. 8D versus 5 180.
D u r i n g 1 9 7 4 — 7 5 , t h e 3 H c o n t e n t o f g r o u n d w a t e r s w a s d e t e r m i n e d a t
3 5 t i m e s — 1 4 o f t h e m w e r e b e l o w 1 T U w i t h t h e r e s t b e t w e e n 2 a n d 7 T U .
F u r t h e r t o t h e s y s t e m a t i c s t u d y o f 3 H c o n t e n t s i n g r o u n d w a t e r s , i t i s n o w
a d m i t t e d t h a t a q u i f e r s m a y b e c o n s i d e r e d a s w e l l - m i x e d r e s e r v o i r s [ 3 — 5 ] . T h u s ,
k n o w i n g t h e 3H c o n t e n t o f t h e r e c h a r g e , t h e 3 H c o n t e n t o f g r o u n d w a t e r s e n a b l e s
u s t o c a l c u l a t e t h e r e c h a r g e c o e f f i c i e n t a ( a n n u a l r e c h a r g e / v o l u m e o f t h e r e s e r v o i r s )
a n d t o i n f e r t h e m e a n r e s i d e n t i a l t i m e o f w a t e r T = 1 / a .
T h e p r e s e n t v a l u e s o f t r i t i u m c o n t e n t s l y i n g b e t w e e n 2 a n d 7 T U r e q u i r e a
r e c h a r g e c o e f f i c i e n t a b e t w e e n 0 . 0 0 1 a n d 0 . 0 0 4 , w h i c h c o r r e s p o n d s t o a n a v e r a g e
c o n f i n e m e n t t i m e T b e t w e e n 2 5 0 a n d 1 0 0 0 y e a r s ( F i g . 8) .
T h e a v e r a g e m a x i m u m d e p t h o f t h e g r o u n d w a t e r i n w h i c h a r e f o u n d c o n t e n t s
b e t w e e n 2 a n d 7 T U i s 2 0 0 m . W i t h a 1 0 % s t o r a g e c o e f f i c i e n t , t h i s c o r r e s p o n d s t o
a w a t e r s e c t i o n o f 2 0 m . T h e r e f o r e , c o n s i d e r i n g t h a t 0 . 0 0 1 < a < 0 . 0 0 4 , t h e a n n u a l
w a t e r r e n e w a l ( m a i n l y t h r o u g h p u m p i n g ) i s b e t w e e n 0 . 0 2 m / a a n d 0 . 0 8 m / a . H e r e
a g a i n a r e t h e m e r e h y d r o l o g i c a l d a t a p r e v i o u s l y m e n t i o n e d , n a m e l y t h e a v e r a g e
r e c h a r g e : 0 . 0 1 m / a ( 3 X 1 0 6 m 3 / 3 0 0 X 1 0 6 m 2 ) a n d t h e a v e r a g e v a l u e o f p u m p i n g s :
0 . 0 8 m / a ( 2 5 X 1 0 6 m 3 / 3 0 0 X 1 0 6 m 2 ) . I n a d d i t i o n t o t h e f a c t t h a t h y d r o l o g i c a l
a n d i s o t o p i c r e s u l t s a g r e e , t h i s c o m p l i a n c e s h o w s t h a t t h e a s s u m p t i o n o f a g o o d
m i x t u r e o f t h e a q u i f e r s ( s h o u l d p u m p i n g b e c a r r i e d o u t ) i s a g a i n c o n f i r m e d , a n d
f o r t h e f i r s t t i m e i n a n a r i d a r e a .
1 9 0 A L L E M M O Z and O L I V E
3 . O T H E R I S O T O P I C D A T A
T h e g r e a t a g e o f t h e d e e p g r o u n d w a t e r s ( 2 0 0 t o 6 0 0 m ) h a s b e e n p r o v e d b y
s e v e n d a t i n g s w i t h 14C — t h e p e r c e n t a g e s o f m o d e r n c a r b o n a r e b e t w e e n 5 a n d 1 0 % .
T h e r e f o r e , i t s h o u l d b e a d m i t t e d t h a t h e r e [ 6 ] , a s i n o t h e r a r i d a r e a s i n A f r i c a , t h e
p r e s e n t r e c h a r g e o f t h e s e d e e p g r o u n d w a t e r s i s n e g l i g i b l e .
T h i s g r e a t a g e o f d e e p a q u i f e r s i s a l s o r e v e a l e d b y a n e x a m i n a t i o n o f D / H a n d
180 / 160 ( F i g . 9 ) . W a t e r s o f s u p e r f i c i a l a n d m i d d l e a q u i f e r s a r e d i s t r i b u t e d o n t h e
s t r a i g h t l i n e o f m e t e o r i c w a t e r s ( 5 D = 8 5 180 + 1 0 % » ) . T h e i r a v e r a g e v a l u e i n
180 , 5 = - 6% o , c o r r e s p o n d s t o a n a p p r o x i m a t e t e m p e r a t u r e o f 1 1 ° C
( 5 18 0 = 0 . 7 t — 1 3 . 6 % o ) , w h i c h i s c l o s e t o t h e p r e s e n t t e m p e r a t u r e p r e v a i l i n g i n
w i n t e r ( w e t s e a s o n ) i n t h e D j e b e l ( F i g . l ) . B y c o n t r a s t , w a t e r s o f d e e p a q u i f e r s a r e
d i s t r i b u t e d a l o n g t h e s t r a i g h t l i n e 6D = 4 . 2 § 180 — 2 6 % c , w h i c h i n d i c a t e s t h a t t h e s e
w a t e r s h a v e b e e n s t r o n g l y e v a p o r a t e d , a s o c c u r s i n n e i g h b o u r i n g a r e a s [ 7 ] . F i r s t
t h e w a t e r h a d a 5 180 s - 9 . 5 % o , w h i c h c o r r e s p o n d s t o a t e m p e r a t u r e o f 6° C . B u t
t h e c u r r e n t c l i m a t o l o g y o f t h e s e a r e a s c a n n o t e x p l a i n s u c h a p a r a m e t e r ( F i g . l ) .
I t i s t h e r e f o r e n e c e s s a r y t o s u p p o s e a v e r y o l d r e c h a r g e [ 8 ] , m a y b e o n t h e u p p e r
P l e i s t o c e n e a n d a w e t t e r p e r i o d t h a n t h e p r e s e n t o n e , t h o u g h r a t h e r d r y a n d
c o l d [ 9 ].
4 . C O N C L U S I O N S
I f t h e w a t e r s t o c k o f d e e p g r o u n d w a t e r s ( m o r e t h a n 2 0 0 m ) c a n b e c o n s i d e r e d
f o s s i l , t h e u p p e r a q u i f e r s ( u p t o 200 m ) a r e n o w f e d w i t h n a t u r a l r e c h a r g e .
T h i s r e c h a r g e o c c u r s m a i n l y i n t h e w a d i b e d s a n d i n t h e p r o v i s i o n a l s t o r a g e
a r e a s a f t e r t h e f l o o d s . T h i s r e c h a r g e , c o r r e s p o n d i n g t o t h e t h i r d o f t h e f l o o d w a t e r s
a p p r o x i m a t e l y , t a k e s p l a c e v e r y q u i c k l y ( a f e w d a y s ) t h r o u g h t h e l a r g e c o n d u i t s
o f t h e u n s a t u r a t e d a r e a ( m a c r o p o r o s i t y ) . D u r i n g w i n t e r t h e p r e c i p i t a t i o n , w h i c h
i s a l m o s t e n t i r e l y r e t r i e v e d b y é v a p o t r a n s p i r a t i o n , m a i n t a i n s a w a t e r r e s e r v e l o c a t e d
i n t h e s o i l m i c r o p o r o s i t y a n d u s e d b y v e g e t a t i o n .
A t p r e s e n t , p u m p i n g a c t i v i t i e s a r e a l m o s t 1 0 t i m e s a s l a r g e a s t h e n a t u r a l
r e c h a r g e , w h i c h r e s u l t s i n a n o b v i o u s d r o p i n p i e z o m e t r i c l e v e l s . O n l y b y t h e
i m m e d i a t e i m p l e m e n t a t i o n o f a w a t e r p o l i c y w i l l a d i s a s t e r b e a v o i d e d i n t h i s a r e a .
R E F E R E N C E S
[1 ] ALLEMMOZ, M., Alimentation des nappes en pays aride. Etude de l’infiltration et del’évapotranspiration en Tripolitaine (Libye), Thesis, Grenoble (1 9 7 6 ) 207 pp.
[2 ] INTERNATIONAL ATOMIC EN ERG Y AGENCY, Environmental IsotopeData: World Survey of Isotope Concentration in Precipitation, Tech. Rep. Series Nos 96 ,117, 129, 147, 165, IAEA, Vienna ( 1 9 5 3 -1 9 7 1 ) .
I A E A - A G - 1 58/13 191
H UBERT, P., MARCE, A., O LIVE, Ph., SIW ERTZ, E ., Etude par le tritium de la dynamique des eaux souterraines, C.R. Hebd. Séances Acad. Sei. Paris 2 7 0 (1 9 7 0 ) 9 0 8 —11. SIEGENTHALER, U., OESCHGER, H., TONGIORGI, E ., “Tritium and oxygen-18 in natural water samples from Switzerland” , Isotopes in Hydrology (Proc. Symp. Vienna, 1970), IAEA, Vienna (1 9 7 0 ) 3 7 3 - 8 5 .ALLISON, G.B., HOLMES, J.W ., The environmental tritium concentration of underground water and its hydrological interpretation, J . Hydrol. 19 (1 9 7 3 ) 1 3 1 —43.SRDOC, D., S LIEP ÍE V IC , A ., OBELIC, В ., HORVATINCIC, N., MOSER, H.,STICHLER, W., “ Isotope investigations as a tool for regional hydrogeological studies in Libyan Arab Jamahiriya” , IAEA-AG-15 8 /1 1 , these Proceedings.CONRAD, G., FONTES, J.C ., “ Hydrologie isotopique du Sahara Nord-Occidental” , Isotopes in Hydrology (Proc. Symp. Vienna, 1970), IAEA, Vienna (1 9 7 0 ) 4 0 5 —19.SALEM, О., V ISSER, J . , D R A Y, M., GONFIANTINI, R ., “ Groundwater flow patterns in the western Libyan Arab Jamahiriya - evaluations from isotopic data” , IA EA -A G -158/12, these Proceedings.ALA YN E STR EET, F ., GROVE, A .T ., Environmental and climatic implications of late Quaternary lake-level fluctuations in Africa, Nature (Lond.) 261 (1 9 7 6 ) 3 8 5 —90.
I A E A - A G - 1 58/14
A S P E C T S O F E N V I R O N M E N T A L IS O T O P E
C H E M I S T R Y IN G R O U N D W A T E R S
IN E A S T E R N J O R D A N
J . W . L L O Y D
H y d r o g e o l o g i c a l S e c t i o n ,
D e p a r t m e n t o f G e o l o g i c a l S c i e n c e s ,
U n i v e r s i t y o f B i r m i n g h a m ,
B i r m i n g h a m ,
U n i t e d K i n g d o m
A b s t r a c t
ASPECTS OF ENVIRONMENTAL ISOTOPE CHEMISTRY IN GROUNDWATERS IN EASTERN JORDAN.
Aspects of the environmental isotope data from a sandstone and a limestone aquifer are given. The data concerning the sandstone are limited but indicate the type of recharge that is occurring in the outcrop area. Although the 14C data indicate some modern recharge in radiocarbon terms the tritium data show evidence of only localized recharge. Stable isotope data from one well indicate that indirect recharge may be important. In the limestone aquifer it is found that tritiated groundwater occurs chiefly in the vicinity of wadis indicating the importance of indirect recharge. The lack of tritiated groundwater in the ‘recharge mounds’ in the interfluve areas is seen as partly a function of sampling but also as indicating the high permeability zones at the tops of the aquifers. The 5 13C data distribution are examined with respect to possible recharge mechanisms.
I N T R O D U C T I O N
T h e m a i n a q u i f e r s o f E a s t e r n J o r d a n c o n s i s t o f L o w e r P a l a e o z o i c s a n d s t o n e s
i n t h e s o u t h w i t h a n e x t e n s i v e C r e t a c e o u s c a r b o n a t e a q u i f e r e x t e n d i n g t h r o u g h o u t
t h e r e s t o f t h e a r e a .
T h e r a i n f a l l t h r o u g h o u t E a s t e r n J o r d a n i s s m a l l , a s s h o w n o n F i g . l , a n d
c a l c u l a t i o n s s h o w t h a t t h e p r e d o m i n a n t d i r e c t r e c h a r g e c o m p o n e n t t o t h e c a r b o n a t e
a q u i f e r o c c u r s o n l y i n t h e w e s t ( L l o y d e t a l . 1 9 6 6 ) [ 1 ] . T h e r e c h a r g e a p p e a r s t o
o c c u r t o a s e r i e s o f m o u n d s w i t h g r o u n d w a t e r f l o w m o v i n g t o w a r d s t h e i n t e r v e n i n g
r i v e r v a l l e y s . I n t h e s o u t h e r n s a n d s t o n e s n o d i r e c t r e c h a r g e i s t h o u g h t t o o c c u r .
1 9 3
1 9 4 L L O Y D
S O U T H E R N S A N D S T O N E A Q U I F E R
T r i t i u m d a t a
T h e i s o t o p e d a t a f o r t h e s a n d s t o n e a q u i f e r a r e l i m i t e d b u t t h e y d o i n d i c a t e
s o m e i n t e r e s t i n g i n f o r m a t i o n p e r t i n e n t t o t h e o u t c r o p p e r i m e t e r s o f l a r g e r e g i o n a l
g r o u n d w a t e r b a s i n s i n a r i d a r e a s . T h e s a m p l e p o i n t s a r e s h o w n o n F i g . 2 a n d t h e
I A E A - A G - 1 58/14 1 9 5
FIG.2. Tritium data distribution fo r the southern sandstone aquifer in Eastern Jordan.
d a t a a r e g i v e n i n T a b l e I . F i r s t l y , i t s h o u l d b e n o t e d t h a t t h e t r i t i u m d a t a i n t h e
r a i n f a l l i n t h e v i c i n i t y a r e o f t h e o r d e r o f 1 0 0 T U a n d t h a t o n l y s o m e w e l l s , S I 9 ,
S 6 , S 5 , P P 9 a n d P P 7 0 h a v e p o s i t i v e t r i t i u m ( > 4 T U ) v a l u e s i n t h e i r g r o u n d w a t e r s .
T h e s e w e l l s a r e l o c a t e d i n t h e o u t c r o p b u t o t h e r o u t c r o p w e l l s a t s i m i l a r g r o u n d w a t e r
e l e v a t i o n s d o n o t h a v e t r i t i a t e d w a t e r . T h i s i s s e e n a s d e m o n s t r a t i n g t h a t m o d e r n
r e c h a r g e o c c u r s , b u t o n l y w i t h a v a r i a b l e d i s t r i b u t i o n i n t i m e a n d l o c a t i o n a n d i s
c o n s i s t a n t w i t h a h y p o t h e s i s t h a t s p o r a d i c r e c h a r g e t o t h e s y s t e m i s t h r o u g h
i n f i l t r a t i o n l o s s f r o m r u n o f f d u r i n g f l a s h f l o o d s .
S t a b l e i s o t o p e d a t a
T h e s t a b l e i s o t o p e d a t a p l o t t e d o n F i g . 3 c l e a r l y s h o w t h a t t h e g r o u n d w a t e r s
i n t h e s a n d s t o n e s d o n o t c o r r e l a t e d i r e c t l y w i t h t h e M e d i t e r r a n e a n r a i n f a l l - t y p e
1 9 6 L L O Y D
T A B L E I . I S O T O P E D A T A F O R S A N D S T O N E A Q U I F E R
Well No. Tritium(TU)
8D
(.%»)5 180(%o)
s 13c(%o)
14C(% modern)
‘Corrected’age-years
S3 3.2 - 3 4 - 6 .2 - 4 .6 40 .0 Modern
S7 0.5 - 4 1 - 6 .5 - 9 .9 40 .0 Modern
S19
OOо
- 4 0 - 5 .7 - 1 4 .6 41 .8 2 5 0 0
S33 1.0, 0 .2 - 4 0 - 6 .1 - 8 .4 12.8 6 0 0 0
S36 1.2, 3 .0 - 3 3 - 6 .1 - 3 .2 16.1 Modern
S43 0 .7 , 2 .2 - 2 4 - 4 .3 - 4 . 0 5.3 8 6 0 0- 3 . 4 1.2 19 500
S49 1.4 - 3 5 -6 .1 - - -
S5 5 .3 , 1.5 - 3 8 - 6 .4 - 7 . 4 18.1 3 70 0
PP70 23 .0 - - - 4 .1 41 .9 Modern
S2 1.7 - 4 4 - 6 .4 - 5 . 0 38.1 Modern
S6 74 .8 - 2 6 - 4 .2 - 1 3 .0 86.8 Modern
PP9 1.3 , 6 .4 - 3 7 - 6 .1 - 7 .0 22 .8 1300
PP22 1.4 - 3 5 - 6 .3 - 7 . 0 22.1 1500
r e l a t i o n s h i p . T h i s m a y i n c e r t a i n c a s e s b e d u e t o d i f f e r e n t c l i m a t i c c o n d i t i o n s i n
t h e p a s t , s u c h a s i n t h e o l d g r o u n d w a t e r s a m p l e S 4 3 ( s e e b e l o w ) , b u t a s m a n y o f
t h e w e l l s a r e i n t h e u n c o n f i n e d s e c t i o n o f t h e a q u i f e r i t i s m o r e l i k e l y t o d e m o n s t r a t e
e v a p o r a t i v e e f f e c t s d u r i n g t h e r e c h a r g e e v e n t . T h i s i s p a r t i c u l a r l y w e l l s h o w n b y
t h e h i g h l y t r i t i a t e d s a m p l e S 6 a n d i n d i c a t e s t h a t a l t h o u g h i n d i r e c t r e c h a r g e o c c u r s ,
t h e w a t e r t h a t i n f i l t r a t e s b e l o w t h e r u n - o f f a r e a s i s s u b j e c t t o h e a v y e v a p o r a t i o n a n d
t h e r e c h a r g e m e c h a n i s m i s c o m p l i c a t e d . I n e f f e c t t h e d e p l e t i o n i n t h e s t a b l e
i s o t o p e s s h o w s t h a t t h e p o s s i b l e a m o u n t o f r e c h a r g e t h a t c a n o c c u r i n a n a l r e a d y
s e v e r e l y r e s t r i c t e d r e c h a r g e e n v i r o n m e n t i s e v e n m o r e c u r t a i l e d .
I n e x a m i n i n g t h e s t a b l e i s o t o p e d a t a i t i s w o r t h c o n s i d e r i n g t h e d a t a a v a i l a b l e
f r o m t h e s e v e n r a i n f a l l s t a t i o n s s h o w n o n F i g . 3 . T h e 5 180 - S D r e l a t i o n s h i p s f o r
t h e s e s t a t i o n s a r e p l o t t e d a s a v e r a g e w i n t e r v a l u e s w e i g h t e d f o r t h e a m o u n t o f
p r e c i p i t a t i o n , a g a i n s t t h e M e d i t e r r a n e a n r e l a t i o n s h i p . T h e n u m b e r o f a n a l y s e s a r e
l i m i t e d b u t i t i s i n t e r e s t i n g t o n o t e t h a t t h e d e s e r t s t a t i o n s o f A z r a q a n d R a m d o
n o t r e l a t e t o t h e s t a n d a r d M e d i t e r r a n e a n r e l a t i o n s h i p a n d R a b b a a l s o i s s o m e w h a t
d i s p l a c e d . T h e d a t a m a y s u g g e s t t h a t m o i s t u r e p a s s i n g o v e r t h e d e s e r t a r e a s i s
s u b j e c t t o e v a p o r a t i v e o r p o s s i b l y v i r g i s e f f e c t s .
IA E A - A G - 1 58/14 1 9 7
s 18 о % °
FIG.3. Stable isotope data fo r precipitation stations and groundwater samples from southern desert sandstones.
C a r b o n i s o t o p e d a t a
T h e 14C d a t a f o r t h e s a n d s t o n e a q u i f e r s h o w n i n T a b l e I a r e w i t h a g e c o r r e c t i o n s
c a r r i e d o u t u s i n g t h e W i g l e y ( 1 9 7 6 ) [ 2 ] t e c h n i q u e . I t w i l l b e s e e n t h a t m o s t o f t h e
o u t c r o p w a t e r s a r e ‘m o d e r n ’ i n 14C t e r m s , w h i c h i s c o n s i s t e n t w i t h t h e r e c h a r g e
h y p o t h e s i s , b u t t h a t o l d w a t e r s a r e p r e s e n t u n d e r o u t c r o p ( P P 9 , S 5 , S I 9 ) a n d t h a t
w a t e r s 8 t o 20 000 y e a r s o l d a r e p r e s e n t i n t h e c o n f i n e d a r e a c l o s e t o o u t c r o p .
H e r e t h e 14C d a t a p o s e t h e d i l e m m a o f r e c h a r g e v e r s u s ‘ f o s s i l g r a d i e n t ’ ( B u r d o n ,
1 9 7 7 ) [ 3 ] . A s s h o w n o n F i g . 2 a w e l l - d e f i n e d h y d r a u l i c g r a d i e n t e x i s t s t h r o u g h o u t
t h e a r e a , a n d a l t h o u g h i t i s u n d o u b t e d l y s u p p o r t e d t o a c e r t a i n e x t e n t b y t h e
m o d e r n r e c h a r g e i t i s u n l i k e l y t h a t t h e i m p l i e d g r o u n d w a t e r f l o w m o v i n g u n d e r
t h e g r a d i e n t c a n b e a c c o u n t e d f o r p u r e l y b y s u c h r e c h a r g e , e s p e c i a l l y i n v i e w o f
t h e v e r y l o w r a i n f a l l a m o u n t s t h a t o c c u r i n t h e a r e a ( L l o y d , 1 9 6 9 ) [ 4 ] . T h e p r e s e n c e
o f f a i r l y o l d g r o u n d w a t e r c l o s e t o o u t c r o p t h e r e f o r e s u p p o r t s t h e v i e w t h a t m i x e d
r e c h a r g e a n d f o s s i l g r a d i e n t d e c a y c o n d i t i o n s p r o b a b l y e x i s t i n t h e a r e a w i t h m u c h
o f t h e o l d w a t e r p o s s i b l y h a v i n g b e e n r e c h a r g e d s o m e 10 —20 000 y e a r s a g o .
19 8 L L O Y D
FIG.4. Sample area over the limestone aquifer showing tritium distributions.
T h e 1 4 C d a t a f r o m w e l l S 4 3 p r o v i d e i n t e r e s t i n g i n f o r m a t i o n . T h e f i r s t
s a m p l e o b t a i n e d a t a p u m p i n g r a t e o f 2 4 m 3/ h g a v e a m a r k e d l y l o w e r a g e
( 8 6 0 0 y e a r s ) t h a n t h e s e c o n d s a m p l e ( 1 9 5 0 0 y e a r s ) w h i c h w a s o b t a i n e d w h e n t h e
w e l l w a s p u m p e d a t 6 0 m 3/ h . T h e o t h e r c h e m i c a l p a r a m e t e r s , h o w e v e r , s h o w e d n o
c h a n g e . T h e h i g h e r p u m p i n g r a t e o b v i o u s l y d r e w w a t e r f r o m a g r e a t e r d e p t h a n d
d i s t a n c e , t h e d e p t h p r o b a b l y b e i n g m o r e s i g n i f i c a n t . T h e i m p l i c a t i o n i s t h e n t h a t
s t r a t i f i c a t i o n o f w a t e r o c c u r s , w h i c h i s r e v e a l e d b y 1 4 C a n a l y s i s b u t n o t f r o m o t h e r
i n f o r m a t i o n . T h e o r i g i n , n a t u r e o r d i s t r i b u t i o n o f t h i s p o s t u l a t e d s t r a t i f i c a t i o n i s
IA E A -A G -1 58/14 1 9 9
I I + I +500 400 300 200 100mm P re c ip ita t io n
be a fra c tio n o f s t ra ta g ra p h ic a l u n it
FIG.5. Conceptual m odel o f groundwater flow in limestone aquifers.
n o t k n o w n , b u t t h e s e 1 4 C d i f f e r e n c e s i n d i c a t e s o m e o f t h e p r o b l e m s o f o b t a i n i n g
d e f i n i t i v e g r o u n d w a t e r a g e d a t a f r o m n o r m a l p u m p e d s a m p l e s . I n t h i s p a r t i c u l a r
c a s e t h e ‘ a g e ’ o b t a i n e d i s r e l a t e d t o t h e y i e l d a n d d e p t h i n f l u e n c e o f p u m p i n g .
L I M E S T O N E A Q U I F E R
T r i t i u m d a t a
I f w e n o w t u r n t o t h e c a r b o n a t e a q u i f e r e n v i r o n m e n t a l i s o t o p e d a t a , s o m e
i n t e r e s t i n g i n f o r m a t i o n i s i m m e d i a t e l y a p p a r e n t f r o m t h e t r i t i u m v a l u e s . O n F i g . 4 ,
i n a r e p r e s e n t a t i v e s a m p l e a r e a o f t h e a q u i f e r , t h e p o s i t i v e t r i t i u m l e v e l s a r e s e e n
t o o c c u r p r e d o m i n a n t l y o u t s i d e t h e m a i n r a i n f a l l z o n e w h e r e t r i t i u m l e v e l s i n
p r e c i p i t a t i o n a r e a r o u n d 1 0 0 T U , a n d w h e r e l a r g e v o l u m e s o f d i r e c t r e c h a r g e a r e
c o n s i d e r e d t o o c c u r a n n u a l l y ( P a r k e r , 1 9 7 0 ) [ 5 ] . F u r t h e r , i n t h e c l a s s i c a l s t y l e t h e
r e c h a r g e m o u n d s o c c u r u n d e r t h e h i g h p r e c i p i t a t i o n a r e a s a n d g r o u n d w a t e r g r a d i e n t s
s l o p e r a d i a l l y f r o m t h e s e a r e a s d e s p i t e t h e f a c t t h a t m u c h o f t h e m o u n d w a t e r i s
n o n - t r i t i a t e d . W h a t t h e n d o t h e t r i t i u m d a t a i n f e r a n d w h a t p o s s i b l e f l o w
m e c h a n i s m s o c c u r i n t h e c a r b o n a t e a q u i f e r ?
2 0 0 L L O Y D
FIG.6. Areas o f groundwaters with differing 8 13C ranges.
F i r s t l y , t h e t r i t i a t e d w a t e r s a w a y f r o m t h e m o u n d a r e a s a r e l o c a t e d i n v a l l e y s
a n d i n d i c a t e t h e i m p o r t a n c e o f i n d i r e c t r e c h a r g e a s t r a n s m i s s i o n l o s s e s f r o m f l o o d s
i n t o t h e f i s s u r e d s e c t i o n s — a p o i n t n o t g e n e r a l l y a p p r e c i a t e d f r o m h y d r a u l i c d a t a .
S e c o n d l y , t h e n o n - t r i t i a t e d w a t e r i n t h e m o u n d a r e a s i m p l i e s t h a t t h e l a r g e v o l u m e s
o f d i r e c t r e c h a r g e c a n o n l y m o v e t h r o u g h a t h i n h i g h p e r m e a b i l i t y u p p e r s e c t i o n o f
t h e a q u i f e r o v e r t h e t o p o f u n d e r l y i n g o l d e r w a t e r s w h i c h a r e k n o w n t o o c c u r
( P a r k e r [ 5 ] ) . T h e b a s i s f o r t h i s s t a t e m e n t i s t h a t t h e t r i t i u m s a m p l e s a r e p u m p e d
s a m p l e s t a k e n f r o m w e l l s p e n e t r a t i n g i n t o t h e d e e p e r z o n e s o f t h e a q u i f e r s o t h a t
i n t e g r a t e d s a m p l e s a r e o b t a i n e d r e s u l t i n g i n r e d u c e d t r i t i u m l e v e l s . T h e h y p o t h e s i s
i s s u p p o r t e d b y o t h e r e v i d e n c e . F r e q u e n t l y i n d r i l l i n g , t o t a l c i r c u l a t i o n l o s s e s
o c c u r a t t h e w a t e r - t a b l e i n d i c a t i n g h i g h p e r m e a b i l i t y , w h i c h i s a c o m m o n f e a t u r e
I A E A - A G - 1 58/14 2 0 1
- 16-
-12
-10 -
- 8 -
\ \ ♦
; ■ \/ ' « / ♦ ' i ♦ \ и 4 ( -i ♦ E \ c • j\ • • ♦ \ ------------- -Чл ♦ N. .
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ь
waters
A etc. re la tes to areas on F igure 6
10 20 30 U 0
14 С % Modern
FIG. 7. Ь1ЪС and 14C%o modern data fo r limestone aquifers.
o f c a r b o n a t e a q u i f e r s w i t h m a r k e d s o l u t i o n o f l i m e s t o n e s r e s u l t i n g f r o m w a t e r -
t a b l e f l u c t u a t i o n . F u r t h e r , l i m i t e d t e m p e r a t u r e - c o n d u c t i v i t y p r o f i l e d a t a i n d i c a t e
g r e a t e r A s s u r i n g i n t h e s h a l l o w e r p a r t o f t h e a q u i f e r t h a n a t d e p t h . W h e r e s e r i a l
s a m p l i n g h a s b e e n c a r r i e d o u t a s a t S h a u b a k , i t h a s b e e n d e m o n s t r a t e d t h a t t r i t i u m
l e v e l s d e c l i n e w i t h i n c r e a s i n g w e l l y i e l d , f o r e x a m p l i n g t r i t i u m r e d u c i n g f r o m 7 t o
0 T U w i t h t h e y i e l d i n c r e a s i n g f r o m 8 0 t o 1 2 0 m 3/ h , w h i c h i s c o n s i s t e n t w i t h a
s t r a t i f i e d g r o u n d w a t e r c o n d i t i o n . U n f o r t u n a t e l y , d e p t h p r o f i l e s o f t r i t i u m a r e n o t
a v a i l a b l e b u t i n a n y c a s e m a y n o t b e o f v a l u e i n f u l l y o p e n w e l l s p e n e t r a t i n g
v a r i a b l e p e r m e a b i l i t y m a t e r i a l s .
2 0 2 L L O Y D
T A B L E I I . R E C H A R G E M E C H A N I S M S I N F E R R E D O N T H E B A S I S O F
S 1 3 C D I S T R I B U T I O N A N D L O C A L S U P E R F I C I A L G E O L O G Y
Area General 8 13C %o characteristic
Recharge feature
A - 9 to - 1 4 Average direct recharge rates, important vegetation cover.
В - 6 to - 7 Local mixing of resident groundwater with incompatible indirect recharge, very poor vegetation cover.
С - 1 3 Average recharge rates, important vegetation cover.
D - 4 t o - 1 0 Large outcrop area with variable recharge rates of both direct and indirect type resulting in mixing of waters. Poor recharge volumes per unit area. Limited vegetation cover.
E - 1 1 to - 1 6 Rapid direct recharge from high rainfall areas. Limited direct recharge elsewhere due to thick low-permeability soil cover. Local indirect recharge in valley beds. Important tree and vegetation cover.
A l t h o u g h u n d o u b t e d l y a h i g h t r a n s m i s s i o n o f r e c h a r g e o c c u r s i n t h e u p p e r
l a y e r s o f t h e a q u i f e r t h e d e a r t h o f p o s i t i v e l y t r i t i a t e d w a t e r i s d i s t u r b i n g a n d m u s t
b r i n g i n t o q u e s t i o n t h e a s s e s s m e n t s o f d i r e c t r e c h a r g e . T h e f a c t t h a t t r i t i u m d a t a
i n d i c a t e l o w r e c h a r g e c a n n o t b e o v e r l o o k e d .
A c o n c e p t u a l m o d e l o f t h e f l o w m e c h a n i s m t h o u g h t t o b e o p e r a t i n g i n t h e
c a r b o n a t e a q u i f e r s b a s e d o n t h e i s o t o p e d a t a a v a i l a b l e i s i l l u s t r a t e d i n F i g . 5 .
C a r b o n i s o t o p e d a t a
T h e 5 13C d a t a f r o m t h e l i m e s t o n e a q u i f e r a r e s u r p r i s i n g l y n e g a t i v e w i t h v e r y
f e w s a m p l e s h a v i n g v a l u e s m o r e p o s i t i v e t h a n - 7 % o , s i g n i f y i n g t h a t a ‘t e m p e r a t e ’
t y p e o f p h o t o s y n t h e s i s h a s b e e n o p e r a t i v e h i s t o r i c a l l y u n l e s s n o r m a l c a r b o n a t e
r e a c t i o n s h a v e n o t t a k e n p l a c e . T h e r o c k m a t r i x 5 1 3 C v a l u e i s a b o u t 0 % o b u t
c o n s i d e r a b l y d i f f e r e n t v a l u e s h a v e b e e n o b t a i n e d f o r s u r f a c e s a m p l e s w h e r e c a l i c h e
d e v e l o p m e n t i s i m p o r t a n t . S u c h s a m p l e s h a v e r e c o r d e d v a l u e s r a n g i n g f r o m
- 8 . 9 t o - 1 0 . 3 % ^ .
IA E A - A G - 1 58/14 2 0 3
T h e d i s t r i b u t i o n o f 6 1 3 C i n t h e a q u i f e r i s i n t e r e s t i n g i n t h a t i t a p p e a r s t o b e
p o s s i b l e t o d e l i m i t f i v e a r e a s a s s h o w n o n F i g .6 w i t h t h e p a r t i c u l a r c h a r a c t e r i s t i c s
g i v e n o n F i g . 7 . T h e d i s t r i b u t i o n s d o n o t a p p e a r t o b e d u e t o l i t h o l o g i c a l d i f f e r e n c e s
i n t h e a q u i f e r a n d a r e n o t e a s i l y r e c o n c i l a b l e w i t h a n y h y d r o c h e m i c a l z o n a t i o n
r e l a t i n g t o c a r b o n a t e u n d e r - s a t u r a t i o n o r i o n e x c h a n g e e t c . I f d i f f e r e n t i a l m o d i
f i c a t i o n t o 5 1 3 C h a s n o t o c c u r r e d b e l o w t h e w a t e r - t a b l e t h e n t h e d i f f e r e n c e s i n
5 1 3 C s h o u l d r e f l e c t p r e d o m i n a n t l y t h e e f f e c t s o f d i f f e r i n g r e c h a r g e m e c h a n i s m s
a n d p o s s i b l y u n d e r - s a t u r a t i o n a n d m a t r i c d i s s o l u t i o n d u e t o t h e m i x i n g o f
i n c o m p a t i b l e r e s i d e n t g r o u n d w a t e r w i t h r e c h a r g e w a t e r . T h e p o s t u l a t e d c o n d i t i o n s
a r e s u m m a r i z e d i n T a b l e I I a n d i t m u s t b e p o i n t e d o u t t h a t t h e y a r e p o s t u l a t e d a s
h a v i n g e x i s t e d o v e r v e r y l o n g p e r i o d s o f t i m e . O b v i o u s l y s u c h p o s t u l a t i o n s a r e
s p e c u l a t i v e b u t t h e e v i d e n c e o f 5 1 3 C d i s t r i b u t i o n s i n d i c a t e s t h a t t h i s i s o t o p e m a y
b e o f v a l u e i n i t s o w n r i g h t a n d n o t j u s t i n c o n j u n c t i o n w i t h 14C .
C O N C L U S I O N S
I n c o n c l u s i o n i t m a y b e s a i d t h a t t h e r e g i o n a l e n v i r o n m e n t a l d a t a f o r E a s t e r n
J o r d a n h a v e p r o v i d e d i n f o r m a t i o n e s s e n t i a l l y r e l a t i n g t o r e c h a r g e m e c h a n i s m s . T h e
i m p o r t a n c e o f i n d i r e c t r e c h a r g e i n t h e l o w r a i n f a l l a r e a s h a s b e e n d e m o n s t r a t e d
b y t r i t i u m a n d t h e p r e s e n c e o f o l d w a t e r , i d e n t i f i e d b y 1 4 C d a t i n g , h a s i n d i c a t e d
t h a t ‘ f o s s i l ’ g r o u n d w a t e r c o n d i t i o n s e x i s t e v e n i n t h e r e c h a r g e m o u n d s . T h e s e
t r i t i u m d a t a s u p p o r t . o t h e r i n f o r m a t i o n c o n c e r n i n g t h e p r e s e n c e o f h i g h p e r m e a b i l i t y
z o n e s a t t h e t o p o f t h e l i m e s t o n e a q u i f e r o v e r l y i n g l e s s p e r m e a b l e z o n e s a t d e p t h .
T h e i s o t o p e s c l e a r l y s h o w t h a t m i x i n g o f g r o u n d w a t e r s i s o c c u r r i n g o w i n g t o
d i f f e r e n t r e c h a r g e i n p u t s s o t h a t s o m e o f t h e ‘d a t i n g ’ m a y b e o p e n t o q u e s t i o n .
S u c h m i x i n g p r e c l u d e s t h e u s e o f t h e i s o t o p e s t o d e t e r m i n e g r o u n d w a t e r v e l o c i t i e s
a n d t r a n s i t t i m e s . T h e s t a b l e i s o t o p e s o f l s O a n d d e u t e r i u m i n t h e s a n d s t o n e
g r o u n d w a t e r s h a v e l a r g e l y b e e n d e p l e t e d , s h o w i n g t h a t t h e i n f i l t r a t i o n h a s b e e n
s u b j e c t e d t o e v a p o r a t i o n a s w o u l d b e e x p e c t e d . T h e <51 3 C d a t a c a n b e d i f f e r e n t i a t e d
o n a r e g i o n a l b a s i s a n d m a y i n d i c a t e v a r i a b l e r e c h a r g e c o n d i t i o n s o v e r t h e a r e a .
T h e i s o t o p e d a t a h a v e n o t p r o v i d e d d e f i n i t i v e o r q u a n t i t a t i v e i n f o r m a t i o n i n
t h e a r e a . T h e y h a v e , h o w e v e r , p r o v i d e d q u a l i t a t i v e i n f o r m a t i o n t h a t w o u l d n o t
h a v e o t h e r w i s e b e e n a v a i l a b l e a n d h a v e m o s t i m p o r t a n t l y q u e s t i o n e d t h e r e l i a b i l i t y
o f t h e d i r e c t r e c h a r g e e s t i m a t i o n s .
A C K N O W L E D G E M E N T S
T h e a u t h o r w o u l d l i k e t o a c k n o w l e d g e t h e s t a f f o f t h e F o o d a n d A g r i c u l t u r e
O r g a n i z a t i o n w h o c o l l e c t e d t h e s a m p l e s f o r i s o t o p e a n a l y s i s . T h e a n a l y s e s w e r e
c a r r i e d o u t b y t h e I n t e r n a t i o n a l A t o m i c E n e r g y A g e n c y , V i e n n a .
2 0 4 L L O Y D
R E F E R E N C E S
[1 ] LLO YD , J.W ., DRENNEN, D.S.H., BEN N ELL, B.M.U., A groundwater recharge study in north-eastern Jordan, Proc. Inst. Civ. Eng. (1 9 6 6 ) 36.
[2 ] W IGLEY, T.M .L., Effect o f mineral precipitation on isotopic composition and carbon-14 dating of groundwater, Nature (London) 2 6 3 (1 9 7 6 ) 2 1 9 - 2 1 .
[3] BURDON, D .J., Flow of fossil groundwater, Q.J. Eng. Geol. 10 (1 9 7 7 ) 9 7 —124.[4 ] LLO YD , J.W ., The Hydrogeology of the Southern Desert of Jordan, FAO Rep.
A G L :S F /Jo r 9 , Tech. Rep. 1 (1 9 6 9 ) 207 pp.[5] PAR KER, D.H., The Hydrogeology of the Mesozoic-Cainozoic Aquifers of the Western
Highlands and Plateau of East Jordan, FAO Rep. A G L :S F/Jor 9. Tech. Rep. 2 (1 9 7 0 ) 285 pp.
I A E A - A G - 1 58 /15
A C O N C E P T U A L H Y D R O C H E M I C A L M O D E L
F O R A L L U V I A L A Q U I F E R S O N
T H E S A U D I A R A B I A N B A S E M E N T S H I E L D
J . W . L L O Y D
H y d r o g e o l o g i c a l S e c t i o n ,
D e p a r t m e n t o f G e o l o g i c a l S c i e n c e s ,
U n i v e r s i t y o f B i r m i n g h a m , B i r m i n g h a m ,
U n i t e d K i n g d o m
P . F R I T Z
D e p a r t m e n t o f E a r t h S c i e n c e s ,
U n i v e r s i t y o f W a t e r l o o , W a t e r l o o ,
O n t a r i o
D . C H A R L E S W O R T H
H y d r o g e o l o g i c a l S e c t i o n ,
J a m e s F . M a c L a r e n L t d . ,
W i l l o w d a l e , O n t a r i o ,
C a n a d a
A b s t r a c t
A CONCEPTUAL HYDROCHEMICAL MODEL FOR ALLUVIAL AQUIFERS ON THE SAUDI ARABIAN BASEMENT SHIELD.
Situated across the Precambrian Basement shield of Saudi Arabia is a sequence of alluvial- filled wadis which contain the main groundwater resources of the area. The groundwater in some of the wadis indicates a central wadi fresh groundwater zone with highly saline groundwater occurring at the edges of the major alluvial thicknesses. The distribution of the chemistry in the groundwaters is examined and a conceptual model is given for one main wadi. Both the major ion chemistry and the environmental isotopes are used to demonstrate that the edge effect high salinity is the result of evaporative action and the concentration of salts in groundwater entering the main wadi from side wadis containing thin alluvial fill deposits.
I N T R O D U C T I O N
T h e P r e c a m b r i a n B a s e m e n t i n S a u d i A r a b i a f o r m s a s h i e l d a r e a o f s o m e
7 7 0 0 0 0 k m 2 w h i c h d o m i n a t e s t h e w e s t e r n p a r t o f t h e c o u n t r y a n d e x t e n d s f r o m
t h e J o r d a n i a n b o r d e r i n t o t h e Y e m e n . D e v e l o p e d u p o n t h e s h i e l d a r e a s e q u e n c e
o f a l l u v i a l - f i l l e d w a d i s w h i c h c o n t a i n t h e m a i n g r o u n d w a t e r r e s o u r c e s o f t h e a r e a .
T h e m a i n p r e c i p i t a t i o n o c c u r s i n t h e h i g h e r p a r t o f t h e w a d i c a t c h m e n t s w i t h t h e
2 0 5
2 0 6 L L O Y D et al.
FIG .l. Rainfall distribution over the Wadi Bisha catchment.
m a j o r r e c h a r g e t o t h e g r o u n d w a t e r i n t h e a l l u v i u m o c c u r r i n g t h r o u g h b e d t r a n s
m i s s i o n l o s s e s d u r i n g f l o o d r u n o f f .
T h e g r o u n d w a t e r i n s o m e o f t h e w a d i s i n d i c a t e s t h e p r e s e n c e o f c e n t r a l w a d i ,
l o w s a l i n i t y g r o u n d w a t e r z o n e s , w i t h h i g h e r s a l i n i t y g r o u n d w a t e r o c c u r r i n g a t t h e
e d g e s o f t h e m a j o r a l l u v i a l t h i c k n e s s e s . T h e d i s t r i b u t i o n o f t h e h i g h e r s a l i n i t y
w a t e r s i s s o m e w h a t u n u s u a l a s i s t h e i r c o m p o s i t i o n , w h i c h i s p r e d o m i n a n t l y o f
‘ s o d i u m s u l p h a t e ’ t y p e .
T o e x a m i n e t h e o r i g i n o f t h e s e h y d r o c h e m i c a l d i s t r i b u t i o n s m a j o r i o n a n d
i s o t o p e d a t a f r o m t h e g r o u n d w a t e r s i n t h e W a d i B i s h a , o n e o f t h e l a r g e s t w a d i s i n t h e
a r e a ( s e e F i g . l ) , h a v e b e e n u s e d . I n t h i s w a d i t h e m a i n t r a n s m i s s i o n l o s s e s f r o m
f l o o d s o c c u r s i n t h e w a d i b e d b e t w e e n H i f a a n d J u n a y n a h . T h e p r e c i p i t a t i o n g i v i n g
r i s e t o t h e f l o o d s o c c u r s p r e d o m i n a n t l y i n t h e u p p e r c a t c h m e n t a s i n d i c a t e d i n
F i g . 1 . E l s e w h e r e i n t h e c a t c h m e n t , h o w e v e r , o v e r a n e x t r e m e l y l a r g e a r e a , w h e r e
t h e m e a n r a i n f a l l r a n g e s b e t w e e n 1 2 5 a n d 1 5 0 m m , o n l y v e r y s p o r a d i c f l o o d i n g i s
k n o w n t o o c c u r .
I A E A - A G - 1 58/15 2 0 7
FIG.2. Groundwater conductivity o f central Wadi Bisha.
H Y D R O C H E M I C A L D A T A
O n F i g . 2 t h e g r o u n d w a t e r c o n d u c t i v i t y f o r t h e m o s t e x t e n s i v e a q u i f e r a r e a o f
t h e W a d i B i s h a i s s h o w n a n d t h e p r e s e n c e o f t h e h i g h e r s a l i n i t y g r o u n d w a t e r s a t t h e
a q u i f e r e d g e i s c l e a r l y d e m o n s t r a t e d . I t i s i m p o r t a n t t o n o t e t h a t t h e a q u i f e r
b o u n d a r y s h o w n o n F i g . 2 r e p r e s e n t s t h e b o u n d a r y o f s i g n i f i c a n t a l l u v i a l t h i c k n e s s
FIG.3. Schematic section across Wadi Bisha showing som e major ion and conductivity data at A-A on Fig.2.
20
8
LLOYD
et al.
IA E A - A G - 1 5 8 /1 5 2 0 9
FIG.4. Durov diagram indicating general major ion characteristics o f Wadi Bisha groundwaters.
( > 5 m ) a n d t h a t a w a y f r o m t h e b o u n d a r y v a s t a r e a s o f v e r y t h i n a l l u v i u m w i t h
g o o d p e r m e a b i l i t y a r e p r e s e n t . C l e a r l y t h e l o w - s a l i n i t y w a t e r s a l i g n w i t h t h e
p r e s e n t w a d i c o u r s e f r o m w h i c h t h e d o m i n a n t r e c h a r g e e l e m e n t s o c c u r .
O w i n g t o e x t e n s i v e g r o u n d w a t e r a b s t r a c t i o n f o r i r r i g a t i o n p u r p o s e s m u c h o f
t h e g r o u n d w a t e r c h e m i s t r y h a s b e e n m o d i f i e d a n d m i x i n g h a s o c c u r r e d ; h o w e v e r ,
t h e h y d r o c h e m i c a l t y p e s p r e s e n t c a n b e d e m o n s t r a t e d a s i s s h o w n i n F i g s 3 a n d 4 .
O n F i g . 3 t h e t o t a l s a l i n i t y , s u l p h a t e a n d c h l o r i d e d a t a a r e s h o w n f o r t h e
s e c t i o n a c r o s s t h e w a d i m a r k e d A - A o n F i g . 2 . F i g u r e 3 s h o w s t h e l o w - s a l i n i t y
i n d i r e c t w a d i r e c h a r g e w a t e r o c c u r r i n g a t t w o l o c a l i t i e s , w h i l e a s u l p h a t e d o m i n a n c e
o v e r c h l o r i d e i s c l e a r l y s h o w n a t t h e e d g e s o f t h e a q u i f e r . W h e r e h i g h - s a l i n i t y
w a t e r o c c u r s c e n t r a l l y i t i s a t t r i b u t e d t o s a l t c o n c e n t r a t i o n b y i r r i g a t i o n r e t u r n
i n f i l t r a t i o n . T w o t y p e s o f r e t u r n s e e m t o o c c u r , o n e i n w h i c h t h e s u l p h a t e w a t e r i s
c o n c e n t r a t e d , a n d t h e o t h e r i n w h i c h c h l o r i d e d o m i n a n t w a t e r d e v e l o p s , p r o b a b l y
a s a r e s u l t o f r e t u r n , t h r o u g h t h e i r r i g a t i o n u s e o f t h e m a i n w a d i r e c h a r g e w a t e r .
NJО
FIG .5. Schematic section across Wadi Bisha showing isotopic data in the vicinity o f A-A (Fig.2.).
Co
nd
uc
tiv
ity
jj
S
IA E A - A G - 1 58 /15 2 1 1
F i g u r e 4 i l l u s t r a t e s t h e t y p e o f m a j o r i o n c h e m i s t r y p r e s e n t i n t h e W a d i B i s h a a n d
i n d i c a t e s s o m e t h i n g o f t h e o r i g i n a n d m o d i f i c a t i o n s o f t h e s e w a t e r s . A s c a n b e
s e e n t h e m a i n w a d i r e c h a r g e i s b i c a r b o n a t e i n c h a r a c t e r w h i l e t h e e d g e w a t e r s a r e
s u l p h a t e w i t h c h l o r i d e w a t e r s d e v e l o p i n g f r o m i r r i g a t i o n r e t u r n .
O n F i g . 5 t r i t i u m a n d 6 1 8 0 d a t a a r e p l o t t e d a g a i n s t c o n d u c t i v i t y a n d g r o u n d
w a t e r t e m p e r a t u r e f o r a w a d i s e c t i o n a d j a c e n t t o A - A . T h e i s o t o p e s s h o w , a s w o u l d
b e e x p e c t e d , t h a t t h e m a i n w a d i r e c h a r g e w a t e r i s m o d e r n w i t h o u t m a r k e d l y
d e p l e t e d 5 1 8 0 l e v e l s . O n e c e n t r a l s a m p l e i n t h e s e c t i o n , h o w e v e r , i s d e p l e t e d
( 5 1 8 0 = — 1 . 3 1 ) a n d i s p r o b a b l y a t t r i b u t a b l e t o e v a p o r a t e d i r r i g a t i o n r e t u r n w a t e r .
I t i s i n t e r e s t i n g t o n o t e t h a t t h e e d g e w a t e r s h a v e m u c h l o w e r t r i t i u m v a l u e s t h a n
t h e c e n t r a l w a t e r s a n d t h a t t h e 5 1 8 0 l e v e l s a r e r e l a t i v e l y d e p l e t e d . L i m i t e d 1 4 C
d a t a t h a t a r e a v a i l a b l e f o r t h e w a d i a l s o s h o w t h a t a c t i v i t y d e c r e a s e s a w a y f r o m t h e
w a d i c e n t r e .
I N T E R P R E T A T I O N
T h e i n t e r p r e t a t i o n o f t h e u n u s u a l g r o u n d w a t e r c h e m i s t r y i n t h e W a d i B i s h a i s
s h o w n d i a g r a m m a t i c a l l y i n F i g . 6 . T h e c h e m i c a l t y p e s i n d i c a t e d r e l a t e t o a D u r o v -
s t y l e h y d r o c h e m i c a l c l a s s i f i c a t i o n . T h e s i g n i f i c a n t f e a t u r e i s t h e i n c o r p o r a t i o n i n t o
t h e h y d r o g e o l o g y o f t h e e f f e c t s o f t h e v e r y t h i n a l l u v i a l s p r e a d s a d j a c e n t t o t h e
m a i n a q u i f e r . C o n c e p t i o n a l l y t h e d e v e l o p m e n t o f t h e h y d r o c h e m i s t r y i s s e e n
a s f o l l o w s :
( i ) P r e c i p i t a t i o n p r o d u c e s m i n o r r e c h a r g e ( p r o b a b l y i n d i r e c t ) i n t o t h e t h i n h i g h l y
r e c e p t i v e a l l u v i a l s p r e a d . T h i s w a t e r i s p r o b a b l y C a ( H C 0 3)2 i n c h a r a c t e r w i t h a
n e g a t i v e g y p s u m s a t u r a t i o n i n d e x ( - S I G ) .
( i i ) T h e w a t e r f l o w s t h r o u g h t h e t h i n a l l u v i u m d i s s o l v i n g g y p s u m t h a t i s k n o w n t o
o c c u r , a n d i s s u b j e c t e d t o p a r t i a l e v a p o r a t i o n . I t a l s o w o u l d a p p e a r t o d i s s o l v e N a c l
t o a c q u i r e a n o v e r a l l ‘ N a 2S 0 4 ’ c l a s s i f i c a t i o n . T h i s c l a s s i f i c a t i o n i s s o m e w h a t
m i s l e a d i n g i n t h a t t h e g r o u n d w a t e r s a r e s i m p l y r i c h i n t h e s a l t s o f N a C l a n d C a S 0 4 .
W i t h t h e e v a p o r a t i o n 6 1 8 0 i s e n r i c h e d . O w i n g t o i t s u n d o u b t e d s l o w m o v e m e n t
s u c h w a t e r w o u l d h a v e l o w t r i t i u m v a l u e s a n d 1 4 C a c t i v i t y w o u l d d e c r e a s e t o w a r d s
t h e m a i n a q u i f e r .
( i i i ) T h e ‘N a 2S 0 4 ’ w a t e r r e c h a r g e s t h e m a i n a l l u v i u m p r o d u c i n g a n e d g e e f f e c t o f
r e l a t i v e l y h i g h s a l i n i t y g r o u n d w a t e r . T h e e d g e e f f e c t i s m a i n t a i n e d b y t h e f l o w o f
t h e g r o u n d w a t e r i n t h e m a i n a l l u v i u m . T h i s w a t e r i s C a ( H C 0 3)2 i n c h a r a c t e r ,
r e l a t i v e l y n o n - e n r i c h e d i n 6 180 a n d e n t e r s t h e a q u i f e r a s i n d i r e c t r e c h a r g e f r o m
f l o o d s .
FIG. 6. Wadi Bisha conceptual flow model.
212 LLO
YD
et al.
I A E A - A G - 1 5 8 /1 5 2 1 3
( i v ) S a l i n i t y a n o m a l i e s a w a y f r o m t h e a l l u v i a l e d g e a r e a t t r i b u t e d t o i r r i g a t i o n
r e t u r n e f f e c t s . ‘ N a 2S 0 4 ’ w a t e r s c a n b e c o n c e n t r a t e d b y t h i s m e a n s a n d N a C l w a t e r s
a r e p r o d u c e d f r o m t h e m a i n w a d i r e c h a r g e . S u c h w a t e r s s h o w e n r i c h m e n t o f 5 1 8 0 .
C O N C L U S I O N S
T h e c o n c e p t u a l m o d e l d e v i s e d p r o v i d e s a r e a s o n a b l e e x p l a n a t i o n o f t h e
h y d r o g e o l o g i c a l m e c h a n i s m s b e l i e v e d t o b e o p e r a t i n g a n d d e m o n s t r a t e s t h e u s e o f
i s o t o p e s t o g e t h e r w i t h c o n v e n t i o n a l m a j o r i o n c h e m i s t r y i n h y d r o c h e m i c a l
i n t e r p r e t a t i o n .
I t i s c o n c l u d e d t h a t o v e r a v a s t a r e a o f t h i n , g o o d p e r m e a b i l i t y a l l u v i u m ,
t h e l o n g - t e r m r e c h a r g e b e c o m e s c o n c e n t r a t e d a s e d g e w a t e r i n t h e m a i n w a d i a n d i s
s e e n t o b e h i g h l y s a l i n e w i t h l o w t r i t i u m v a l u e s a n d e n r i c h e d 5 1 8 0 . A l t h o u g h t h i s
r e c h a r g e i s s i g n i f i c a n t o v e r l o n g d i s t a n c e s a l o n g t h e e d g e s o f t h e m a i n a l l u v i a l
a q u i f e r , t h e f a c t t h a t i t i s r e t a i n e d i n i t s p o s i t i o n b y t h e m a i n r e c h a r g e p u l s e s a l o n g
t h e c e n t r a l w a d i b e d i n d i c a t e s t h a t t h e f l o w v o l u m e o f t h e s a l i n e g r o u n d w a t e r i s
r e l a t i v e l y s m a l l . T h e r e f o r e , t h e u n i t r e c h a r g e u n d e r p r e s e n t - d a y c o n d i t i o n s i n t h e
t h i n a l l u v i a l a r e a s , w h e r e t h e m e a n r a i n f a l l r a n g e s b e t w e e n 1 2 5 a n d 1 5 0 m m , m u s t
b e v e r y s m a l l .
A C K N O W L E D G E M E N T S
T h e a u t h o r s w o u l d l i k e t o t h a n k J a m e s F . M a c L a r e n L t d . , C a n a d a , f o r
p e r m i s s i o n t o p u b l i s h t h e p a p e r w h i c h i n c l u d e s d a t a c o l l e c t e d f o r t h e M i n i s t r y o f
A g r i c u l t u r e a n d W a t e r i n S a u d i A r a b i a . T h e i s o t o p e a n a l y s e s w e r e m a d e i n t h e
D e p a r t m e n t o f E a r t h S c i e n c e s , U n i v e r s i t y o f W a t e r l o o , C a n a d a .
I A E A - A G - 1 58/16
I S O T O P E M E T H O D S A S A T O O L
F O R Q U A T E R N A R Y S T U D I E S IN
S A U D I A R A B I A
H . H Ô T Z L * , C . J O B * * , H . M O S E R + , W . R A U E R T +,
W . S T I C H L E R * J . G . Z Ö T L * ,
* I n s t i t u t f ü r G e o l o g i e ,
U n i v e r s i t ä t K a r l s r u h e ,
F e d e r a l R e p u b l i c o f G e r m a n y
* * I n s t i t u t f ü r B a l n e o l o g i e ,
U n i v e r s i t ä t I n n s b r u c k , A u s t r i a
+ I n s t i t u t f ü r R a d i o h y d r o m e t r i e d e r
G e s e l l s c h a f t f ü r S t r a h l e n - u n d U m w e l t f o r s c h u n g ,
N e u h e r b e r g , F e d e r a l R e p u b l i c o f G e r m a n y
H+ A b t e i l u n g f ü r H y d r o g e o l o g i e ,
T e c h n i s c h e U n i v e r s i t ä t G r a z , A u s t r i a
A b s t r a c t
ISOTOPE METHODS AS A TOOL FO R Q UATERN ARY STUDIES IN SAUDI ARABIA.Within the framework of a combined sedimentological, hydrogeological, hydrochemical,
geomorphical and climatological investigation in central and eastern Saudi Arabia various environmental isotope studies were performed. 14 С measurements on shells from different shell banks in the Gulf coastal region confirmed the existence of sea-level fluctuations during the Würm time and during the Holocene. Measurements of 2H, 180 , and 3H contents of water samples from the Al Qatif and Al Hasa areas gave valuable information on the origin and the fate of these waters as well as on hydraulic connections between aquifer strata. 14C measurements on calcite crusts in the region As Sulb Plateau confirm that the precipitation of the “Neolithic Pluvial” reactivated the karst development in this region. In its southwestern part, different I4C values of stalactites and of duricrust cementing the stalactites show the development of duricrust during late Pleistocene and/or Holocene. In the upper part of Wadi Ar Rimah, a close insight into the causes of salination o f groundwaters was obtained by studying their contents o f 2H, 180 , and 3H in combination with hydrochemical data. It is shown that secondary and tertiary evaporation during irrigation and infiltration must be taken into consideration. In Wadi Ad Dawasir and its hinterland the 2H and 180 enrichment and that of the mineralization in hand-dug wells were explained by an admixture of recent shallow groundwater with old sandstone water.
2 1 5
2 1 6 H Ö T Z L et al.
The results reported here were elaborated within the framework of a combined sedimentological, hydrogeological, hydrochemical, geomorphical and climatological investigation in central and eastern Saudi Arabia. The project was carried out by an interdisciplinary team of 20 scientists from Saudi Arabia, Austria, the United States of America and the Federal Republic of Germany under the guidance of Professor Zötl. The whole investigation was recently published.1
In regional investigations environmental isotope studies contributed to solving problems concerned with sea-level fluctuations during the Quaternary period, with climatic variations and with hydrochemistry. The measurements of 2 H and 180 contents in water samples were performed by W. Stichler. W. Rauert measured the 3H and 14C contents in water samples; while 14C contents in sedimentary rocks were determined partly by W. Rauert and H. Felber2 , and the 34S contents byE. Pak2. The interpretation of the isotope contents resulted from discussions with C. Job (hydrochemist), H. Hötzl and V. Maurin (hydrogeologists)3.
1. QUATERNARY SEA-LEVEL FLUCTUATIONS INVESTIGATED IN THE GULF COASTAL REGION
To obtain a good knowledge of the Quaternary sea-level fluctuation along the east coastal zone, three areas were investigated (Fig. 1): the coastal area south of Al Jubayl and the shorelines north of Ras Tannurah and near the border with Qatar. 14C measurements on shells from different shell banks (Table I) gave some information on sea-level fluctuations during the Würm time and during the Holocene. A marine transgression during the interstadial period of the Middle Würm (40 000—26 000 years BP), where the maximum water level reached a few metres higher than the present sea level, proved to be possible by 14C measurements.
The post-glacial transgression began about 15 0Ö0 years ago BP. The sea-level rise occurred in stages interrupted by brief standstill phases of and partly by regression.
The existence of the Holocene transgression and a higher sea level during the Middle Holocene in the east coastal zone of Saudi Arabia provides a well-developed abrasion terrace in the area north of Ras Tannurah where well-cemented shells were collected. 14C measurements revealed a 14C age of about 4000 years BP (Table I, Nos 1,2, 3).
I N T R O D U C T I O N
1 AL-SAYARI, S.S., ZÖTL, J.G . (Eds), Quaternary Period in Saudi Arabia, Vol. 1, Springer Verlag Vienna-New York (1978).
2 Institut für Radiumforschung und Kernphysik der Österreichischen Akademie der Wissenschaften, Vienna, Austria.
3 Institut für Geologie, Universität Karlsruhe, Federal Republic of Germany.
IA E A - A G - 1 58/16 2 1 7
И Р ;« !
E S 3
Qaternary ( in general )Sabkhahs
sandstone, mafP1 and limestoneHofuf Formation
Dam Formation
Hadrukh Formations 47*
Dammam Formation
Dammam and Rus FormationRus Formation
\Umm er Radhuma Formation /Al AlahJabal Midra At JanubiJabaI Umm Ar Run
Ю0 km
4ST Ar Rub' Al Khali'¿ ¡ ¡y s o 1 5Г long
FIG .l. Generalized geological map o f the G ulf coastal region and its hinterland.
2 . I S O T O P E S A S A T O O L F O R I N V E S T I G A T I N G A L Q A T I F A N D A L H A S A
W A T E R S O U R C E S
T h e A l Q a t i f o a s e s c o v e r a n a r e a o f a b o u t 7 5 k m 2 b e t w e e n A d D a m m a m a n d
R a s T a n n u r a h ( F i g . 1 ) . F i g u r e 2 s h o w s ’ A y n A l L a b a n i y a , o n e o f t h e l a r g e s t s p r i n g s
o f t h e Q a t i f a r e a . F o r t h e e n t i r e c u l t i v a t e d a r e a o n e m a y a s s u m e a t o t a l d i s c h a r g e
o f 8 0 0 000 m 3 / d f r o m s p r i n g s a n d b o r e h o l e s .
T h e o a s i s o f A l H a s a i s a b o u t 7 0 k m t o w a r d s t h e i n t e r i o r o f t h e c o u n t r y f r o m
t h e G u l f c o a s t . A l H a s a i s o n e o f t h e l a r g e s t o a s e s i n t h e w o r l d a n d c o v e r s a
c u l t i v a t e d a r e a o f a b o u t 2 0 0 k m 2 . W i t h i n i t s m a i n t o w n , A l H o f u f , a n d a b o u t
6 0 v i l l a g e s a r e a b o u t 2 5 0 0 0 0 i n h a b i t a n t s . I n 1 9 7 7 t h e t o t a l d i s c h a r g e o f t h e s p r i n g s
a n d s o m e b o r e h o l e s w a s a b o u t 1 0 8 0 000 m 3/ d .
M e a s u r e m e n t s o f 2H , 180 , a n d 3 H c o n t e n t s o f w a t e r s f r o m t h e A l Q a t i f a n d
A l H a s a a r e a s g a v e v a l u a b l e i n f o r m a t i o n o n t h e o r i g i n a n d t h e f a t e o f t h e s e w a t e r s .
I n t h e A l H a s a a r e a , w a t e r s a m p l e s w e r e o n l y c o l l e c t e d i n s p r i n g 1 9 7 3 ( T a b l e I I ) .
T A B L E I . R E S U L T S O F 1 4 C M E A S U R E M E N T S O F S A M P L E S C O L L E C T E D I N 1 9 7 3 A T T H E S H O R E L I N E O F T H E
A R A B I A N G U L F A N D A L H A S A
LocalityGeographicposition
Laboratory Reg. No. Material14 С age (years BP)
1. Shoreline,wave-cut bench Lat. 26° 53’ N Shells of
Long. 49° 56' E IRMa Lab. 3 6 4 9 Cardies and Pectes 4 6 7 0 ± 190
2. Shell bank Lat. 26° 50' N Shells ofLong. 50° 00' E IRM Lab. 3650 Cardies and Pectes 3 3 8 0 ± 180
3. Shell bank Lat. 26° 30' N Cemented shells ofLong. 50° 00' E IRKWb VRI-406 Cardies and Pectes 3990 ± 90
4. Shells 1.5 kmwest of the shoreline Lat. 24° 45 ’ N
Long. 50° 45' E IRM Lab. 3652 Oyster shells > 3 8 800
5. Small hill in Al Hasa Lat. 25° 30' NLong. 49° 37' E IRM Lab. 3653 Snail shells 14 280 + 430
6. Al Hasa,2 m below surface Lat. 25° 30' N
Long. 49° 40' E IRM Lab. 3654 Snail shells 2180 + 210
7. Small hill in Al Hasa Lat. 25° 30' NLong. 49° 37' E IRKW VRI-405 Peat and charcoal 8290 ± 120
a Institut für Radiohydrometrie, Munich. b Institut für Radiumforschung und Kernphysik, Vienna.
218 H
ÖTZL
et al.
I A E A - A G - 1 58/16 2 1 9
FIG.2. ’Ayn Al Labaniyah, Al Qatif oases (photo taken by J.G. Zötl in 1975).
I n t h e A l Q a t i f a r e a t h r e e s e r i e s o f s a m p l e s w e r e c o l l e c t e d i n t h e s p r i n g o f 1 9 7 3 ,
1 9 7 4 a n d 1 9 7 5 . T h e r e s u l t s ( T a b l e I I I ) s h o w t h a t t h e d a t a o b t a i n e d a r e i n d e p e n d e n t
o f s a m p l i n g t i m e a n d i t w a s a s s u m e d t h a t t h i s i s a l s o t h e c a s e f o r t h e w a t e r s a m p l e s
t a k e n f r o m A l H a s a .
F r o m t h e ¿>2 H - 5 1 8 0 r e l a t i o n o f A l Q a t i f a n d A l H a s a w a t e r s ( F i g . 3 ) i t c a n b e
s e e n t h a t t h e v a l u e s l i e d i s t i n c t l y b e l o w t h e l i n e 5 2 H = 8 ; 6 1 8 0 + 1 0 , v a l i d f o r
p r e c i p i t a t i o n i n t e m p e r a t e c l i m a t e s , e . g . c e n t r a l E u r o p e . T h e r e c a n b e n o d o u b t
t h a t t h e e q u a t i o n l i n e o f t h e A l H a s a a n d A l Q a t i f w a t e r s a m p l e s i s b a s e d u p o n a
c l i m a t i c e f f e c t . T h e w a t e r s r e c e n t l y d i s c h a r g i n g f r o m s p r i n g s i n t h e A l H a s a a n d
A 1 Q a t i f o a s e s o r i g i n a t e f r o m p r e c i p i t a t i o n s e v a p o r a t e d f r o m t h e s e a u n d e r c l i m a t i c
c o n d i t i o n s o t h e r t h a n t h o s e t o d a y . T h e l o w 2 H e x c e s s v a l u e s , t ( d e f i n e d b y
ô 2 H = m 6 1 8 0 + t ) , l e a d t o t h e c o n c l u s i o n t h a t t h e w a t e r v a p o u r o f t h i s c o n d e n s a t e
w a s i n a b e t t e r e q u i l i b r i u m w i t h t h e s e a - w a t e r t h a n i s t h e c a s e n o w . T h e e v a p o r a t i o n
r a t e w a s e v i d e n t l y l o w e r t h a n t o d a y , a n d t h i s c o u l d b e d u e t o a c o o l e r a s w e l l a s a
m o r e h u m i d c l i m a t e t h a n t h a t t o d a y .
B y c o n t r a s t t o t h e A l Q a t i f a n d A l H a s a w a t e r s , t h e w a t e r s a m p l e s f r o m
W a d i H a n i f a h s h o w , a s e x p e c t e d f o r r e c e n t w a t e r s , t h e ô 2H - ô 1 8 0 r e l a t i o n o f r e c e n t
w a t e r s i n t h e M e d i t e r r a n e a n r e g i o n . T h e s t a b l e i s o t o p e r e s u l t s a r e c o n f i r m e d b y
T A B L E I I . S T A B L E I S O T O P E D A T A F R O M A L H A S A S P R I N G S 1 9 7 3
2 2 0 H Ö T Z L et al.
Location No. of sample 52 H(°/oo) 5180 (%„)
24 -39.8 -5.3625 -40.1 -5.2426 -39.9 -5.27
<oca 27 -39.8 -5.26caca
к28 -39.9 -5.07
< 29 -40.1 -5.30e 30 -39.8 -5.4000.5i_ 31 -40.0 -5.45a
32 -39.8 -5.26’Ayn Mansur 33 -39.6 -5.07
34 -40.1 -5.123о'С 35 -39.1 -5.07•> 35 -39.9 -5.10
36 -39.2 -4.87Agricultural farm 37 -39.8 -5.36’Ayn Sumbor 38 -40.0 -5.05’Ayn Haql 39 ' -39.4 -5.12’Ayn Shabah 40 -39.6 -5.20’Ayn Khudud 41 -38.8 -4.85’Ayn Najm 42 -40.0 -5.32Al ’Uyun 43 -36.7 -4.86
3 H a n d 1 4 C m e a s u r e m e n t s — t h e A l Q a t i f a n d A l H a s a w a t e r s a r e p r a c t i c a l l y
t r i t i u m - f r e e ( T a b l e s I V a n d V ) , w h e r e a s s o m e o f t h e W a d i H a n i f a h w e l l s o r s p r i n g s
( 4 6 , 4 8 , 4 9 , 5 0 ) h a v e h i g h e r t r i t i u m c o n c e n t r a t i o n s , t h u s s h o w i n g a m o r e o r l e s s
p r o n o u n c e d c o n t r i b u t i o n o f p r e s e n t r a i n f a l l s t o t h e f e e d i n g o f s h a l l o w g r o u n d w a t e r
w e l l s . T h e s e w a t e r s a m p l e s h a v e t h e h i g h e s t 2 H a n d 1 8 0 c o n t e n t s . L a t e r t h e
f e e d i n g m e c h a n i s m o f t h e w a d i w a t e r s i s d i s c u s s e d i n m o r e d e t a i l ( S e c t i o n 5 ) .
F o u r 1 4 C m e a s u r e m e n t s o f A l Q a t i f a n d A l H a s a w a t e r s ( T a b l e V I ) s h o w o l d w a t e r s ,
w h i c h i n f i l t r a t e d a c c o r d i n g t o t h e 180 a n d 2 H c o n t e n t s u n d e r c l i m a t i c c o n d i t i o n s
d i f f e r e n t f r o m t h o s e p r e v a i l i n g n o w .
T A B L E I I I . S T A B L E I S O T O P E D A T A O F A L Q A T I F A R E A I N 1 9 7 3 - 1 9 7 5
I A E A - A G - 1 58/16 2 2 1
Location No. of sample 62H(%o) 6 ,sO (°/oo)
’Ayn Al Labaniyah I 1973 2 -26.3 -3.631974 57 -26.4 -3 .771975 126 -26.3 -3 .82
’Ayn Al Labaniyah II 1973 3 -26.5 -3 .591974 58 -26.4 -3.831975 127 -26.3 -3 .84
’Ayn Hussain 1973 4 -26.2 -3 .721974 59 -26.3 -3 .701975 128 -26.5 -3 .76
’Ayn Subia I 1973 5 -25.7 -3.411974 64 -26.2 -3 .681975 129 -26.3 -3 .76
’Ayn Subia II 1973 13 -26.9 -3 .591974 65 -27.2 -3 .751975 130 -26.6 -3 .78
’Ayn Al Jaiamiyah 1973 15 -27.3 -3 .881974 61 -27.1 —3.75
Drainage near1975 131 -27.2 -3 .89
’Ayn Al Jaiamiyah 1973 n.s. n.s. n.s.1974 62 -25.8 -3 .601975 132 -25.6 -3 .69
’Ayn Muhairig 1973 14 -27.7 -3 .751974 63 -28.2 -3.831975 133 -27.2 -3 .95
’Ayn Obede (Al Ajam) 1973 11 -31.7 -4.111974 70 -31.7 -4 .4 01975 141 -31.4 -4.41
Hammam Balousa 1973 17 -26.8 -3 .341974 78 -27.3 -3 .441975 143 -24.6 -3 .44
Sea-water Dawhat Zaloom 1973 n.s. n.s. n.s.1974 67 +24.2 +5.041975 145 +27.6 +4.84
As Sabacha 11 (Al Ajam) 1973 n.s. n.s. n.s.1974 69 -30 .0 -4 .161975 123 -29.4 -4 .18
’Ayn Hussain (Al Ajam) 1973 n.s. n.s.' n.s.1974 71 -32.2 -4 .291975 124 -31.5 -4.36
’Ayn Towairit (Al Ajam) 1973 9 -27.8 -4 .681974 73 -28.1 -3 .921975 125 -28.5 -3 .89
n.s. — no sample.
2 2 2 H Ö T Z L et al.
СГ2Н % .
FIG.3. Relationship between S2H and 8 1S0 contents in water samples taken from Wadi Hanifah and Al Qatif oases and the Al Hasa area, in 1973.
I n t h e A l Q a t i f r e g i o n a v e r y s t r o n g p o s i t i v e c o r r e l a t i o n e x i s t s b e t w e e n 5 2 H a n d
s a l t c o n t e n t ( c o r r e l a t i o n c o e f f i c i e n t r = 0 . 9 ) a s w e l l a s s u l p h a t e c o n t e n t ( r = 0 . 9 5 ) .
B y c o n t r a s t , t h e r e i s a n e g a t i v e c o r r e l a t i o n b e t w e e n n i t r a t e a n d 5 2 H ( r = — 0 . 8 5 ) .
T h i s m a k e s i t p o s s i b l e t o e s t i m a t e t h e c o m p o s i t i o n o f t h e m i x i n g c o m p o n e n t s .
A n i n t e n s i v e s t u d y b y C . J o b o n t h e s e m i x i n g e f f e c t s c a n b e s u m m a r i z e d a s f o l l o w s :
A l l a q u i f e r s o f t h e l i m e s t o n e l a y e r s o f t h e T e r t i a r y s t r a t a a r e h y d r a u l i c a l l y c o n n e c t e d .
T h e s t r a t a s e r i e s c o n t a i n w a t e r s o f d i f f e r e n t s a l t c o n t e n t o w i n g t o l o c a l d i f f e r e n c e s
i n t h e g e o c h e m i c a l c o m p o s i t i o n o f t h e r o c k m a t e r i a l . S e a - w a t e r i n t r u s i o n a n d
s e e p a g e o f d r a i n a g e w a t e r s h a v e n o s i g n i f i c a n t i n f l u e n c e o n t h e s a l t c o n t e n t o f t h e
k a r s t w a t e r s . S a l t y u n d e r g r o u n d w a t e r s ( h i g h S 2 H v a l u e s ) m i x w i t h w a t e r s o f a
l o w e r s a l t a n d h e a v y i s o t o p e c o n t e n t . I n g e n e r a l t h e s a l t y w a t e r s c o m e f r o m N ,
w h e r e a s t h e f r e s h w a t e r c o m e s f r o m S ( F i g . 4 ) . S i m i l a r c o m b i n e d h y d r o c h e m i c a l -
i s o t o p e i n v e s t i g a t i o n s h a v e a l s o b e e n m a d e b y C . J o b i n o t h e r o a s e s .
I A E A - A G - 1 58/16
T A B L E I V . 3 H - C O N T E N T I N A L Q A T I F W A T E R S I N 1 9 7 3 - 1 9 7 5
2 2 3
LocationSample number
1973 1974 1975 19733H content (TU ) 1974 1975
’Ayn Al Labaniyah I 2 57 126 < 0 .8 < 3 .5 0.1 ± 1.3
’Ayn Al Labaniyah II 3 58 127 < 0 .8 < 4 .5 3 .3 ± 1.3
’Ayn Hussain 4 59 128 < 2 .3 < 2 .6 1.0 ± 1.4
’Ayn Subía I 5 64 129 < 0 .9 < 2 .2 0 .6 + 1.3
’Ayn Subia II 13 65 130 < 2 .3 < 4 .4 1 .0 ± 1.3
’Ayn A1 Jalamiyah 15 61 131 < 2 .7 < 4 .5 1.3 ± 1.2
Drainage near’Ayn Al Jalamiyah n.s. 62 132 n.s 3 .4 ± 2 .2 1.5 ± 1.3
’Ayn Muhairig 14 63 133 < 2 .7 < 4 .0 0 ± 1 .2
’Ayn Obede 11 70 141 < 0 .9 < 2 .2 0 .6 ± 1.0
Hammam Balousa 17 78 143 < 2 .2 2.5 ± 1.8 1.2 ± 1.0
Sea-water Dawhat Zaloom n.s. 67 145 n.s. 2 4 .4 ± 2.7 1 3 .2 ± 1.3
As Sabacha II (Al Ajam) n.s. 69 123 n.s. < 2 .2 0 .4 ± 1.4
’Ayn Hussain (Al Ajam) n.s. 71 124 n.s. < 3 .4 0 .5 ± 1.4
’Ayn Towairit (Al Ajam) 9 73 125 < 2 .7 < 4 .6 0.3 ± 1.4
3 . K A R S T I F I C A T I O N I N V E S T I G A T E D I N T H E R E G I O N O F T H E A S
S U L B P L A T E A U
T h e t h i c k c o m p l e x o f l i m e s t o n e s i n A s S u l b P l a t e a u ( F i g . l ) a r e s t r o n g l y
k a r s t i f i e d , b u t k a r s t p h e n o m e n a l i e b e l o w t h e s u r f a c e . I n t h e c a v e s o f t h e n o r t h e r n
a n d e a s t e r n p a r t n e i t h e r s t a l a c t i t e s n o r s t a l a g m i t e s w e r e f o u n d , o n l y s m a l l w a r t y
c a l c i t e d e p o s i t s . 1 4 C m e a s u r e m e n t s r e v e a l e d a 1 4 C a g e o f 5 0 0 0 y e a r s B P f o r t h e s e
c a l c i t e c r u s t s . F r o m t h e s e d a t a i t m a y b e c o n c l u d e d t h a t t h e p r e c i p i t a t i o n o f t h e
H o l o c e n e “ N e o l i t h i c P l u v i a l ” i n f i l t r a t e d i n t o t h e u n d e r g r o u n d , t h u s p r o v i n g t h e
a c t i v i t y o f k a r s t i f i c a t i o n u n t i l t h e H o l o c e n e .
I n t h e s o u t h w e s t e r n p a r t o f t h e P l a t e a u s t a l a c t i t e s c e m e n t e d i n t o d u r i c r u s t
h a v e b e e n f o u n d . F o r t h e s e t h e f o l l o w i n g g e n e t i c d e v e l o p m e n t m a y b e d e d u c e d :
D e v e l o p m e n t o f t h e s t a l a c t i t e s o n t h e r o o f o f a c a v e ;
F i l l i n g o f t h e c a v e b y a e o l i a n s a n d ;
C e m e n t a t i o n o f t h i s s a n d b y c a r b o n a t e s o l u t i o n s , ( r e ) - c r y s t a l l i z a t i o n o f c a l c i t e ; a n d
E r o s i o n o f t h e l i m e s t o n e o v e r l y i n g t h e c a v e s a n d r e a c t i v a t i o n o f k a r s t i f i c a t i o n .
2 2 4 H Ö T Z L et al.
T A B L E V . 3H - C O N T E N T I N A L H A S A W A T E R S A N D O F W E L L S I N
W A D I H A N I F A H ( 1 9 7 3 )
SampleNo.
Al Hasa
(TU)SampleNo.
(TU)
Wadi Hanifah Sample No.
24 < 0 .7 35 < 2 .5 44 < 1 2
25 < 2 .6 35a < 2 .6 45 < 4
26 < 2 .3 36 < 2 .3 46 7.0 ± 2
27 < 2 .5 37 < 2 .6 47 < 3 .8
28 < 0 .5 38 < 0 .3 48 74 ± 10
29 < 2 .5 39 < 2 .6 49 6 7 + 8
30 < 1 .2 40 < 1 .2 50 16 ± 2
31 < 2 .8 41 < 2 .4 51 < 4
32 < 0 .9 42 < 1 .9 52 < 0 .9
33 < 0 .9 43 < 2 .9 53 < 3 .8
34 < 2 .6 54 < 3 .4
a Twofold standard deviation
T A B L E V I . R E S U L T S O F 14C , 1 3 C A N D 3 H M E A S U R E M E N T S O F W A f E R
S A M P L E S C O L L E C T E D F R O M T H E A L Q A T I F A N D A L H A S A A R E A S ( 1 9 7 3 )
Place or No. of sample14C content“ (°/o modern)
14 С age (years BP, un corrected)
13 Cb
(°/oo)3H content (TU)
’Ayn Al Labaniyah I(Al Qatif) < 5 .9 > 2 2 000 - 9 .0 < 0 .8
’Ayn Al Labaniyah II(Al Qatif) < 1 .2 > 3 4 500 - 8 .8 < 2 .7
No. 32 (Al Hasa) < 1 .4 > 3 3 000 -1 0 .3 < 0 .9
’Ayn Mansur (Al Hasa) < 1 .4 > 3 3 000 - 1 0 .0 < 0 .9
a An initial 14C-content of 85% modem was assumed.b The I3C content is given as the relative per mille deviation from the limestone standard PDB.
I A E A - A G - 1 58 /16 2 2 5
О 50 100 150— — — ............P IP E L I N E
K IL O M E T E R S
FIG.4. Mineralization and flow directions o f underground waters between the outcrop region o f the Umm Er Radhuma Formation and the Gulf coastal area.
A s w a s e x p e c t e d , n o 1 4 C c o u l d b e m e a s u r e d i n t h e s t a l a c t i t e s ( > 3 7 0 0 0 y e a r s ) .
D u r i c r u s t s h o w e d , h o w e v e r , a s m a l l 1 4 C c o n t e n t ( u n c o r r e c t e d 3 5 0 0 0 y e a r s B P ) .
D e s p i t e s o m e u n c e r t a i n t i e s — c o n t e n t o f f o s s i l c a r b o n a t e , v o l u m e a n d t y p e o f
r e c r y s t a l l i z a t i o n — t h e s e 1 4 C m e a s u r e m e n t s s h o w a p a r t l y p r o g r e s s i v e d e v e l o p m e n t
o f d u r i c r u s t d u r i n g t h e l a t e P l e i s t o c e n e a n d / o r H o l o c e n e a n d a r e p e a t e d r e a c t i v a t i o n
o f t h e c a v e s y s t e m .
4 . M I N E R A L I Z A T I O N O F G R O U N D W A T E R D E M O N S T R A T E D B Y T H E
E X A M P L E O F W A D I A R R I M A H
T h e A r a b i a n P e n i n s u l a s h o w s t h r e e l a r g e o l d r i v e r s y s t e m s c r o s s i n g t h e e n t i r e
A r a b i a n S h e J f f r o m w e s t t o e a s t — W a d i A r R i m a h , W a d i A s S a h ’ b a ( w i t h i t s
f o r m e r u p p e r p a r t , t h e p r e s e n t W a d i B i r k ) , a n d W a d i A d D a w a s i r .
T h e Q u a t e r n a r y s t u d i e s i n t h e u p p e r p a r t o f W a d i A r R i m a h w e r e c a r r i e d o u t
o v e r a d i s t a n c e o f a b o u t 2 5 0 k m w e s t o f B u r a y d a h a n d ’ U n a y z a h , a l o n g a b o u t
L a t . 2 6 ° N , b e t w e e n L o n g . 4 2 ° 3 0 ’ a n d L o n g . 4 4 ° E ( F i g . 5 ) .
2 2 6 H Ö T Z L et al.
FIG.5. Generalized geological map o f the area o f Wadi Ar Rimah, with numbers o f wells or water samples, Qes, Qs, Qg = Quaternary aeolian sand, silt and gravel; S, Saq = sandstone;Gn = Gneissic gray granite; Gp = red and pink granite; Mu = Murdama schist; Di = diorite.
R i y a d h A l K h a b r a i n c l u d e s a f e w s m a l l o a s e s i n W a d i A r R i m a h . W a t e r s a m p l e s
w e r e c o l l e c t e d b y C . J o b f r o m d r i l l e d w e l l s ( 6 0 - 7 0 m ) , r e a c h i n g d o w n t h e S a q
s a n d s t o n e , w h e r e t h e w a t e r l e v e l l i e s 1 6 t o 2 0 m b e l o w g r o u n d s u r f a c e . W a t e r
t e m p e r a t u r e i s 2 7 . 5 ° C a n d t h e w a t e r i s s a t u r a t e d w i t h o x y g e n . T h e d e g r e e o f
m i n e r a l i z a t i o n i s h i g h l y v a r i a b l e ( 1 . 6 — 8 . 7 g / l t r ) a n d i n c r e a s e s w i t h t h e a g e o f t h e ,
w e l l s ( F i g . 6 ) . T h e s a l t c o n t e n t ( N a - C a - C l - S 0 4- s o l u t i o n , n i t r a t e u p t o 3 7 0 m g / l t r
a n d b o r i c a c i d u p t o 1 0 m g / l t r ) d e c r e a s e s , h o w e v e r , d u r i n g d a i l y p u m p i n g . ( F i g . 6 ) .
T h e e n r i c h m e n t o f v a r i o u s i o n s d o e s n o t i n c r e a s e r e g u l a r l y w i t h t h e t o t a l
c o n c e n t r a t i o n . T h u s , i t i s m o s t s t r i k i n g t h a t b r o m i d e a n d n i t r a t e a r e l e s s c o n c e n t r a t e d
t h a n c h l o r i d e , a l t h o u g h b o t h b r o m i d e a n d n i t r a t e a r e v e r y s o l u b l e . A c l o s e i n s i g h t
i n t o t h e c a u s e s o f m i n e r a l i z a t i o n o f t h e s e g r o u n d w a t e r s i s o b t a i n e d b y s t u d y i n g
t h e i r 2H , 1 8 0 a n d 3 H c o n t e n t s . I n t h i s c a s e , b e s i d e s p r i m a r y e v a p o r a t i o n o f s e a
w a t e r , t h e s u b s e q u e n t s e c o n d a r y a n d t e r t i a r y e v a p o r a t i o n m u s t b e t a k e n i n t o
c o n s i d e r a t i o n — i f a p a r t o f t h e r a i n - w a t e r o r s u r f a c e w a t e r e v a p o r a t e s a g a i n , a n
e n r i c h m e n t o f h e a v y i s o t o p e s o c c u r s i n t h e l i q u i d p h a s e . I n t h i s c a s e t h e s l o p e ,
a n d t h e r e f o r e a l s o t h e 2 H e x c e s s t i n t h e ô 2H - ô 1 8 0 r e l a t i o n , a r e n o l o n g e r c o n s t a n t
b u t d e p e n d o n s e v e r a l c l i m a t i c f a c t o r s ( q u a n t i t y a n d i n t e n s i t y o f r a i n f a l l , h u m i d i t y ,
a i r t e m p e r a t u r e , w i n d v e l o c i t y e t c . ) w h i c h d e t e r m i n e t h e e x t e n t o f t h i s s e c o n d a r y
e v a p o r a t i o n . V a r y i n g i s o t o p e c o n t e n t s o f p r e c i p i t a t i o n i n f i l t r a t i n g i n t o
g r o u n d w a t e r p r o d u c e a n a v e r a g e c o n t e n t b y m i x i n g a n d e x c h a n g e . I f g o u n d -
w a t e r a g a i n r e a c h e s t h e l a n d s u r f a c e t h r o u g h w e l l s a n d s p r i n g s , k i n e t i c a l l y
I A E A - A G - 1 58/16 2 2 7
RIYADH AL KHABRA
Watertable 1 6 - 2 0 m b.s. Borehole 60 - 75 m
N
FIG. 6. Parts o f the well area o f Riyadh Al Khabra with numbers o f wells or water samples.The tabulated data give sample number, age o f the well (i.e. time since extraction by pumping), time interval between the start o f the pumping and sample collecting (= sampled hours after start), water temperature, oxygen content (mg/ltrj, total mineralization in g/ltr (TDS) and content o f stable isotopes (8 2H, 8 isO). b.s. = below surface.
i n f l u e n c e d e v a p o r a t i o n w i l l b e c o m e a c t i v e a g a i n ( t e r t i a r y e v a p o r a t i o n ) . I n f i l t r a t i n g
o n c e m o r e , t h e s e w a t e r s h a v e l o w e r t v a l u e s i n t h e r e l a t i o n 6 2 H = 8 5 1 8 0 + t 8 ( w h e n
m = 8 , t w i l l b e w r i t t e n a s t 8 i n t h e f o l l o w i n g t e x t ) . I n t h i s i n f i l t r a t i o n w a t e r t h e
s a l t c o n c e n t r a t i o n w i l l b e h i g h e r , t o o ; t h i s i s n o t s o m u c h a r e s u l t o f e v a p o r a t i o n a s
t h a t d u e t o t h e h i g h s a l t c o n t e n t o f a r i d s o i l s . W h e n t h e s e w a t e r s a r e r e c y c l e d
i n t o w e l l s , t h e e n r i c h m e n t o f t h e - s t a b l e i s o t o p e c o n t e n t b y t e r t i a r y e v a p o r a t i o n w i l l
t h e r e f o r e b e c o m b i n e d w i t h a n i n c r e a s i n g s a l t c o n c e n t r a t i o n .
R e g a r d i n g t h e c o n t e n t o f d e u t e r i u m a n d 180 , t h e w a t e r s o f R i y a d h A l K h a b r a
d i f f e r s t r i k i n g l y f r o m t h e i s o t o p i c a l l y l i g h t e r o l d k a r s t w a t e r s . T h e 5 2 H v a l u e s a r e
b e t w e e n - 8 . 5 a n d — 1 0 . 9 ° / o o ; a n d t h e S 180 v a l u e s b e t w e e n - 1 . 2 5 a n d - 3 . 3 7 °/00 ( F i g . 7 ) .
T h e d e u t e r i u m c o n t e n t s h o w s n o s i g n i f i c a n t r e l a t i o n t o t h e t o t a l m i n e r a l i z a t i o n .
H o w e v e r , t h e 5 1 8 0 v a l u e s b e c o m e s i g n i f i c a n t l y h i g h e r , c o r r e s p o n d i n g t o t h e
i n c r e a s i n g d e g r e e o f t o t a l m i n e r a l i z a t i o n . T h e d i f f e r e n c e b e t w e e n t h e § 2 H a n d
6 1 8 0 v a l u e s d e p e n d s o n t h e f a c t t h a t t h e 5 2 H v a l u e s , i n r e l a t i o n t o m e a s u r e m e n t
a c c u r a c y , a r e m u c h l e s s i n f l u e n c e d b y t h e k i n e t i c s o f e v a p o r a t i o n t h a n 5 1 8 0 v a l u e s .
I n t h e r e l a t i o n S 2 H = 8 5 1 8 0 + t 8 t h e t 8 v a l u e i s s i g n i f i c a n t l y c o r r e l a t e d w i t h t h e
s a l t c o n t e n t . W e f o u n d t h a t t h i s i s t y p i c a l f o r w a t e r s o f t e r t i a r y e v a p o r a t i o n .
2 2 8 H Ö T Z L et al.
FIG.7. 82H-SlsO relation o f recent well waters in the area o f Riyadh Al Khabra (...), in Wadi Ar Rimah (A), and in Wadi Maraghan (xxx). Samples with a 3H content o f ~>4 TU are marked by circles. Note the different slopes o f the equation lines o f Riyadh Al Khabra waters (influenced by evaporation) and the mixed waters o f Wadi Ar Rimah.
T h e v e r y l o w t r i t i u m c o n t e n t s h o w s t h a t r e c e n t r a i n - w a t e r c a n n o t b e t h e m a i n a g e n t
w a s h i n g s o i l s a l t s i n t o t h e w e l l s .
S u m m a r i z i n g a l l o b s e r v a t i o n s m a d e i n t h e a r e a o f R i y a d h A l K h a b r a t h e
f o l l o w i n g a r g u m e n t s s p e a k i n f a v o u r o f a r e f l u x o f d r a i n a g e w a t e r s i n t o t h e w e l l s
( r e c y c l i n g ) : T h e d e c r e a s i n g d e g r e e o f m i n e r a l i z a t i o n o f t h e w a t e r d u r i n g p u m p i n g
( i . e . t h e l o w e r i n g w a t e r l e v e l ) i s e v i d e n c e t h a t t h e u p p e r g r o u n d w a t e r z o n e h a s a
h i g h e r s a l t c o n c e n t r a t i o n t h a n t h e l o w e r o n e . C o n s e q u e n t l y , t h e s a l t i n f i l t r a t i o n
m u s t c o m e f r o m a b o v e . F u r t h e r m o r e , m i n e r a l i z a t i o n p r o v e d t o b e c o r r e l a t e d w i t h
i s o t o p i c e n r i c h m e n t . F i n a l l y , t h e f a c t t h a t t h e i n c r e a s i n g d e g r e e o f m i n e r a l i z a t i o n
c o r r e s p o n d s t o t h e a g e o f t h e w e l l s u p p o r t s t h e t h e o r y o f r e c y c l i n g ; t h e l o n g e r t h i s
r e c y c l i n g p r o c e s s l a s t s , t h e m o r e s u r f a c e - e v a p o r i t e s a c c u m u l a t e d i n t h e Q u a t e r n a r y
s e d i m e n t s a r e d i s s o l v e d a n d c a r r i e d i n t o t h e a q u i f e r .
I n t h e W a d i A r R i m a h a n d W a d i M a r a g h a n a r e a ( a t r i b u t a r y t o W a d i A r R i m a h
w e s t o f R i y a d h A l K h a b r a ) t h e m i n e r a l i z a t i o n d e c r e a s e s w i t h i n c r e a s i n g 3 H c o n t e n t .
T h e t 8 v a l u e s d e c r e a s e s i g n i f i c a n t l y w i t h t h e m i n e r a l i z a t i o n . I n c o m p a r i s o n w i t h
t h e w a t e r s o f R i y a d h A l K h a b r a i t s e e m s t h a t t h e w a t e r s o f h i g h e r m i n e r a l i z a t i o n m a y
h a v e u n d e r g o n e a s t r o n g e r e v a p o r a t i o n p r o c e s s .
H o w e v e r , a s c a n b e s e e n f r o m s a m p l e N o s 9 3 — 9 8 i n T a b l e V I I a n d F i g . 7 ,
a d e c r e a s i n g t 8 v a l u e i s n o t l i n k e d t o a n i n c r e a s i n g 180 c o n t e n t — a s i t i s i n t h e
R i y a d h A l K h a b r a w a t e r s w h e n e v a p o r a t i o n i s i n v o l v e d — b u t m a i n l y t o a
d e c r e a s i n g d e u t e r i u m c o n t e n t . I n t h i s c a s e i t s e e m s t h a t t h e c h a n g e o f t 8 v a l u e - a n d
IA E A - A G - 1 58/16 2 2 9
TABLE VII. ISOTOPE CONTENTS OF WATER SAMPLES COLLECTED IN THE AREAS OF RIYADH AL KHABRA, WADI AR RIMAH,WADI MARAGHAN AND RAIN-WATER SAMPLE FROM A THUNDERSTORM AT ’UNAYZAH, 18 MARCH 1974 (AIR TEMPERATURE 18.5°C). WELL- WATER SAMPLES COLLECTED BETWEEN 18 AND 25 MARCH 1974
fs calculated fro m 6 2H = 8 X 8 1S0 + ts.
No. SJH i0/«,) 5lsO (°/oo) Í8 TU
87 -10.9 —2.37 8.06 0.8 ± 1.78 8 -9.7 —2 , 2 0 7.90 0.2 ± 1.789 -8.5 -1.73 5.34 2.5 ± 1.990 -9.3 -2.24 8.62 0 ± 1.791 -9.2 -2.18 8.24 1.3 ± 1.8 Riyadh Al Khabra
1 0 0 -9.6 -2.35 9.20 0.4+ 2.310 1 -8.5 -1.25 1.50 2.2 ± 1.91 0 2 -9.6 - 2 . 1 1 7.28 1.9 ± 2.1103 -9.9 -2.06 6.58 0.6 ± 2.3
92 +0 . 6 -0.18 2.04 2.9 ±1.893 -1.9 - 1 . 6 6 11.38 5.4 ± 1.894 ' -1.5 -1.99 14.42 59.1 ±3.9 Wadi Ar Rimah95 -4.0 -2.05 12.40 9.1 ± 1.996 - 2.1 -1.98 13.74 40.2 ±3.1
97 -5.6 - 2 . 2 1 12.08 23.0 ±3.198 -5.7 -1.81 8.78 22.7 ±3.1 Wadi Maraghan99 -7.7 -2.05 8.70 1.7 ± 2.3
8 6 +9.2 - 0 . 0 2 9.36 33.5 ± 2.8 Rain-water
therefore also the differences in mineralization — depends on the fact that older waters, free from tritium, poor in deuterium and more strongly mineralized, become diluted by younger waters of a lower mineralization, carrying tritium and an enriched deuterium content. Therefore, a decreasing t8 value combined with an increasing mineralization is not always a proof of tertiary evaporation, but can also occur in mixed waters. The gradient of the 52H-S180 relations shows which of these two possibilities is effective.
On the other hand, recycling is not always the only reason for a high degree of total groundwater mineralization. Thus, the high salinity of the wells in Wadi Maraghan (samples 97 and 98) is probably attributable to a contamination by an
2 3 0 H Ö T Z L et al.
100 km
FIG. 8. Geological map o f the recharge area o f Wadi Ad Dawasir (generalized after US Geol. Survey and Arabian American Oil Company, 1963). 1. Precambrian metamorphic rocks, mainly schists; 2. Basic igneous rocks, partly metamorphic; 3. Granite and granite gneiss;4. Permian (Wajid sandstone and K hu ff limestone) and Triassic (mainly silt and sandstone);5. Jurassic (mainly limestone); 6. Cretaceous (mainly sandstone); 7. Alluvium and related surficial deposits; 8. Aeolian sand; 9. Tertiary and Quaternary basalts; 10. Fault.
i n f i l t r a t i o n o f r i v e r w a t e r , w h i c h i n f i l t r a t e s i n t o t h e w e l l t h r o u g h s a l i n a t e d
Q u a t e r n a r y d e p o s i t s . T h e i n f l o w o f w a t e r s o f h i g h s a l t c o n t e n t f r o m Q u a t e r n a r y
d e p o s i t s e x p l a i n s t h e o c c u r r e n c e o f h i g h l y m i n e r a l i z e d w a t e r i n l e s s s o l u b l e r o c k s ,
e . g . g r a n i t e , s c h i s t s a n d s a n d s t o n e .
T h i s g e n e r a l c o n c l u s i o n f r o m h y d r o c h e m i c a l i n v e s t i g a t i o n s m a k e s i t c l e a r t h a t ,
f o r t h e a g r i c u l t u r a l d e v e l o p m e n t o f a r i d r e g i o n s , n o t o n l y t h e w a t e r s u p p l y i s
i m p o r t a n t f o r i r r i g a t i o n b u t a l s o a w e l l - c o n t r o l l e d d r a i n a g e s y s t e m . F r o m t h i s p o i n t
o f v i e w c u l t i v a t i o n s h o u l d g e n e r a l l y n o t u s e w a d i f l o o r s a n d b a s i n s b u t h i g h e r
I A E A - A G - 1 58/16 23 1
l o c a t e d a r e a s a n d f l a t s l o p e s w h e r e t h e r e i s a f r e e r u n o f f o f i r r i g a t i o n w a t e r , a n d
w h e r e n o i n f i l t r a t i o n o f s a l t w a t e r f r o m Q u a t e r n a r y a c c u m u l a t i o n s m a y o c c u r .
T o c a l c u l a t e t h e d e p t h o f b o r e h o l e s a n d t h e i r c a s i n g , t h e p o s s i b i l i t i e s o f g r o u n d w a t e r
m i n e r a l i z a t i o n t h r o u g h r e f l u x o r i n f l o w o f h i g h l y m i n e r a l i z e d w a t e r s f r o m Q u a t e r n a r y
w a d i s e d i m e n t s s h o u l d b e t a k e n i n t o c o n s i d e r a t i o n .
5 . E N R I C H M E N T O F S T A B L E I S O T O P E S I N H A N D - D U G W E L L S A S A G A I N S T
A R T E S I A N W E L L S A N D C O R R E L A T I O N B E T W E E N M I N E R A L I Z A T I O N
A N D I S O T O P E C O N T E N T , I N V E S T I G A T E D I N W A D I A D D A W A S I R A N D
I T S H I N T E R L A N D
W a d i A d D a w a s i r i s t h e o l d r i v e r s y s t e m c r o s s i n g t h e c u e s t a l a n d s c a p e o f
S a u d i A r a b i a i n i t s s o u t h e r n p a r t . T h e m a i n t r i b u t a r i e s a r e t h e p r e s e n t W a d i T a t h l i t h ,
W a d i B i s h a h , W a d i R a n y a h a n d W a d i S u b a y ’ ( F i g . 8) . T h e n a m e W a d i A d D a w a s i r i s
g e n e r a l l y u s e d d o w n s t r e a m o f t h e j u n c t i o n o f W a d i T a t h l i t h a n d W a d i B i s h a h .
W a d i R a n y a h h a s c u t i t s w a y t h r o u g h a b a s a l t f l o w . T h i s b a s a l t f l o w s h o w s
i n t e n s i v e w e a t h e r i n g . L a r g e a r e a s o f t h e a c c u m u l a t i o n p l a i n , e s p e c i a l l y s o u t h o f t h e
w a d i c h a n n e l , c o n s i s t o f a u t o c h t h o n o u s b a s a l t d e t r i t u s a n d a e o l i a n s a n d o f s m a l l
t h i c k n e s s . T h e u n d e r l y i n g c l a y e y w e a t h e r e d s o i l r e a c h e s a t h i c k n e s s o f a b o u t 4 m .
I n s o m e p i t s u s e d f o r r e m o v i n g m a t e r i a l f o r b u i l d i n g s . s e v e r a l l i m e y s o i l h o r i z o n s ,
s i g n s o f f o r m e r g r o u n d w a t e r l e v e l s a r e d e t e c t a b l e .
T h e s e o l d s o i l В h o r i z o n s , t o g e t h e r w i t h t h e c l a y e y l a y e r s , e m p h a s i z e t h e
i n t e n s i v e c h e m i c a l w e a t h e r i n g p r o c e s s t h a t f o l l o w e d t h e e r u p t i o n o f t h e b a s a l t s .
S a m p l e s f o r 1 4 C m e a s u r e m e n t s w e r e c o l l e c t e d f r o m t h e l i m e y c r u s t s a t d e p t h s o f
0 . 7 a n d 1 . 0 m b e l o w t h e l a n d s u r f a c e . T h e m e a s u r e m e n t s r e v e a l e d 1 4 C a g e s o f
2 6 4 0 0 a n d 2 9 8 4 0 y e a r s B P f o r t h e o r i g i n o f t h e s e c a l c i t e h o r i z o n s ( s e e T a b l e V I I I ) .
I n c o n t r a s t t o o u r e x p e c t a t i o n s , t h e s e r e s u l t s l e a d t o t h e c o n c l u s i o n t h a t t h e r e w a s
e n o u g h p r e c i p i t a t i o n d u r i n g t h e W ü r m i n t e r s t a d i a l t o p r o d u c e s u c h s o i l c a l i c h e f o r
t h i s p e r i o d .
W h i l e t h e p l a i n b e t w e e n t h e v i l l a g e s o f M u q a b i l a n d R a w d h a h s h o w s a c e r t a i n
r e l i e f c a u s e d b y b a s a l t f l o w s a n d p e d i m e n t s w i t h o u t c r o p p i n g r o c k s a n d d i f f e r e n t
d e t r i t u s c o v e r s , t h e f l a t b a s i n e a s t o f R a w d h a h o a s i s t a k e s o n m o r e a n d m o r e t h e
c h a r a c t e r o f a n a c c u m u l a t i o n p l a i n t o w a r d s t h e e a s t . T h e a l l u v i a l f a n s f r o m t h e
s o u t h s t i l l s h o w a n o r t h w a r d s f l o w d i r e c t i o n , i . e . t o w a r d s t h e o l d e a s t - w e s t t r e n d i n g
w a d i . N o r t h o f t h e w a d i t h e a c c u m u l a t i o n p l a i n d i s p l a y s m o r e a n d m o r e a
h o m o g e n e o u s e a s t w a r d s d i p . A t p r e s e n t t h e o l d w a d i c h a n n e l i s a c c o m p a n i e d b y
d u n e s o n b o t h s i d e s . T h e s e a e o l i a n a c c u m u l a t i o n s d e m o n s t r a t e t h e w i n d t r a n s p o r t
o f f i n e m a t e r i a l s b l o w n o u t f r o m t h e u p p e r a r e a s o f t h e a c c u m u l a t i o n p l a i n .
I n m o s t c a s e s a t W a d i A d D a w a s i r , h a n d - d u g w e l l s s u p p l y w a t e r . W a t e r o c c u r s
8 - 1 2 m b e l o w g r o u n d s u r f a c e w i t h a t e m p e r a t u r e o f 2 7 - 3 1 ° C a n d a s t r o n g
m i n e r a l i z a t i o n , 2 8 - 1 0 4 m e q / l t r . C h e m i c a l l y , t h e s e a r e m o s t l y N a - C a - C l - S 0 4 w a t e r s ,
T A B L E V I I I . R E S U L T S O F 1 4 C M E A S U R E M E N T S I N T H E A R E A O F W A D I A D D A W A S I R A N D W A D I R A N Y A H
( 1 9 7 5 )
Locality Geograph.position
Laboratory Reg.-No. Material 14 С age (years BP)
1 Bir Juqjuq Lat. 21° 18' N IRMa Lab.-No. 4 231 Weathered limey crust, > 3 5 000Long. 43° 42' E soil horizon in terrace
2 Bir Juqjuq Lat. 21° 18'- N IRM Lab.-No. 4 232 Limey crust in terrace 10 820 ± 3 2 0 bLong. 43° 42' E
3 Loam pit Muqabil Lat. 21° 14' N IRM Lab.-No. 4 233 Limey weathered material 26 400 ± 1970Wadi Ranyah Long. 42° 46' E overlying basalt; sample
0.7 m below land surface
4 Loam pit Muqabil Lat. 21° 14' N IRM Lab.-No. 4 234 Limey weathered material 29 840 ± 2600Wadi Ranyah Long. 42° 46' E overlying basalt; sample
1.0 m below land surface
5 Sha’ib Hathag Lat. 21° 15' N IRM Lab.-No. 4 235 Calcite sinter 6 7 0 0 ± 280NW of Al Jirthamiyah Long. 42° 44' E
6 Sha’ib Hathag Lat. 21° 15' N IRM Lab.-No. 4 236 Calcite sinter 6 1 1 0 + 250NW of Al Jirthamiyah Long. 42° 44' E
a Institut für Radiohydrometrie, Munich. b Twofold standard deviation.
23
2
HÖ
TZL et
al.
I A E A - A G - 1 58/16 2 3 3
T A B L E I X . I S O T O P E C O N T E N T S O F T H E W A T E R S O F W A D I A D D A W A S I R
( S A M P L E S T A K E N B E T W E E N 2 1 A N D 2 7 F E B R U A R Y 1 9 7 5 )
i8 is c o m p u te d f r o m 8 2H = 8 X 8 l s O + t 8
No.of sample S2 H(°/oo) 5180(°/oo) *8
3H content (TU)
164 -46.5 -6.52 5.66 2 . 8 ± 2 . 0
166 -42.9 -6.06 5.58 1.9 ±2.5167 -56.5 - 8 . 0 2 7.66 1 . 6 ± 2 . 0
170 -49.0 -6.98 6.84 1.5 ± 2.0171 -47.9 -6.73 5.94 0.7 ± 1.9
157 -38.5 -5.60 6.30 0.3 ± 1 -158 -44.8 -6.08 3.84 0.7 ± 1.9159 -44.3 -5.68 1.14 0.9 ± 2.0160 -41.2 -5.64 3.92 0 ± 2 . 0
161 -35.7 -4.93 3.74 2.3 ± 2.0162 -26.3 -4.39 8.82 0.5 ± 2.0
163 -33.7 -4.89 5.42 1 1 . 0 ± 2 . 2
165 -26.3 -3.61 2.58 1.5 ± 1.9168 -5.7 - 2 . 0 0 10.30 45 ±3169 -31.4 -4.71 6.28 2 . 8 ± 2 . 0
151 - 6 . 8 -2.48 13.04 28 ± 2
152 -5.7 -1.97 10.06 35 ± 2153 -7.7 -2.03 8.54 44 ± 2.3154 -4.8 -1.87 10.16 30± 2155 - 6 . 2 -1.76 7.88 30 ± 2
p a r t l y w i t h a h i g h M g c o n t e n t . S a m p l e N o . 1 6 8 ( T a b l e I X ) c o n t a i n s s t r o n g p o r t i o n s
o f r e c e n t r a i n - w a t e r a c c o r d i n g t o t h e 3 H a n d s t a b l e i s o t o p e c o n t e n t s . A l l o t h e r
w a t e r s b e l o n g t o a c o n s i d e r a b l y o l d e r s t o r a g e a r e a w i t h w i d e l y r a n g i n g v a l u e s .
T h e a r t e s i a n w e l l s c a n b e d i f f e r e n t i a t e d b y t h e i r g y p s u m c o n t e n t . T h e y h a v e
a l o w d e g r e e o f m i n e r a l i z a t i o n ( 8 — 1 2 m e q / l t r ) . A s c a n b e e x p e c t e d , a l l a r t e s i a n
w a t e r s a r e f r e e o f 3H . T h e i s o t o p e d a t a s h o w n o r e l a t i o n t o t h e t e m p e r a t u r e s .
M o s t h a n d - d u g w e l l s s h o w l e s s n e g a t i v e 5 - v a l u e s t h a n a r t e s i a n w e l l s ( F i g . 9 ) .
A t f i r s t s i g h t o n e m i g h t , t h e r e f o r e , b e t e m p t e d t o e x p l a i n t h e p h e n o m e n o n o f
i s o t o p e e n r i c h m e n t a s r e s u l t i n g f r o m a n e v a p o r a t i o n p r o c e s s , b e g i n n i n g i n a r t e s i a n
2 3 4 H Ö T Z L et al.
<Г*0'А'
FIG.9. 5 2Я -5180 relation o f the waters o f Wadi Ad Dawasir and Wadi Ranyah, together with the equation lines fo r artesian waters ( l j and waters from hand-dug wells (2). The broken line stands fo r mixtures o f artesian waters with the groundwater originating in the Shield.
w e l l s a n d c o n t i n u i n g i n h a n d - d u g w e l l s t o a n e v e r - i n c r e a s i n g e x t e n t . A n o t h e r
a r g u m e n t s e e m s t o b e t h e p o s i t i v e r e l a t i o n s h i p b e t w e e n t h e d e g r e e o f m i n e r a l i z a t i o n
a n d t h e e n r i c h m e n t o f d e u t e r i u m a s w e l l a s 180 .
T w o q u e s t i o n s a r e o f d e c i s i v e i m p o r t a n c e c o n c e r n i n g t h e s e w a t e r s — f i r s t ,
w h a t i s t h e r e a s o n f o r t h e e n r i c h m e n t o f s t a b l e i s o t o p e s i n h a n d - d u g w e l l s a s a g a i n s t
a r t e s i a n w e l l s ? S e c o n d , h o w c a n t h e c o r r e l a t i o n b e t w e e n m i n e r a l i z a t i o n a n d i s o t o p e
c o n t e n t b e e x p l a i n e d i f t h e r e i s n o e v a p o r a t i o n ? I t h a s b e e n o b s e r v e d t h a t d r i l l e d
w e l l N o . 1 6 2 ( t h e w a t e r o f w h i c h i s n o t c o n n e c t e d w i t h t h e u p p e r l a y e r s o f t h e
w a d i f l o o r o w i n g t o a c a s i n g r e a c h i n g d o w n 3 0 m ) h a s a r e l a t i v e l y l o w d e g r e e o f
m i n e r a l i z a t i o n y e t i t i s s t r o n g l y e n r i c h e d i s o t o p i c a l l y . T h i s e n r i c h m e n t c a n n o t h a v e
o c c u r r e d i n t h e w a k e o f e v a p o r a t i o n b e c a u s e o f i t s h i g h t s v a l u e . I t w a s i n d i c a t e d
i n S e c t i o n 4 t h a t i s o t o p e s h i f t i n g , t o g e t h e r w i t h c h a n g e s i n t h e e x t e n t o f
m i n e r a l i z a t i o n m a y b e e x p l a i n e d a s d u e n o t o n l y t o e v a p o r a t i o n b u t a l s o t o t h e
m i x i n g o f w a t e r s .
I n W a d i A d D a w a s i r s u c h a p o t e n t i a l i t y a r i s e s t h r o u g h a s h a l l o w g r o u n d w a t e r
f l o w c o m i n g f r o m t h e v a s t r e c h a r g e a r e a i n t h e c r y s t a l l i n e r o c k o f t h e s h i e l d ,
f o l l o w i n g a c h a n n e l f i l l e d w i t h Q u a t e r n a r y s e d i m e n t s . W a t e r s t a k e n f r o m v a l l e y
f i l l i n g s o f t h e s h i e l d i n W a d i R a n y a h s i t u a t e d s o m e 2 0 0 k m f r o m A l K h a m a s i n w e r e
p o o r l y m i n e r a l i z e d ( 4 — 1 3 m e q / l t r ) a n d s h o w e d a m e d i u m d e u t e r i u m c o n t e n t o f
S 2 H = — 6 % 0 a n d a m e d i u m 1 8 0 c o n t e n t o f 5 1 8 0 = — 2 % 0 . A s s u m i n g t h a t t h e s e
i s o t o p e v a l u e s w e r e l i k e w i s e v a l i d f o r t h e g r o u n d w a t e r o f W a d i A d D a w a s i r o r i g i n a t i n g
I A E A - A G - 1 58/16 2 3 5
i n t h e s h i e l d , a n d a s s u m i n g t h a t a r t e s i a n w e l l N o . 1 6 4 ( T a b l e I X ) i s r e p r e s e n t a t i v e
o f t h e o l d s a n d s t o n e w a t e r i n t h e a r e a o f A l K h a m a s i n , t h e m i x t u r e s o f t h e s e t w o
c o m p o n e n t s s h o u l d t h e n l i e o n t h e b r o k e n l i n e d r a w n i n F i g . 9 . T h i s i s i n d e e d t r u e
f o r t h e i s o t o p e v a l u e s o f w e l l N o . 1 6 2 , w h i c h i s p r o v i d e d w i t h a c a s i n g d o w n t o
l o w e r p a r t s o f t h e w a d i f i l l i n g . B y c o n t r a s t , e x c e p t N o . 1 5 7 , a l l h a n d - d u g w e l l s
( p r o b a b l y b e c a u s e o f s t r o n g e r e v a p o r a t i o n ) a r e g r o u p e d a r o u n d a s l i g h t l y f l a t t e r
e q u a t i o n l i n e d r a w n b n t h e r i g h t i n F i g . 9 ( 0 2 H = 7 . 9 5 X 5 1 8 0 + 3 . 8 9 ) . T h i s e q u a t i o n
l i n e s h o w s t h a t t h e s h i f t i n g o f i s o t o p e c o n t e n t m a y o c c u r p a r a l l e l t o 5 2 H = 8 X 5 1 8 0 ,
o w i n g t o a c o m b i n a t i o n o f m i x i n g a n d e v a p o r a t i o n , a l t h o u g h i t h a s g e n e r a l l y b e e n
a s s u m e d t h a t a s l o p e o f m = 8 i s a s u r e s i g n t h a t n o k i n e t i c a l l y i n f l u e n c e d e v a p o r a t i o n
h a s t a k e n p l a c e .
T h e s e d e l i b e r a t i o n s l e a d t o t h e c o n c l u s i o n t h a t t h e e n r i c h m e n t o f i s o t o p e s a s
w e l l a s t h e m i n e r a l i z a t i o n i n h a n d - d u g w e l l s m a y b e e x p l a i n e d b y a n a d m i x t u r e o f
r e c e n t s h a l l o w g r o u n d w a t e r t o o l d s a n d s t o n e w a t e r , p r o v i d e d t h a t t h e r e c e n t w a t e r
i n t h e l o w e r p a r t s o f t h e w a d i f i l l i n g d i s p l a y s a p o o r e r d e g r e e o f m i n e r a l i z a t i o n t h a n
t h a t i n t h e u p p e r p a r t s .
I A E A - A G - 1 5 8 /1 7
E N V I R O N M E N T A L I S O T O P E S T U D Y
O F G R O U N D W A T E R S Y S T E M S IN
T H E R E P U B L I C O F D J I B O U T I
J . C h . F O N T E S *
L a b o r a t o i r e d e G é o l o g i e D y n a m i q u e ,
U n i v e r s i t é P i e r r e e t M a r i e C u r i e , P a r i s
P . P O U C H O N
L a b o r a t o i r e d e G é o d y n a m i q u e ,
U n i v e r s i t é d e B o r d e a u x I I I , T a l e n c e
J . F . S A L I E G E , G . M . Z U P P I *
L a b o r a t o i r e d e G é o l o g i e D y n a m i q u e ,
U n i v e r s i t é P i e r r e e t M a r i e C u r i e , P a r i s ,
F r a n c e
A b s t r a c t
ENVIRONMENTAL ISOTOPE STUDY OF GROUNDWATER SYSTEMS IN THE REPUBLIC OF DJIBOUTI.
Environmental isotopes and hydrogeochemistry are being used to shed new light on the occurrence of present-day recharge and on the origin of groundwater systems in the Republic of Djibouti. Furthermore, an attempt is also being made to evaluate palaeohydrological conditions during the past 6000 years. From stable isotope data which he along a correlation line at a slope o f 8 in the diagram 6 2H-6180 , it can be concluded that recharge occurs by rapid seepage in fractured rocks without evaporation. Some waters from hot springs show an oxygen shift, indicating the occurrence of an exchange process with rocks at high temperatures. The following conclusions can be reached from tritium and 14C content of waters. Groundwaters can be divided into two groups: one deriving from recent recharge (last five or six years) corresponding to water with rather fast circulations in fractured media; and a second group, pre-bomb recharged corresponding to water with low flow rates in porous media. Only one sample (Yoboki) seems to derive from about 10-year-old recharge. In the case of Abhè hot spring, a 14C age of about 1200 years may be evaluated. The calcite concretions of the Abhè Lake Basin are believed to have formed as a result of the mixing of lake water (sodium-carbonate type) with groundwater (sodium-chloride, calcium-sulphate type). From the 13C and 14C content it appears that the dissolved carbon of present-day lake water is in, or close to, equilibrium with the atmosphere. Consequently, it is assumed that such was also the case during the whole Holocene. The 180 content of palaeolake water, evaluated from the calcite isotopic composition with the palaeo- temperature equation, was originally more negative than the present one. This is interpreted as due to the fact that the Holocene lake was fed by large floods and that significant seepage occurred through the lake bottom with a consequent reduction of the evaporation effects.
* The present address of Mr. Fontes and Mr. Zuppi is given in the List of Participants.
2 3 7
2 3 8 F O N T E S et al.
T A B L E I . C L I M A T I C C O N D I T I O N S I N T H E R E P U B L I C O F D J I B O U T I
Location Altitude(m )
Annualrainfall(average)(mm )
Rainy days Annualevaporation(average)(mm)
Annualtemperature(average)
- (°C )
Djibouti + 10 177 17 2 8 1 0 29 .9
Arta 700 24 6 28 25 0 0 26.1
FIG .l. Location map o f Djibouti area and distribution o f sampling points.
1 . H Y D R O G E O L O G I C A L F R A M E W O R K A N D P R O B L E M S
T h e R e p u b l i c o f D j i b o u t i ( F i g . l ) i s l o c a t e d i n t h e a r i d z o n e o f e a s t e r n A f r i c a
( 1 1 t o 1 3 ° l a t . N ) . P r e c i p i t a t i o n i s r a n d o m l y d i s t r i b u t e d o n t h e c o a s t a n d b e c o m e s
m o n s o o n i n t y p e i n t h e i n l a n d ( s u m m e r r a i n s ) . T e m p e r a t u r e s a r e h i g h a n d s h o w
l i t t l e m o n t h l y v a r i a t i o n s . C l i m a t i c d a t a ( a v e r a g e a n n u a l v a l u e s ) a r e s u m m a r i z e d
i n T a b l e I . T h e r a t i o p r e c i p i t a t i o n / e v a p o r a t i o n w h i c h c a n b e u s e d t o d e f i n e t h e
a r i d i t y v a r i e s b e t w e e n 0 . 0 5 ( D j i b o u t i ) a n d 0 . 1 ( A r t a ) a c c o r d i n g t o t h e a l t i t u d e
o f t h e s t a t i o n .
T h e w h o l e c o u n t r y i s p r a c t i c a l l y c o v e r e d b y l a v a a n d v o l c a n o d e t r i t a l d e p o s i t s
f r o m T e r t i a r y a n d Q u a t e r n a r y a g e s , s o m e o f t h e m v e r y r e c e n t . T h i s v o l c a n i c
a c t i v i t y i s d u e t o t h e l o c a t i o n o f t h e a r e a o n t h e s o u t h e r n p a r t o f t h e A r a r r i f t .
IA E A - A G - 1 5 8 /1 7 2 3 9
V e r t i c a l m o v e m e n t s a l o n g t h e f a u l t s o f t h e r i f t s t r u c t u r e g i v e r i s e t o i s o l a t e d
b a s i n s w i t h i n t e r n a l d r a i n a g e ( G r a n d B a r a , A b h è , H e n l é , G o b a a d , A s a l ) . D u r i n g
t h e U p p e r P l e i s t o c e n e a n d u n t i l a b o u t 4 0 0 0 y e a r s a g o , l a c u s t r i n e e p i s o d e s t o o k
p l a c e i n t h e s e d e p r e s s i o n s [ 1 , 2 ] . A t p r e s e n t t w o b a s i n s a r e s t i l l a c t i v e , b o t h
o c c u p i e d b y h i g h l y s a l i n e l a k e s . L a k e A b h è i s t h e t e r m i n a l l a k e o f t h e A w a s h
R i v e r s y s t e m w h i c h c o m e s f r o m t h e E t h i o p i a n P l a t e a u . I t i s a s o d i u m - c a r b o n a t e
t y p e w a t e r ( T D S ~ 1 5 0 g ) . L a k e A s a l i s m a i n l y s u p p l i e d b y s e a - w a t e r u n d e r f l o w
a n d r e a c h e s s a t u r a t i o n i n s o d i u m c h l o r i d e [ 3 ] .
T h e s u r f a c e n e t w o r k a t p r e s e n t c o n s i s t s o f i n t e r m i t t e n t w a d i s . G r o u n d w a t e r ,
s y s t e m s a r e g o v e r n e d b y g e o l o g i c a l a n d s e d i m e n t o l o g i c a l f e a t u r e s . P r e c i p i t a t i o n
a n d / o r f l o o d s c a n i n f i l t r a t e t h r o u g h h i g h l y p e r m e a b l e f r a c t u r e d l a v a s a n d a c c u m u l a t e
w i t h i n s e d i m e n t s i n t h e d e p r e s s i o n s w h e r e p o r o s i t y a n d p e r m e a b i l i t y a r e e x t r e m e l y
l o w o w i n g t o t h e o c c u r r e n c e o f a h i g h l o a d i n c l a y m i n e r a l s .
P i e z o m e t r i c s u r f a c e s a r e v e r y d e e p a n d r a n g e f r o m 2 0 0 m b e l o w g r o u n d l e v e l
i n t h e n o r t h o f t h e c o u n t r y t o 3 0 m i n t h e c o a s t a l r e g i o n o f D j i b o u t i . B e c a u s e o f
t h e h i g h g e o t h e r m a l f l u x i n t h e r i f t a r e a , g r o u n d w a t e r s a r e w a r m ( 3 0 t o 4 0 ° C f o r
s h a l l o w s y s t e m s t o b o i l i n g p o i n t f o r d e e p s y s t e m s ) . T h e p u r p o s e s o f t h e e n v i r o n
m e n t a l i s o t o p e i n v e s t i g a t i o n w e r e t o d e t e r m i n e :
T h e o c c u r r e n c e a n d i m p o r t a n c e o f t h e p r e s e n t - d a y r e c h a r g e ;
T h e o r i g i n o f g r o u n d w a t e r ( l o c a l i n f i l t r a t i o n , s u p p l y f r o m h i g h e r a l t i t u d e ,
o l d w a t e r s , i . e . p a l a e o w a t e r s ) ;
T h e i n f l u e n c e o f e v a p o r a t i o n b e f o r e o r d u r i n g t h e r e c h a r g e ; a n d
T h e a g e o f p a l a e o g r o u n d w a t e r s , i f a n y , w h i c h c o u l d b e i n v o l v e d i n g r o u n d
w a t e r s y s t e m s .
T h e i r r e g u l a r r e g i m e o f a 2 5 t o 2 2 4 m m p r e c i p i t a t i o n f o r a n n u a l r a i n f a l l
i n t h e c i t y o f D j i b o u t i ( o v e r t h e p e r i o d 1 9 0 1 — 1 9 4 5 ) ; t h e p o o r k n o w l e d g e o f
é v a p o t r a n s p i r a t i o n v a l u e s ; a n d t h e d i f f i c u l t y o f g r o u n d w a t e r f l o w s t u d y b y
c o n v e n t i o n a l m e a n s i n f r a c t u r e d r o c k s a n d i n b a s i n s o f i n t e r n a l d r a i n a g e , m a k e
t h e h y d r o g e o l o g i c a l s t u d y d i f f i c u l t a n d t h u s i t s i s o t o p i c c o m p l e m e n t a t t r a c t i v e .
F u r t h e r m o r e , s o m e l a c u s t r i n e c o n c r e t i o n s o f H o l o c e n e p r o v i d e s u i t a b l e m a t e r i a l
f o r a n a t t e m p t i n t h e f i e l d o f p a l a e o h y d r o l o g y .
P r e v i o u s i s o t o p i c i n v e s t i g a t i o n s i n t h i s a r e a w e r e c a r r i e d o u t o n s u r f a c e w a t e r s
a n d l a k e s y s t e m s [ 3 , 4 ] , a n d a g e n e r a l r e c o n n a i s s a n c e s u r v e y o n t h e s t a b l e i s o t o p e
c o n t e n t o f g r o u n d w a t e r w a s r e p o r t e d b y S c h o e l l a n d F a b e r [ 5 ] .
2 . H Y D R O G E O L O G Y
2 . 1 . S a m p l i n g
S a m p l e s w e r e c o l l e c t e d f r o m a v a i l a b l e b o r e h o l e s f o r t h e w h o l e c o u n t r y .
S p e c i a l a t t e n t i o n w a s p a i d t o t h e b o r e h o l e f i e l d ( s e e T a b l e I I ) , w h i c h s u p p l i e s t h e
T A B L E II . A Q U I F E R O F W A D I A M B O U L I ( W A T E R S U P P L Y O F D J I B O U T I )
No. Date ô 13o/ show 62h/ smow 6 3H TU 613C PDB 14C $ mod. pHA lk a l .
meq. kg
E 1 02-73 - 1-54 - 11.83 5 2 .З + 4 .5 8.1 4 .1
E 2 02-73 - 1.66 - 1 .2 24 + З
E 3 02-73 - 1 .45 + 0 .7 1 3 + 3
E 4 02-73 - 11.15 7 9 .З + 2.7 3 .4 (4 .2 )
E 5 02-73 — i . 66 - 3 .3 28 + 3 73 .4 + 1 .8
E 6 02-73 - 1.70 - 1 .4 11 + 3
02-71 - 1 .45 - 8. ó
E 7 02-73 - 1.49 + 2 .0
02-71 - 1 .45 . + 2 .9
E 8 02-73 - 1 .54 + 0 .6 43 + 3 - 12.03 7 3 . 4 + 2 .2 7 .5 (3 .9 )
E 10 02-73 - 1 .75 + 0 .6 23 + 3 - 1 З.ОЗ 7 5 .6 + 2 .0 8 . 4 - 4 .O
02-71' - 1 .62 - 3 .3
E 11 02-73 - 1.62 - 4 .0 2 6 + 3
E 12 02-73 - 1.73 - 0 .3 13 + 3
02-71 - 1.87 - 0 .4
E 13 06-76 - 1 .62 1 3 + 3 - 12.45 8 6 . 6 + 7 . 0 7 .24 3 .35
E 1 5 06-76 - I . 4 8 i7 + 3 7 З . 5 + 3 .0 7 .1 2 3.30
E 17 06-76 - I .6 9 6 + 1 - 11.45 7 9 .6 + 4 . 0 6 .9 2 3.45
RG 2 02-73 - 1 .75 1.1 27 + 3
RG 3 02-73 - 1 .75 1.1 27 + 3
RG 4 02-71 - 1.37 2.7
RG 6 02-73 - 1.80 - 1 2 .3 2 77.1 + 7 .0 8.1 (4 .0 )
Values in brackets were obtained during a separate sampling. Temperatures are uniformly close to 40°C.
240 FO
NTES
et al.
I A E A - A G - 1 58 /1 7 241
c i t y a n d t h e h a r b o u r o f D j i b o u t i w i t h f r e s h w a t e r ( c o n s u m p t i o n 2 4 X 1 0 3 m 3/ d )
f r o m a s l i g h t l y c o n f i n e d a q u i f e r . S e v e r a l s p r i n g s g e n e r a l l y c o r r e l a t e d t o f a u l t s
a n d h o t c i r c u l a t i o n s w e r e a l s o s a m p l e d .
W h e n p o s s i b l e , t h e p H , t e m p e r a t u r e , c o n d u c t i v i t y a n d a l k a l i n i t y w e r e m e a s u r e d
o n l o c a t i o n . C h e m i c a l d e t e r m i n a t i o n s 1 w e r e d o n e o n u l t r a f i l t r a t e d s a m p l e s w i t h
b o t h a c i d i f i e d a n d n o n - a c i d i f i e d a l i q u o t s . R e s u l t s w i l l b e p u b l i s h e d i n d e t a i l
e l s e w h e r e .
2 . 2 . T h e p r o b l e m o f p r e s e n t - d a y r e c h a r g e
T r i t i u m m e a s u r e m e n t s ( s e e T a b l e I I a n d f o l l o w i n g ) s h o w v a l u e s f r o m b a c k
g r o u n d t o 4 3 ± 3 T U . M o s t o f t h e s a m p l e s t h u s c o n t a i n a p a r t o f r e c e n t r e c h a r g e .
B e c a u s e m e a s u r e m e n t s o f 3 H a c t i v i t y i n r a i n - w a t e r a r e n o t a v a i l a b l e , t h e
t r i t i u m c o n t e n t o f g r o u n d w a t e r i s c o m p a r e d w i t h p r e c i p i t a t i o n d a t a o f t h e I A E A
N e t w o r k [ 6 - 1 1 ] . T h e s e l e c t e d s t a t i o n s ( T a b l e I I I ) o f K h a r t o u m , M i n i c o y ,
B a h r a i n a n d J e d d a h m a y b e n o t f u l l y r e p r e s e n t a t i v e o f t h e D j i b o u t i a r e a . F o r
i n s t a n c e , K h a r t o u m r a i n s s h o w a r a t h e r h i g h 3H c o n t e n t d u e t o t h e c o n t i n e n t a l e f f e c t .
H o w e v e r , t h e r e c o r d w a s c h o s e n t h e r e b e c a u s e i t s c o m p l e t e n e s s a l l o w s r e l a t i v e
e s t i m a t i o n s . F r o m t h e c o m p a r i s o n o f t h e s e r e s u l t s , i t a p p e a r s t h a t t h e f o l l o w i n g
a v e r a g e r a n g e s o f v a l u e s c a n b e p r o p o s e d a s r o u g h g u i d e l i n e s t o e s t i m a t e t h e t r i t i u m
c o n t e n t o f r e c h a r g e i n t h e D j i b o u t i a r e a :
I n M a y 1 9 7 6 t h e w a t e r s o f t h e A s a l l a k e h a d a h o m o g e n e o u s t r i t i u m c o n t e n t
o f a b o u t 9 T U [ 3 ] , w h i c h r e f l e c t s t h e a v e r a g e o f 3H a c t i v i t y o f t h e e n v i r o n m e n t a l
a t m o s p h e r i c m o i s t u r e s i n c e t h i s a c t i v i t y i s o n l y g a i n e d b y m o l e c u l a r e x c h a n g e .
D u r i n g t h e s a m e p e r i o d , t h e t a i l o f t h e f l o o d o f t h e i n t e r m i t t e n t W a d i A m b o u l i ,
n e a r D j i b o u t i , e x h i b i t e d a 3H c o n t e n t o f 7 ± 1 T U w h i c h i s a l s o a t t r i b u t e d t o t h e
e x c h a n g e w i t h a t m o s p h e r i c v a p o u r r a t h e r t h a n t o r a i n o r t o s o i l w a t e r .
T a k i n g i n t o a c c o u n t t h e a g e e f f e c t , o n e w o u l d e s t i m a t e t h a t a l l g r o u n d w a t e r
s h o w i n g a t r i t i u m c o n t e n t r a n g i n g f r o m 1 0 t o 4 3 T U ( T a b l e s I I , I V , V ) w a s
r e c h a r g e d d u r i n g t h e l a s t 5 t o 1 0 y e a r s . S i n c e s u c h t r i t i u m c o n t e n t s a r e o b s e r v e d
i n d i f f e r e n t u n c o n f i n e d a q u i f e r s — 7 5 / 3 , 7 6 / 6 D i k k i l , 7 5 / 3 9 D o r r a , 7 6 / 2 T a d j o u r a h
M a g a l e , 7 6 / 3 1 G o u r a b o u s , 7 6 / 3 3 Y o b o k i , a n d m o s t o f t h e s a m p l i n g p o i n t s o f t h e
a q u i f e r o f W a d i A m b o u l i ( T a b l e I I ) ■ - i n t h e w h o l e r e g i o n o n e m u s t c o n c l u d e t h a t
t h e s e a q u i f e r s a r e r a p i d l y r e c h a r g e d t h r o u g h f r a c t u r e s a n d c r a c k s o f t h e b a s a l t i c
o u t c r o p s . F u r t h e r m o r e , t h e o c c u r r e n c e o f r e c e n t w a t e r s i n t h e d i s c h a r g e i n d i c a t e s
1 Chemical studies involve collaboration with the US Geol. Survey (B .F . Jones) and the University of Aachen (H .R . Langguth).
1 9 6 3 - 1 9 6 5 p e a k
1 9 6 5 - 1 9 7 0
1 9 7 0 - 1 9 7 6
1 0 0 t o 2 0 0 T U
4 0 t o 5 0 T U
10 t o 2 0 T U
T A B L E I I I . A N N U A L A V E R A G E T R I T I U M C O N T E N T O F R A I N S O F S O M E S E L E C T E D S T A T I O N S F O R
C O M P A R I S O N W I T H T H E D J I B O U T I A R E A . F r o m R e f . [6 t o 1 1 ].
Station Co-ordinates Alt.(m)
1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976
Khartoum 15°60'N;32°55'E 382 89 71 379 1432 771 387 269 171 104 219 130 149.5 82 45.1 53.7 50.1 35.3
Minicoy 8°30'N; 73°00'E 1 67 53 56 26
Bahrain 26°27'N;50°62,E 2 370 222 242 172 59.8 37.6 97.7 96.5 34.4 32.1 24.3 33.7 17.8
Jeddah 21°50'N;39o20'E 11 19 15
24
2
FON
TES et
al.
T A B L E I V . G R O U N D W A T E R S F R O M T H E R E P U B L I C O F D J I B O U T I
( S a m p l i n g 1 9 7 5 )
No. NameDrainage
Area Type t°C pH
0
/iScm "1
A lk a l.
. -1meq kg13
6 0 SMOW 62H SMOW 3H TU 613C PDB14„
С prac
75/1 Doubdoubbol e l e Grand Bara В 90m 36.6 7.62 3500 2.33 - 4 .0 6 -3 2 .0 < 4 - З.4 0 27.6 + 2.7
75/2 Mouloud Grand Bara B BOm 36.3 7.40 2400 4 .З 4 - 3 .9 3 - 32 .О . <: 4 -10.79 5 2 .2 + 2 .5
75/3 D ik k il Henlé В 110m 4 2 .0 7.30 2150 6 .1 5 - 1 .3 6 2 3 + 2 -11.31
75/4 Abhè A lay lou Abhè Lake 30.1 9 .3 0 + 3 .7 3 - З.5 0 37 + 7 + 0.63 110.0 + 1.1
75/17 Abhè Spring Abhè Spring 70.0 3.26 0.72 -2 .77 -2 2 .0 < 4 -17 .05 30.0 + 5 .0
75/19 Abhè Spring Abhè Spring 99.0 - З . 5З -3 2 .5
75/19 Abhè Spring Abhè Spring 36.0 -3 .4 9 -З 1 .9
75/20 Abhè Spring Abhè Spring 33.0 -З .7 7 -3 1 .6
75/21 Àbhè Spring Abhè Spring 100.0 -З .4 0 -3 4 .3
75/22 Abhè Spring Abhè Spring 95.0 -3 .77 -31 .0 .
75/33 K o r i l i Asal Spring 55.0 7 .0 5 O .9O -1 .23 + 9-7 < 5 - З.З9
75/34 Agna Henlé Spring 60.0 3.34 3.20 -3 .3 5 Í 4 - 2.61 49-7 + 4 .0
75/35 H a llo i Asal Spring 69.0 7 . 4 I 0.66 - 2 .1 4 - 1 4 .З £ 5 -17.44
75/36 A sal SE2 Asal Spring 59.0 7 . 4 I O.4 9 -1 .2 8 -4 2 .5 ¿5 - 6.99
75/37 Obock S o u b la li PiedmontP la in
В 120m 39.5 7.39 5. 4 I - 2 .О4 - I 7 .5 8 + 2 -12 .34 9O. 9 + 2 .5
75/33 PK 50 CoastalA qu ife r
В 39*0 ( 6 . 50 ) 1.30 - I .2 9 7 t 1 -11 .60 <60
75/39 Богга PiedmontP la in
B 250m 43.0 3.60 4 . I 3 -0 .4 3 - 3.6 24 + 5 - I O .4 6 63.3 + 2.0
75/40 Andaba PiedmontP la in
В 43.0 8.12 2.13 -3 .6 8 <5 - 3.33 30.3 + 2 .5
В = Borehole and depth
IAEA
-AG
-158/17 243
T A B L E V . G R O U N D W A T E R S F R O M T H E R E P U B L I C O F D J I B O U T I
( S a m p l i n g 1 9 7 6 )
Conduct. Alkal.6180 SMOW6 ^ SMOW 3H TU 6 13C PDBNo. Name Drainage Area Type t°C pH /iS-cm-1 , -1 raeq. kg pmc
76/1 Ambouli Coastal Aquifer Wadi flood - 3 . 0 1 7 ± 17 6 / 2 Tadjourah
MagaleCoastal Aquifer В 100m 37 7 . 5О 4.7 1 8 + 2 -12.00 9О. 7 + 1 . 1
76/i Tadjourah PK.9 Coastal Aquifer В 100m 35 7 . 2 2 4.1 - 1 . 5 8 5 + 1 - 1 3 . 2 0 9 З. 9 t 1л76/4 Pit 50 Piedmont Plain В 40 7.65 II50 5 .З - 1 . 5 7 -I I . 6 9 80.4 + 1.076/5 Abhè Spring Abhè Spring 69 0.7 - 2.61 <3 -I 7 .O5 9 2 . 0 + 8.076/6 Dikkil Henlé В 110m 7.22 3 100 5-5 - 1.66 1 5 + 2 -11.81 37.7 + 1.776/7 Korili Asal Spring 80 7.42 О. 5 2 - 1.44 - 11.8 0.3 + 0 . 3 - 8.53 <3776/8 Manda Aeal Spring 32 7 . 5 1 2 . 2 2 + 0.68 + 6.3 3 . 1 + 0 . 2 - 9 . 2 6
76/14 Hallol Asal Spring 70 7 . 3 2 0.35 - 2 . 4 0 - 17.3 0 . 8 + 0.2 -12.18 1 0 1 + 1876/19 Gubbet Coast Coastal Aquifer Spring 45 + 0.33 4 + 176/30 Daddato Piedmont Plain Spring 37 (7 . 5 0) (10.0) - 1.42 19 + 2 - 1 5 . 0 0 9 8 . 2 + 2 . 1
76/31 Gourabous Piedmont Plain В 27 7.65 1 5 0 6 6 . 4О 2 0 + 2 -11.99 76.1 + 3.076/32 Oulma Obock Coastal Aquifer B 120m (40) 7.07 367 2.39 - 2.37 ' - 7.33 7 З. 4 + 1 . 6
76/33 Yoboki Henlé B 70m (40) 7.55 6 0 2 (6.39) 43 + 3 - 7.46 43.1 + 0.776/34 Minkile HenlÔ Spring 64 300 1.44 - 2.06 -17.3576/35 Koutabouya Gobaad В 90m 40 8 . 1 0 2188 (2.44) 5 ± 1 - 5-62 3 0 . 6 + 1 . 7
Type: WadiB : Borehole and depthValues in brackets were obtained during a separate sampling
244 FO
NTES
et al.
IA E A - A G - 1 58 /1 7 2 4 5
t h a t t h e s t o r a g e i s l o w a n d t h a t t h e p o r o u s m e d i u m , w h i c h m a y e x i s t i n t h e s e
l o c a t i o n s , d o e s n o t c o n t r i b u t e t o t h e r e c h a r g e a s e x p e c t e d f r o m s e d i m e n t o l o g i c a l
c r i t e r i a ( l a r g e c o n t e n t o f c l a y m i n e r a l s p r o v i d e d b y w e a t h e r i n g o f b a s a l t i c r o c k s ) .
N e v e r t h e l e s s , s o m e s a m p l e s — 7 5 / 3 7 O b o c k S o u b l a l i , 7 5 / 3 8 P K 5 0 ,
7 6 / 3 T a d j o u r a h P K 9 , 7 6 / 1 9 C o a s t a l S p r i n g i n t h e b o t t o m o f t h e G u l f o f T a d j o u r a h ,
7 6 / 3 5 K o u t a b o u y a , a n d t h e b o r e h o l e E 1 7 o f D j i b o u t i w a t e r s u p p l y ( i n 1 9 7 6 ) —
s h o w t r i t i u m a m o u n t s w h i c h a r e a t t h e l o w e r l i m i t s o f a n t i c i p a t e d a c t i v i t y f o r
r e c e n t g r o u n d w a t e r s . T h e s e l o w t r i t i u m c o n t e n t s c o u l d b e e x p l a i n e d b y a
m e c h a n i s m o f r e c h a r g e i n v o l v i n g s i n g l e e p i s o d e s o f t r i t i u m - p o o r m o n s o o n r a i n s .
T h i s k i n d o f r e c h a r g e i s c o m p a t i b l e w i t h l o c a l c l i m a t i c a n d h y d r o g e o l o g i c a l
c o n d i t i o n s , i . e . s p o r a d i c e p i s o d e s o f h e a v y r a i n s a n d r a p i d i n f i l t r a t i o n i n f r a c t u r e d
l a v a . S u c h a m e c h a n i s m w o u l d a l s o i m p l y i s o l a t e d c i r c u l a t i o n s a n d l o w s t o r a g e
c a p a c i t i e s . B e c a u s e o f t h e l o c a t i o n o f t h e s e b o r e h o l e s i n r a t h e r t h i n d e t r i t a l
d e p o s i t s a n d t h e f a c t t h a t E 1 7 w a s s a m p l e d d u r i n g d r i l l i n g o p e r a t i o n s , t h e
o c c u r r e n c e o f m i x i n g b e t w e e n “ o l d ” ( t r i t i u m - f r e e ) a n d r e c e n t w a t e r s c o u l d a l s o
b e i n v o k e d . I n t h a t c a s e , “ o l d ” m e a n s a g e d a t l e a s t m o r e t h a n 2 0 y e a r s a n d w o u l d
c o r r e s p o n d t o p o o r l y c i r c u l a t i n g w a t e r b o d i e s .
S o m e w a t e r s a r e c l e a r l y o l d a s i n d i c a t e d b y t h e b a c k g r o u n d l e v e l i n t r i t i u m
a c t i v i t y . T h i s i s t h e c a s e f o r h o t a n d t h u s d e e p c i r c u l a t i o n s i n t h e t h e r m a l s p r i n g
o f L a k e s A b h è a n d A s a l ( s a m p l e s 7 6 / 5 , 7 6 / 7 , 7 6 / 1 4 , 7 5 / 1 7 , 7 5 / 3 3 , 7 5 / 3 5 , a n d
7 5 / 3 6 ) , b u t a l s o f o r n o r m a l g r o u n d w a t e r s i n l a r g e b a s i n s f i l l e d w i t h a r g i l l a c e o u s
s e d i m e n t s ( s a m p l e s 7 5 / 1 D o u b d o u b b o l e l e , 7 5 / 2 M o u l o u d , 7 5 / 3 4 A g n a , 7 5 / 4 0
A n d a b a ) . T h i s r e i n f o r c e s t h e h y p o t h e s i s o f t h e o c c u r r e n c e o f w a t e r s o l d e r t h a n
2 0 y e a r s i n t h e m a i n d e p r e s s i o n s . F u r t h e r m o r e , b e c a u s e t h e a q u i f e r s a r e u n c o n
f i n e d , t h e l a c k o f 3H i n t h e s e b a s i n s o f i n t e r n a l d r a i n a g e s u g g e s t s t h a t t h i n s e d i
m e n t s a l l o w a n a c t i v e e v a p o r a t i o n a n d é v a p o t r a n s p i r a t i o n o f r a i n a n d i n t e r m i t t e n t
f l o o d s i n t h e l o w e s t p o i n t s o f t h e s e s y s t e m s w h i c h w o u l d n o t b e n e f i t a n y s i g n i f i c a n t
v e r t i c a l s u p p l y .
2 . 3 . O r i g i n a n d m e c h a n i s m s o f g r o u n d w a t e r r e c h a r g e
T h e s t a b l e i s o t o p e c o n t e n t o f g r o u n d w a t e r s i s r e p o r t e d i n T a b l e s I I a n d V I
a n d F i g s 2 a n d 3 .
F r o m t h e d i s t r i b u t i o n o f t h e p o i n t s o n t h e d i a g r a m 5 2 H - 5 1 8 0 o f 1 9 7 2
s a m p l i n g , w h i c h i s t h e m o s t d e t a i l e d ( F i g . 2 ) , i t c a n b e c o n c l u d e d t h a t :
M o s t o f t h e v a l u e s f a l l o n a r a n g e w i t h a s l o p e o f 8 i n d i c a t i n g t h a t n o
s i g n i f i c a n t e v a p o r a t i o n o c c u r r e d b e f o r e o r d u r i n g i n f i l t r a t i o n ; t h i s f a c t i s
i n a g r e e m e n t w i t h t h e c o n c e p t o f a f a s t i n f i l t r a t i o n t h r o u g h f r a c t u r e d l a v a s ;
A l o n g w i t h t h i s t e n d e n c y , t h e v a l u e s r a n g e b e t w e e n — 4 . 6 a n d a b o u t 0 %o
i n 180 c o n c e n t r a t i o n s ; i n t e r p r e t e d i n t e r m s o f a l t i t u d e e f f e c t , t h i s r a n g e
No,
1
2
3
4
5
6
7
8
9
1 0
11
1 2
13
14
15
16
17
18
19
2 0
2 1
2 2
23
24
25
26
27
28
29
30
31
32
F O N T E S et al.
Î L E V I . G R O U N D W A T E R S F R O M R E P U B L I C O F D J I B O U T I
n p l i n g 1 9 7 2 )
Name Type t ° c Conductivity/uS-сп Г 1
5 lsO SMOW 5 2H SMOW
Seik Sabir W 3 .0 m 2 8 0 - 0 . 2 + 3 .0
Teoao W 2.5 m 4 8 0 0 .0 - 2 . 4
Moutrous W 2 .0 m 4 .8 X 103 + 2 .6 + 4 .2
Goma hele W 2 .0 m 3.5 X 103 - 1 . 0 - 1 0 .8
Guidoli W 2 .0 m 2.8 X 103 + 1.7 + 5 .3
Halouli S - 1 .6 - 2 . 4
Mouri Hani S 3.3 X 103 - 1 .7 - 1 3 .7
Lahibouya S 6 70 + 1.5 + 5.1
Boule yare W 2.5 m 6 70 - 1 .1 - 1 . 4
Nihile S 1,5 X 103 - 1 . 9 - 1 . 3
Gourabous В 2 0 m 540 - 0 .2 - 0 . 9
Djibouti R 2 7 /0 4 /7 2 - 1 .5 - 6 .5
Djibouti R 2 1 /0 9 /7 2 + 2 .4 - 8 . 2
Yoboki R 0 8 /7 2 + 4 .5 - 6 .5
Oueah В 25 m - 0 .7 - 7 . 0
Hindi W 2 .0 m - 0 . 4 + 5 .1
PK 50 В - 0 .5 - 3 . 0
Ambouli В 30 m - 1 .1 + 4 .0
Douda В 30 m - 1 .5 - 7 .5
Koussourale W 0.0 - 7 . 0
Galerie Ambouli В - 0 . 7 + 1.8
Hallol HE 7 S 3.1 X 103 - 3 .5 - 2 7 .1
Hallol HE 11 S 3.85 X 103 - 3 .5 - 2 8 .0
Hallol HE 19 S 7.1 X 103 - 3 . 4 - 2 3 .0
Hallol HE 2 - 1 S 2 .9 X 103 - 3 .1 - 2 4 .0
Hallol HE 2 - 2 S 2 .9 X 103 - 3 . 6 - 2 9 .8
HaUol S III S 80 .0 15 X 103 - 2 .7 - 1 4 .9
Canal Sakallol S + 0 .2 1 OO 00
Harallol В s 45 .0 7 .4 X 103 - 2 . 4 - 2 0 .0
Harallol A s 50 .0 8 .7 X 103 - 2 . 6 - 1 9 .8
Harallol IIH S s 6.7 X 103 - 2 . 6 - 2 0 .9
Oued Toha s - 1 . 3 - 5 .1
IA E A -A G -1 5 8 /1 7 2 4 7
T A B L E V I . ( c o n t . )
No. Name Type TeC Conductivity/LiS-cm”1
S 1!0 SMOW 6 2H SMOW
33 Bondarra W 6 .0 m + 1.3 + 5 .4
34 Ali Sabieh R 0 8 /7 2 + 5 .2 + 6 .3
35 Korili S - 0 .3 + 14.2
36 Canyon SE 1 S 4 7 .0 76 X 103 - 0 . 9 - 1 .3 5
37 Crevasse Manda S 30 .0 36 X 103 + 1.9
38 Manda 3 s 3 7 .0 51 X 103 + 2 .0 + 15.6
39 Asal N 1 s 105 + 2 .7 - 3 8 .3
40 Asal SE 2 s 5 8 .0 8 2 .103 - 0 .6 - 3 9 .3
41 Hallol A s 4 0 .0 1 5 .103 - 2 . 4 - 1 7 .5
42 Hallol B s 2 9 .0 + 6 .5 + 2 3 .2
43 Hallol C s 46.5 - 2 .5 - 2 0 .7
44 Hallol D s 79.5 - 2 . 4 - 1 5 .9
45 Hallol E s 4 8 .0 - 4 . 6 - 3 4 .6
46 Hallol F s 5 2 .0 - 4 . 0 - 3 1 .3
Symbols: В = Borehole and depth; R = Rain and data; S = Spring; W = Dug well and depth.
s u g g e s t s t h a t i n f i l t r a t i o n s o c c u r r e d o n m o r e t h a n 1000 m o f v e r t i c a l d e v e l o p
m e n t i f o n e t a k e s i n t o a c c o u n t a n i s o t o p i c g r a d i e n t o f 0 . 3 t o 0 . 5 % o p e r
100 m ; i t m a y b e n o t e d t h a t t h e s t a b l e i s o t o p e c o n t e n t o f g r o u n d w a t e r s
f r o m t h e E t h i o p i a n h i g h l a n d s a t a l t i t u d e s h i g h e r t h a n 2 0 0 0 m , r e p o r t e d b y
S c h o e l l a n d F a b e r [ 5 ] , d o n o t s h o w v a l u e s l o w e r t h a n i n t h e l o w l a n d s
o f D j i b o u t i ( m a x i m u m e l e v a t i o n o f t h e e s c a r p m e n t s ~ 1 3 0 0 m ) ; t h i s s u g g e s t s
t h a t t h e a l t i t u d e e f f e c t i s n o t r e s p o n s i b l e f o r t h e o b s e r v e d r a n g e o f v a r i a t i o n s
i n g r o u n d w a t e r s w h i c h c o u l d r a t h e r b e d u e t o s p o r a d i c r e c h a r g e b y i s o l a t e d
e p i s o d e s o f m a j o r r a i n s ;
T h e i n t e r c e p t o f t h e c o r r e l a t i o n ( d e u t e r i u m e x c e s s , d ) , l o w e r t h a n t h e
p r e s e n t - d a y t y p i c a l v a l u e o f + 10 f o r o c e a n i c r a i n s , i s s i m i l a r t o w h a t w a s
f o u n d f o r h o t s p r i n g s o f t h e a r e a [ 5 ] b u t q u i t e d i f f e r e n t f r o m t h e i n t e r c e p t
e n c o u n t e r e d b y t h e s e a u t h o r s i n g r o u n d w a t e r s f r o m E t h i o p i a n h i g h l a n d s
( d = + 1 5 ) ;
I s o l a t e d s a m p l e s o f r a i n - w a t e r f a l l b e l o w t h e l o c a l m e t e o r i c w a t e r l i n e a n d
a r e t h u s c l e a r l y e v a p o r a t e d d u r i n g r a i n f a l l ; t h i s d e v i a t i o n s h o w s t h a t o n l y
h e a v y e p i s o d e s ö f n o n - e v a p o r a t e d r a i n s c a n c o n t r i b u t e t o t h e r e c h a r g e ;
FIG.2. D-ibO correlation fo r 1972 — 73 sampling. The large range o f values is attributed to sporadic recharge by single heavy episodes o f rain and floods in fractured rocks and, to a minor extent, to altitude effect. Evaporation does not a ffect the waters except in som e shallow wells and in available rain samples: only major rains, non-evaporated, contribute to the recharge. Values obtained by Schoelland Faber [5] fall within the observed range and the distinction between “Wadi-type”and “R ift-type” water is not obvious, even i f coastal samples (e.g. Wadi Ambouli aquifer j show a high heavy isotope content associated with a deuterium excess close to +10, which could characterize the first stages o f condensation o f monsoon rains. G eothermal exchange with rock materials a ffects samples 39 and 40 from the Asal R ift system, which show an “oxygen sh ift” towards positive values.
IA E A - A G - 1 58 /17 2 4 9
S a m p l e s f r o m t h e a q u i f e r o f W a d i A m b o u l i ( T a b l e I I ) f a l l w i t h i n a v e r y n a r r o w
r a n g e ( — 1 . 9 < 5 1 8 0 < — 1 . 5 ; —8 < 5 2 H < + 3 ) , c l o s e t o t h e m e t e o r i c w a t e r
l i n e w i t h a n i n t e r c e p t o f + 1 0 s u g g e s t i n g t h a t r e c h a r g e o c c u r r e d f r o m h e a v y
n o n - e v a p o r a t e d m o n s o o n r a i n s w h i c h g e n e r a t e d h e a v y f l o o d s a n d r e c h a r g e d
t h e a q u i f e r ;
S o m e s a m p l e s c l e a r l y s h o w t h e e f f e c t o f e v a p o r a t i o n , t h e i r h e a v y i s o t o p e
c o n t e n t i s s h i f t e d b e l o w t h e c o r r e l a t i o n l i n e ; t h e s e s a m p l e s a r e g e n e r a l l y
s h a l l o w w e l l s - s a m p l e s 3 , 5 , a n d 3 3 ( d u g w e l l s ) ; o r s p r i n g s — s a m p l e s 8 a n d
2 8 — w h i c h m a y h a v e u n d e r g o n e e v a p o r a t i o n n e a r t h e s o i l s u r f a c e ;
E x c e p t f o r s o m e s a m p l e s o f t h e L a k e A s a l b a s i n ( s a m p l e s 3 9 a n d 4 0 ) , t h e r m a l
w a t e r s d o n o t s h o w a s i g n i f i c a n t 180 s h i f t t o w a r d s p o s i t i v e v a l u e s , w h i c h
r e v e a l s a n e x c h a n g e w i t h t h e m i n e r a l s o f t h e a q u i f e r . T h e s a m p l e s w h i c h
s h o w s u c h a n o x y g e n s h i f t h a v e t h e l o w e s t d e u t e r i u m c o n t e n t o f t h e w h o l e
s a m p l i n g a n d t h u s w o u l d c o r r e s p o n d t o t h e s u p p l y c o m i n g f r o m t h e h i g h e s t
a l t i t u d e c o r r e l a t e d t o t h e d e e p e s t c i r c u l a t i o n ; f o r s a m p l e s 4 1 , 4 3 , 4 4 , 4 5 , a n d
4 6 w h i c h c o r r e s p o n d t o w a t e r s c o m i n g f r o m t h e h i g h e s t p a r t s o f t h e L a k e
A s a l b a s i n , a n a l t i t u d e e f f e c t c o u l d a c c o u n t f o r t h e t o t a l d i f f e r e n c e o f 2 % o
i n t h e 180 c o n t e n t o f t h e s e w a t e r s .
T h e s t a b l e i s o t o p e a n a l y s e s r e p e a t e d i n 1 9 7 5 ( F i g . 3 ) a n d 1 9 7 6 o n s o m e o f t h e
s a m e l o c a t i o n s b a s i c a l l y c o n f i r m t h e p r e v i o u s s t a t e m e n t s . O n e c a n d e f i n e a l o c a l
m e t e o r i c w a t e r l i n e w i t h a n i n t e r c e p t c l o s e t o z e r o . T h e g e o t h e r m a l e x c h a n g e i s
s t i l l m a r k e d i n t h e A s a l b a s i n ( s a m p l e 7 5 / 3 6 ; A s a l S E 2 ) b u t a s l i g h t i s o t o p i c
e x c h a n g e b e t w e e n w a t e r a n d r o c k s i s n o t i c e a b l e i n t h e b o i l i n g s a m p l e s 7 5 / 1 8 t o
7 5 / 2 1 f r o m t h e A b h è L a k e s y s t e m .
I t i s i n t e r e s t i n g t o n o t e t h a t t h e w a t e r f r o m t h e K o r i l i S p r i n g s t i l l e x h i b i t s a
d e u t e r i u m e x c e s s g r e a t e r t h a n 1 0 . T h i s i s o l a t e d f e a t u r e m a y b e d u e t o a n e x c h a n g e
w i t h f l u i d s b e a r i n g H S - o r H 2S . T h e c i r c u l a t i o n o f t h i s h o t s p r i n g ( 7 0 ° C ) i s l o c a t e d
n e a r t h e g y p s u m d e p o s i t s o f t h e L a k e A s a l b a s i n , w h i c h c o u l d b e r e d u c e d a t d e p t h
i n t h e p e r i - v o l c a n i c e n v i r o n m e n t o f t h e A s a l r i f t g i v i n g r i s e t o c o m p o u n d s d e p l e t e d
i n 2H , w h e r e a s t h e w a t e r i s e n r i c h e d i n t h i s i s o t o p e a n d t h e r e p r e s e n t a t i v e p o i n t o f
t h e s a m p l e s h i f t e d a b o v e t h e m e t e o r i c w a t e r l i n e i n a 6 2 H - 5 1 8 0 d i a g r a m . .
I n s e v e r a l s p r i n g s o r b o r e h o l e s , t h e s t a b l e i s o t o p e c o n t e n t s h o w s v a r i a t i o n s
b e t w e e n t h e s a m p l i n g , i . e . P K 5 0 ( 5 1 8 0 = — 0 . 5 i n 1 9 7 2 ; — 1 . 2 9 i n 1 9 7 5 , a n d
- 1 . 5 7 i n 1 9 7 6 ) , D i k k i l ( 0 1 8 0 = - 1 . 3 6 i n 1 9 7 5 , a n d - 1 . 6 6 i n 1 9 7 6 ) . T h e s e
v a r i a t i o n s f a v o u r t h e h y p o t h e s i s o f a s p o r a d i c r e c h a r g e . O n t h e o t h e r h a n d , t h e
a q u i f e r o f W a d i A m b o u l i a p p e a r s m o r e h o m o g e n e o u s i n e a c h b o r e h o l e : E 6 ( 0 1 8 0 = — 1 . 4 5 i n 1 9 7 1 , - 1 . 7 0 i n 1 9 7 3 ) ; E 7 ( S 1 8 0 = - 1 . 4 5 i n 1 9 7 1 , - 1 . 4 9 i n
1 9 7 3 ) ; E 1 0 ( 6 1 8 0 = - 1 . 6 2 i n 1 9 7 1 , - 1 . 7 5 i n 1 9 7 3 ) ; E 1 2 ( S 1 8 0 = - 1 . 8 7 i n 1 9 7 1 ,
— 1 . 7 3 i n 1 9 7 3 ) . T h i s i s a t t r i b u t e d t o t h e t e x t u r e o f t h e a q u i f e r ( F i g . 4 ) w h i c h c a n
b e c o n s i d e r e d o f m i x e d ( p o r o u s a n d f r a c t u r e d ) t y p e . T h e f l o o d s c a n p e r c o l a t e
toо
FIG.3. D-n O correlation fo r 1975 sampling. The range o f values o f Fig.2 (1972 - 73) is confirm ed fo r hypotherm al and hydro- thermal systems. The occurrence o f tritium in most samples indicates that the present-day m eteoric water line with a deuterium excess (d = 52tf-88180 = 0) is representative o f present-day recharge in the inland.
I A E A - A G - 1 58 /17 251
N SELEVATION mo.s.l.
FIG.4. Geological setting o f the Wadi Ambouli confined aquifer.
t h r o u g h t h e a n c i e n t a l l u v i a l d e p o s i t s o f t h e w a d i o r t h r o u g h t h e f r a c t u r e d o l d
( P l i o c e n e ) b a s a l t o f t h e s o u t h e r n r i v e r b a n k .
I n c o n c l u s i o n , r e g a r d i n g t h e s t a b l e i s o t o p e s t u d y i t a p p e a r s t h a t i n f i l t r a t i o n
o c c u r s o n l i m i t e d a n d l o c a l i z e d c a t c h m e n t a r e a s a n d t h a t g r o u n d w a t e r s u n d e r g o
r e d u c e d m i x i n g w i t h i n t h e a q u i f e r w h e r e t h e i s o t o p i c v a r i a t i o n s o f t h e i n p u t a r e
p r e s e r v e d . T h e s e v a r i a t i o n s a r e m a i n l y d u e t o t h e i n d i v i d u a l l a b e l l i n g o f s i n g l e
e p i s o d e s o f r e c h a r g e . H o w e v e r , a c o n t r i b u t i o n o f w a t e r s f r o m t h e e s c a r p m e n t o f
t h e E t h i o p i a n p l a t e a u i s f e l t t o e x p l a i n t h e m o s t d e p l e t e d h e a v y i s o t o p e c o n t e n t s .
T h e v a r i a b i l i t y o f t h e s t a b l e i s o t o p e c o n t e n t o f g r o u n d w a t e r c o n f i r m s t h e
i n t e r p r e t a t i o n d r a w n f r o m 3H m e a s u r e m e n t s o n a l i m i t e d s t o r a g e .
2 . 4 . “ A g e ” o f g r o u n d w a t e r
T h e i n i t i a l 1 4 C a c t i v i t i e s o f t o t a l d i s s o l v e d c a r b o n ( T D C ) w e r e d e t e r m i n e d
u s i n g 13 C c o n t e n t o f T D C , p H a n d f i e l d a l k a l i n i t y v a l u e s a c c o r d i n g t o t h e c l o s e d
s y s t e m t r e a t m e n t p r o p o s e d b y F o n t e s a n d G a m i e r [ 1 2 , 1 3 ] , a n d t h e r e s p e c t i v e
a p p r o a c h e s o f T a m e r s [ 1 4 ] , P e a r s o n [ 1 5 ] , a n d M o o k [ 1 6 ] . F o r c a l c u l a t i o n s , o n e
h a s t o a s s u m e p a r a m e t r i c v a l u e s f o r t h e 1 3 C a n d l 4 C c o n t e n t o f s o i l C 0 2 a n d s o l i d
c a r b o n a t e . T o l e a v e t h e m i n i m u m o f f r e e d o m t o t h e m o d e l s , t h e s a m e v a l u e s w e r e
a d o p t e d f o r t h e s t a b l e i s o t o p e c o n t e n t o f s o i l C 0 2 ( — 2 1 % o ) a n d o f s o l i d c a r b o n a t e
TABLE VII. CARBON ISOTOPES GEOCHEMISTRY SAMPLES FROM VARIOUS AQUIFERS FROM THE INLAND
No. a X 103 a/4. c X 103 mCM
Ameas.
A o (M) /1 (F and G)0 * 0( r ) A o (T ) Remarks
Î5/1 O . I I 52 ■ 0 .0461 0 .006 2.50 I . I 9 27.6 4 .9 ЗО.9 4 0 .0 52.4 A0( g ) = 100#
75/2 0 .3483 О.О742 0.006 4 .6 9 2.17 52.2 30.0 49 .7 51.4 53.7 A0(g ) = 100$
75/17 О.ОО85 O . O I I5 0.010 0 .738 0.36 80.0 93 .6 91.9 81.2 50.5 О<
= 100$
75/34 0.0290 O.OO9O 0.042 З .27 I .60 49 .7 38.4 47 .2 56.2 7 5 .5 Ao ( s ) = 100, A (1 )=50
75/37 0 .4379 0 .0748 0.008 5.86 2.705 9O.9 88.1 86.0 79 .5 7O.O A0(g ) = 130$
75/39 0 .0203 0 .0048 0 .1 0 3 4 .25 2.O65 68 .3 57.7 63.4 ' 64 .8 6 6 .8 A0 (g ) = 130$
75/40 0.0317 0 .0145 0 .0 1 8 2 .18 I. O 65 30.3 15.2 32.3 39.7 51.1 A 0 (g ) = 100$
76/2 0.2992 О.О598 O.OO9 5.O I 2.35 90.7 96 .4 98.1 95.7 92 .5 AQ(g ) = 130, A o ( l ) - 5 0
76/3 0.5040 O .IO 94 0 .004 4 .61 2.05 93 .9 106 .4 104.6 100.3 94 .4 A0 ( g ) = 130$,Ao ( l) = 5 0 $
76/4 О.2352 О.О424 0 .0143 5 .55 2.65 80 .4 71.6 75.4 72.4 6 7 .9 A 0 (g ) = 130$
76/5 0 .0081 0 .0114 0 .010 0 .720 0.35 92.O 94 .3 91.9 81.2 51.4 A0 (g ) = 100$
76/6 O .65I 8 O .IO 59 0 .006 6.16 2.75 87.7 90.5 95.5 95 .0 94 .3 A (g ) = 130 ,A o ( l ) = 5 0 $
76/7 0 .0471 0 .0829 0.001 О.568 0.26 Í 37 33.8 37.0 40 .6 54.2
i 0
<
= 100$
76/14 0 .0358 О.О928 0 .001 0.386 0 .175 101 7 8 .0 79 .5 79 .0 77 .3 A0 (g ) = 100 ,A o ( l) = 5 0 $
76/ЗО 0 .6367 О.О598 0 .1 8 2 IO .65 5.000 98.2 103 .0 84.8 71 .4 69 .0 A 0 (g ) = 100$
76/31 O.1986 0 .0 3 0 0 0 .022 6 .62 3.20 76. I 8 5 .0 80 .4 74.2 67 .2 A0 (g ) = 130$
76/32 0 .4032 0 .1443 0 .002 2 .79 I . I 95 78 .4 42 .7 6 8 .8 79 .8 95 .7 A0 (g ) = 130$,Ao ( l ) - 5 0 $
76/ЗЗ 0 .3 5 7 0 О.О528 0 .0 1 4 6 .76 3.195 4 3 .1 — 38.3 56 .8 84 .4 A0 (g ) = 160$
76/35 0 .0384 O .O I54 0 ,0 1 8 2.50 1.22 30.6 28 .8 57.9 71 .4 9I .0 A0 (g ) = 130$,Ao ( l) = 5 0 $
Sym bols: a, c , 2 : Calculated activities o f aqueous CO^ + H j CO j , carbonate ion and to ta l dissolved carbon, respectively, according to M ook [1 6 ] .mCT , mCM: Calculated m olalities (m illim oles) o f to ta l dissolved carbon and o f carbon o f inorganic origin according to F o n tes and G am ier [1 2 , 13].A meas.- Measured 14C content o f to tal dissolved carbon.
252 FO
NTESetaJ.
IAEA-AG-158/17 253
(0%o). It was checked (Fontes, unpublished data) that the 13C content of lacustrine carbonate of this area does not differ significantly from that of marine . carbonates because of a nearby complete equilibrium with atmospheric C 02. According to the respective locations, two hypotheses were adopted, taking into account the fact that solid carbonate can be either “dead” with respect to 14C decay (case of old marine carbonate) or “active” (case of carbonates of Holocene lacustrine origin). In the latter case, a 14C content of 50 pmc was adopted according to numerous data obtained on these deposits [1 ,3].
The 14C content of soil C 02 was fixed at 100% when the tritium content of the water was lower than 4 TU and was thus regarded as “pre-bomb” carbon. When tritium is present, even in amounts as low as 7 TU, a 14C content of 130% was adopted for soil C 02, i.e. all the dissolved carbon of organic origin was attributed to what could be expected for air C 02 activity at these latitudes during the ’seventies. An exception was made for sample 76/33 (Yoboki in the Henlé basin) whose 3H content (43 TU) is clearly higher than in any other location and corresponds to the period 1965—1970 on the basis of the available tritium record in rains (see Section 2.2). A value of 160 for the 14C content of soil C 0 2 was thus adopted by comparison with the values published by Nydal et al. [17] for the temperate zone, assuming a slight correction because of the latitude.
From this review of the parametric values, it appears obvious that the application of the various models could be easily refined in each single case, but our purpose was mainly to discuss the general value of the 14C age estimations in a given area with expected and measured values rather than to observe the parameters “reasonably”.
Taking into consideration the statistical uncertainty on the 14C measurements, it appears that the measured 14C content of total dissolved carbon of inland groundwater fits generally within about ± 6 pmc with calculated initial activities calculated according to the treatment by Fontes et Garnier (see Table VII).
The first conclusion of this global treatment is that no contradiction appears between 3H and model 14C contents. This means that one deals with two basic groups of recent groundwater: a group which accepts a 14C content close to 130% in the atmosphere, i.e. which is aged less than 5 or 6 years; and a group which corresponds to a 14C content of 100% within the atmosphere, i.e. which infiltrated before nuclear tests (1952).
The first group corresponds to samples 75/37 (Obock Soublali, coastal aquifer); 75/39 (Dorra, piedmont plain); 76/2 (Tadjourah Magale, coastal aquifer);76/3 (Tadjourah PK 9, coastal aquifer); 76/4 (PK 50, piedmont plain), 76/6 (Dikkil, closed basin of Henlé); 76/31 (Gourabous, piedmont plain); 76/32 (Oulma Obock, coastal aquifer). These groundwaters correspond to systems in which fracture porosity plays a greater role than matrix porosity and where the storage capacity is low. Groundwaters are thus rapidly renewed by drainage, withdrawal from wells, and boreholes and évapotranspiration.
TABLE VIII. CARBON ISOTOPE GEOCHEMISTRY Aquifer o f Wadi Ambouli ( Confined aquifer)
Parameters
613C (soil COj) -2\%o (thom-tree steppe)613C (carbonate) 0 %0 (marine carbonate)A14C (soil C 02) 130 %o (post 1970 recharge)A14C (carbonate) 0 %0 (preholocene marine carbonate)
No. a X 103 А/ mGp mCM A (m eas.) A0 (M ) Ao (F and G ) A0 (P) A o (T )
E l 0 .0 5 2 0 .0 1 5 4 4 .20 2 .05 52 .3 7 8 .6 78 .3 73 .5 66.5
E4 0 .0 2 7 0 .0 0 7 7 4 .3 0 2 .1 0 79 .3 6 6 .6 70 .7 6 9 .0 66 .5
E8 0 .1 9 6 0 .0 5 8 9 4 .15 1.95 7 3 .4 75 .5 79 .3 74 .5 6 7 .3
ЕЮ 0 .0 0 8 0 .0 0 7 7 4 .07 2 .0 0 7 5 .6 1 0 0 .0 90 .5 8 0 .7 ' 66 .1
E 13 0 .3 5 1 0 .1 0 2 3 4 .29 1.93 8 6 .6 7 7 .3 80 .7 77.1 7 1 .7
E 17 0 .6 5 7 0 .1 9 2 4 4 .27 1.73 7 9 .6 4 7 .8 6 6 .4 7 0 .9 77 .5
RG 6 0 .0 5 0 0 .0 1 5 4 4 .09 2 . 0 0 77.1 8 6 .4 81.1 7 6 .3 69.1
Sym bols: a: calculated activity o f aqueous C 0 2 plus HaC 0 3A/: calculated activity ratio o f aqueous C 0 2 p lu sH 2C 0 3 versus to ta l dissolved carbon: Mook [1 6 ]mCT : calculated m olality (m illim oles) o f to ta l dissolved carbon F o n tes and Garnier [1 2 , 13]mCM: calculated molality (m illim oles) o f dissolved carbon from inorganic origin F on tes and G am ier [1 2 , 13]A (m eas.): measured 14C activity (pm c) o f total dissolved carbonAq : calculated 14C initial activity (pm c) according to the models o f M ook [1 6 ] : A0 (M ); F on tes and G arnier [1 2 , 13]:
Ao (F and G ); Pearson [1 5 ]: Ao (P); Tamers [1 4 ] : A0 (T )Ao : calculated initial 14C content (pm c) o f the to ta l dissolved carbon according to M ook [1 6 ] : Aq (M ); F o n tes and
G arnier [1 2 , 13]: Ao (F and G ); Pearson [1 5 ] : Ao (P); and Tam ers [1 4 ] : Aq (T ).Param eters:5 !3C (soil C O j) = — 21%o
A 14C (soil C O j) = 100% when 3H activity < 4 T U , 130% for tritium-bearing waters except for sample No. 76/33 (Y o b o k i)where a value o f 160% was adopted because the high ^ content (4 3 T U ) implies a recharge during the 1 9 6 5 —19 7 0 period.
0 13C (soil carbonate) = 0%o for both basins containing m arine carbonates and lacustrine H olocene carbonates ( l3C content controlled by a close to com plete exchange w ith th e atm osphere in b o th cases).
A 14C (soil carbonate) = 0%o for basins containing marine carbonates and 50% for basins containing H olocene carbonates.
IAEA-AG-158/17 255
The second group corresponds to samples 75/1 (Doubdoubbolele, Grand Bara basin); 75/2 (Mouloud, Grand Bara basin); 75/17 and 76/5 (Abhé thermal spring, Abhé Lake basin); 75/34 (Agna, Henlé basin); 75/40 (Andaba, piedmont plain); 76/7 (Korili thermal spring, Asal Lake basin); 76/14 (Hallol, Asal Lake basin); 76/30 (Daddato, piedmont plain); 76/35 (Koutabouya, Gobaad basin). These systems are those where groundwater circulations take place through porous media. Most of these waters are modern, i.e. no age affect can be identified and the recharge is no older than some tens of years, or at most one or two centuries. However, the Abhè spring (75/17 and 76/15), which is probably supplied on the edges of the catchment area, shows in 1975 and 1976 a similar difference in 14C content with the initial modelled activity (Ameas = 80% A0 = 92%). Interpreted in terms of age, this difference is equivalent to about 1200 years.
Between these two groups the samples from the Yoboki borehole (76/33) would be the only ones aged around 10 years. The sample from Koutabouya is difficult to interpret because the alkalinity was not measured on an aliquot of the sample used for the isotope study.
Despite its homogeneous stable isotope content, its restricted extension and its confinement under an argillaceous level (Fig.4), the Wadi Ambouli aquifer does not, in any of the models, correspond to a single set of parameters (see Table VIII) for the evaluation of the initial 14C activity of the total dissolved carbon. This observation, which is in agreement with the variations observed in pH and tritium contents, is interpreted in terms of heterogeneity of the recharge and of differences in the processes of carbon mineralization according to the various recharge areas in the Wadi bank. It is even possible that in some places magnetic or volcanic C 02, coming through the fractured basalt, contributes to the total dissolved carbon. It must be pointed out that such a supply of “dead” deep C02 (at 40°C, a 5 13C of about —7%o) could only be identified on the basis of a decrease in 14C activity since its contribution to the 13C content would be similar to that of the marine carbonate. The supply of deep C 02 would lead to a 5 13C value close to —1 for the dissolved carbon, if the pH tends to be increased towards values greater than 8 under the influence of alkaline and earth-alkaline, as it should in the case of lava; when isolated from the C 02 source by groundwater flow, this dissolved carbon would give a contribution to the total dissolved carbon which would be indistinguishable from that of leaching of solid carbonate from marine origin. Such an effect can be significant in groundwater systems from rift structures and volcanic areas like the southern Afar rift.
In conclusion, regarding 14C age estimation, it appears that most of the hypothermal waters are very recent and can be attributed either to post-1970 or to pre-1950 recharge. Although the dissolved carbon content is very low in hot waters, and thus the measurement is rather difficult (with large uncertainties — see Tables II and V), it seems that these waters are no more than some centuries old (Abhè spring, for which one calculates an age of 1150 years with Ameas = 80% and A0 s 92%), or could even be recent (Sakallol).
2 5 6 FONTES et al.
As inferred from 3H measurements, it is likely that hot water circulations through fractured lava are rather rapid as compared with circulations in porous media.
3. PALAEOHYDROLOGY
3.1. The concretions of Lake Abhè
On the southern shore of Lake Abhè numerous accumulations of porous carbonate (stoichiometric calcite) deposits can be observed. These accumulations, clearly linked to major fracture directions, can reach some tens of metres in elevation. As indicated by the occurrence of boiling springs (99°C at 450 m) in their vicinity and sometimes by vapour emanations at their surface, these masses are correlated to thermal fluid circulations drained by faults. However, no construction of that kind can be observed from the area of maximal extension of the palaeolake indicated by its upper shore line. Thus, it appears that these hydro- thermal concretions were also strictly correlated to the aquatic environment of the lake. Nowadays, brines from Lake Abhè correspond to the concentration by evaporation of waters from the River Awash. These sodium-rich waters give rise to high pH values and to an active reaction with atmospheric carbon dioxide.The result is a sodium-carbonate type water. On the other hand, the boiling springs which are observed at the basis of some major hydrothermal concretions are sodium-chloride, calcium-sulphate in type.
Using the WATEQ programme [18], it can be shown that when river and spring waters are mixed they are supersaturated with respect to calcite even for small amounts (5%) of any of the two components [19]. Thus, the hypothesis is made that the formation of the hydrothermal concretions was due to the precipitation of calcite when sublacustrine hot springs mixed with lake water.
3.2. Principles of the use of carbonate concretions for palaeohydrologicalreconstruction
If the carbonate ion which gives a solid carbonate is in open system equilibrium with the atmosphere it means that: (i) the temperature of crystallization can be calculated, assuming a value for the gaseous C 02 and knowing the thermodependence of the enrichment factor e = [(RcaC0 3/^C0 2^ 4 * 1 0 0 0 , where R = 13C/12C; and (ii) the 14C ages measured for the solid carbonate approximate sideral ages.
If, furthermore, the carbonate précipitants are at equilibrium with the environmental water, it means that the knowledge (or any assumption) of the temperature of crystallization will allow the isotopic composition of the mother water of the
IAEA-AG-158/17 2 5 7
FIG.5. Schematic representation o f the formation o f hydrothermal concretions from the Abhè Lake area. The mixing o f calcium-sulphate and sodium-carbonate type waters is saturated with respect to calcite. I f the total dissolved carbon o f lake water is in equilibrium with atmospheric C 02, one can calculate the temperature o f crystallization using the fractionation between COi gas and calcite. The isotopic composition o f the mother water will then be determined using the paleotemperature equation. Furthermore, the calcite o f the concretion will represent a suitable material fo r 14C dating.
precipitation to be calculated. The origin of palaeowaters at a given time indicated by 14C measurement can thus be discussed.
Basic requirements for this isotopic treatment can be investigated on the basis of present-day isotopic composition of Lake Abhè total dissolved carbon (sample 75/4 on Table IV). The 5 13C of the total dissolved carbon (mainly CO32 ions at pH 9.8) is +0.63% o vs PDB at 30°C and is thus very close to equilibrium with atmospheric C 02. One concludes that calcite precipitation within the lake should at present occur in equilibrium with the atmospheric reservoir. It is assumed that such was also the case during the Holocene.
From an aerial reconnaissance survey of concretions at present forming in lake waters (but which could not be sampled), it was checked that they reach the lake surface. Thus, precipitation occurs near the surface where thermal water comes up through previously precipitated carbonate and mixes with lake water.The precipitation mechanism of these carbonate concretions is shown on Fig.5.
3.3. Data and interpretation
Carbonate samples were taken on the highest concretion and measured for 13C, lsO, and 14C contents (Table IX). For calculation of temperature and the stable isotope content of mother water, the following equations were used:
t°C = 147.7-14.8 (513Ccarb - 0 13CCo 2gas) + 0.266 (S13Ccarb.^S13Cbo2gas)2
according to Fontes et al. [20] from fractionation factors calculated by Bottinga [21 ].
TABLE IX. PALAEOHYDROLOGY: ISOTOPIC COMPOSITION,14C AGE OF THE CALCITE FROM HYDROTHERMAL CONCRETION OF ASBAHALTOCalculated temperature and isotopic composition of the water of palaeolake Abhè
SampleLocation
ô13cvs FDB
»ieovs PDB
Calculated Water Temperature Range
Calculated Water 1Я
S O Range vs SMOW14C Ages BP Uncorrected Remarks
314 m (top) +2.30 - 1 0 .5 8 19 to 39°C - 9 .9 to - 5 . 8°/oo — 1
314 m (top) +1.85 - 1 1 . 6 0 23 to 44°C - 1 0 . 0 to - 6 . 0°/oo 1570 + 60 2
295 m +1 . 2 8 - 7 .40 28 to 50°C - 4 .7 to - 0 . 7 % ° — 1
295 m +1.15 - 7 .80 30 to 51°C - 4 .8 to -0 .9 ° /o o 236 0 + 80 2
285 m +1.79 - 7 .8 6 24 to 44°C - 6 . 2 to - 2 . 1 °/oo — 1
285 m +1.19 - 9 .30 30 to 51°C - 6 .4 to -2 .4 ° /o o 2 7 2 0 + 1 2 0 2
255 и (ta s e ) +1.63 - 6 . 1 0 25.1 to 48°C - 4 . 1 to - 0 . 1 °/oo — 1
255 и (Ъаэе) - 1 . 8 8 -1 3 .8 9 6 1 .4 "to 86°C - 5 .3 to - 1 . 7 % ° 6290 + 1 2 0 2
The range o f tem perature values is calculated using the range —6 .4 to — 8.5%o for ,3C con ten t o f atm ospheric CO ; . These values correspond respectively to the general steady-state 13C co n ten t o f atm ospheric CO : estimated by Craig and Keeling [2 3 ] and to a value corrected for the contribution o f C 0 2 that originates from fossil fuel use during the last century. T h e right value is thus certainly close to -6.4%> for the tim e interval, 6 3 0 0 to 160 0 BP.1 = Average o f 2 m easurem ents from th e same sample2 = M easurem ents from F on tes and Pouchan [1 9 ]
258 FON
TES et al.
IAEA-AG-158/17 259
t°C = 16.9-4 .2 (S l8Ocarb - 5 18Owater) + 0.3 (5 18Ocarb - 5 l8Owater)2
according to Craig [22].Further available data since previous measurements [19] are reported. All
calculated temperatures are obtained from the 13C palaeotemperature equation.The values used for the 13C content of equilibrating atmospheric C 02 are
—6.4%c and — 8.5%o, respectively. The former corresponds to the 13C content of the atmospheric carbon-dioxide free of any contamination with C 0 2 produced by fossil fuel combustion. Hence, this value is likely to apply to the time interval of crystallization (6300 to 1600 BP). The value —8.5% o is taken as a reference for an average 13C content of present-day carbon dioxide from the atmosphere; the corresponding calculated temperatures are thus minimum values.
Results suggests that waters were rather warm and even hot in the vicinity of the thermal springs. On top of the concretion, where the lake was more than 60 m higher than its present-day level, a temperature effect is still noticeable in the precipitating carbonate. This would indicate that thermal springs discharge was very important at lake maximum. At the base of the concretion, two facies of carbonate can be distinguished. One is very hot and suggests that thermal water mixed with lake water at, or near, boiling point.
The calculated 180 content of mother water of carbonates show rather large variations. From base to an elevation of 40 m on the body of the concretion, the 18 О content of the solution is high and corresponds to an active evaporation (—2 < S180 < O). However, the enrichment is far from that of the present-day lake (5180 = +3.73, sample 75/4 in Table IV). This suggests that the residence time of water within the lake was lower or that the relative humidity was higher than at present. On top of the concretion the calculated stable isotope content is low (518 О = - 6%o). The interpretation is that, during high-level stage, the water was actively renewed and leakage occurred from the lake towards the Henlé basin. This isotopic composition corresponds to a supply of water precipitated under rather cool conditions. Thus, the recent (historical) high (+314 m) level of Lake Abhè was probably correlated to the southern migration of polar-front rains rather than to exceptional events of monsoon rains which would have had a high 180 content owing to the vicinity of the sea. At the same time, the thermal spring should have had a higher discharge and a stable isotope content lower than at present (some litres per second, 6 180 = 2 .8 %o).
3.4. Conclusions
From a review of this set of isotope data, one can draw hydrogeological and methodological conclusions. General information is obtained on regional groundwater systems characterized by severe aridity and only sporadic heavy rain
2 6 0 FONTES et al.
episodes, high hydraulic gradients, internal drainage conditions, and circulations in fractured and porous media.
(i) Recent recharge is significant through fractured rocks, whereas evaporation counteracts vertical infiltration in the porous media in the centre of the closed basins;
(ii) Infiltration is due to major episodes of rains and floods which, without evaporation, give rise to direct seepage through the cracks and probably through the upper bed of the flood channels which are not filled by thin sediments;
(iii) The sporadic recharge conditions are generally preserved within the aquifers where low mixing actually occurs;
(iv) Recent recharge of hypothermal and mesothermal waters, including the confined aquifer of Wadi Ambouli, is confirmed by radiocarbon analyses which show also that storage is generally low because of the low porosity of the fractured lava; and
(v) Some water from geothermal circulations can be some tens of years old, and one of them was about 12 centuries old.
Methodological considerations are:
(i) The consistency of the results given by an estimation of 14C initial content indicates that unconfined aquifers can behave as closed systems with respect to carbon chemistry;
(ii) The comparison between 14C and 3H data shows that waters with a 3H content as low as 5 or 6 TU can be entirely recent in inter-tropical maritime regions of monsoon climate where condensing vapour has extensively exchanged with ocean masses and lost much of its tritium. Since the radiocarbon excess of the atmosphere is frequently submitted to this kind of decrease by exchange with the oceanic reservoir, it appears that, in these areas as well as in the southern hemisphere where 3H fallout is low, 14C analyses could represent a more suitable tool than 3H for evaluating recent recharge provided that 13C, hydrochemistry, and field pH and alkalinity are available; and
(iii) An attempt at interpreting the stable isotope contents of inorganic carbonate in terms of palaeo-environmental indicators shows that such an approach is promising provided it is possible to determine that the carbonates were precipitated in equilibrium with the atmosphere.
IAEA-AG-158/17 261
REFERENCES
FONTES, J.Ch., MOUSSIE, C., POUCHAN, P., WEIDMANN, M., Phases humides au Pléistocène Supérieur et à l’Holocène dans le sud de l’afar (T.F.A.I.), C.R. Hebd. Séances Acad. Sei. 277, Ser. D (1973).GASSE, F., “L’évolution des lacs de l’afar central (Ethiopie et T.F.A.I.) du Plio — Pléistocène à l’actuel”, Reconstitution des paléomilieux lacustres à partir de l’étude des Diatomées, Thèse Doctorat, Paris (1975) 406.FONTES, J.Ch., FLORKOWSKI, T., POUCHAN, P., ZUPPI, G.M., “Preliminary isotopic study of Lake Asal system (Republic of Djibouti)”, Isotopes in Lake Studies (Proc. Advisory Group Meeting, Vienna 1977), IAEA, Vienna (1979) 163.GONFIANTINI, R., BORSI, S., FERRARA, G., PANICHI, C., Isotopic composition of waters from the Danakil Depression (Ethiopia), Earth Planet. Sei. Lett. 18 (1973) 13. SCHOELL, M., FABER, E., Survey on the isotopic composition of waters from northeast Africa, Geol. Jahrb. D17 (1976) 197.INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Isotope Data No.l : World Survey of Isotope Concentrations in Precipitation 1953—1963, Tech. Rep. Ser.No. 96, IAEA, Vienna (1969).INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Isotope Data No. 2: World Survey of Isotope Concentrations in Precipitation 1964—1965, Tech. Rep. Ser.No. 117, IAEA, Vienna (1970).INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Isotope Data No. 3: World Survey of Isotope Concentrations in Precipitation 1966—1967, Tech. Rep. Ser.No. 129, IAEA, Vienna (1971).INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Isotope Data No. 4: World Survey of Isotope Concentrations in Precipitation 1968—1969, Tech. Rep. Ser.No. 147, IAEA, Vienna (1973).INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Isotope Data No. 5: World Survey of Isotope Concentrations in Precipitation 1970-1971, Tech. Rep. Ser.No. 165, IAEA, Vienna (1975).INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Isotope Data No. 6: World Survey of Isotope Concentrations in Precipitation 1972—1975, Tech. Rep. Ser.No. 192, IAEA, Vienna (1979).FONTES, J.Ch., GARNIER, J.M., “Determination of the initial 14C activity of the total dissolved carbon: Age estimation of waters in confined aquifers”, Proc. 2nd Int. Symp. on Water-Rock Interaction, (H. PAQUET, Y. TARDY, Eds) 1 (1977) 363.FONTES, J.Ch., GARNIER, J.M., “Determination of the initial 14C activity of the total dissolved carbon: A review of the existing models and a new approach, Water Resour. Res., in press.TAMERS, M.A., “Surface water infiltration and groundwater movement in arid zones of Venezuela”, Isotopes in Hydrology (Proc. Symp. Vienna 1967), IAEA, Vienna (1967) 339.PEARSON, “Use of C-13/C-12 ratios to correct radiocarbon ages of material initially diluted by limestone”, Proc. 6th Int. Conf. on Radiocarbon and Tritium Dating, Pulmann (1965) 357.MOOK, W.G., “The dissolution-exchange model for dating groundwater with 14C”, Interpretation of Environmental Isotope and Hydrochemical Data in Groundwater Hydrology, (Proc. Advisory Group Meeting, Vienna 1975), IAEA, Vienna (1976) 213.
[17] NYDAL, R., LOVSETH, K., SYRSTAD, D., Bomb C-14 in the human population,Nature 232 (1971) 418.
[18] TRUESDELL, A.H., JONES, B.F., WATEQ, a computer program for calculating chemical equilibria of natural waters, J. Res. US Geol. Surv. 2 (1974) 233.
[19] FONTES, J.Ch., POUCHAN, P., “Les cheminées du lac Abhè (T.F.A.I.): stations hydroclimatiques de l’Holocène”, C.R. Hebd. Séances Acad. Sei. 280 Ser. D (1975) 383.
[20] FONTES, J.Ch., LEPVRIER, Cl., MELIERES, F., PIERRE, C., “Isotopes stables dans les carbonates évaporitiques du Miocène Supérieur, de Méditerranée occidentale”,Proc. Messinian Events in the Mediterranean 1973, Konkndljke Nederlands Adad. Van Wetenschappen, Amsterdam (1978) 91.
[21] BOTTINGA, Y., Calculation of fractionation factors for carbon and oxygen isotopic exchange in the system calcite-carbon dioxide-water, J. Phys. Chem. 72 (1968) 800.
[22] CRAIG, H., “The measurement of oxygen isotope paleotemperatures”, Stable Isotopes in Oceanographic Studies and Paleotemperatures (TONGIOGI, E., Ed.), Pisa, Consiglio Nazionale delle Ricerche, Laboratorio di Geologia Nucleare (1965) 161.
[23] CRAIG, A., KEELING, C.D., The efforts of atmospheric N20 on the measured composition of atmospheric C 02, Geoch. Cosm. Acta 27 (1963) 549,
2 6 2 FONTES et al.
L IST OF PART IC IPAN TS
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Airey, P.L. Australian Atomic Energy Commission,PO Box 41, Coogee 2034, New South Wales
AUSTRIA
Zötl, J.G. Technische Universität Graz,Abteilung für Hydrogeologie, Rechbauerstrasse 12, 8010 Graz
CANADA
Fritz, P. University of Waterloo,Department of Earth Sciences, Waterloo, Ontario N2L 3G1
FRANCE
Fontes, J.Ch. Laboratoire d’hydrologie et de géochimie isotopique,Université de Paris-Sud,Bât. 504, F-91405 Orsay Cedex
Guizerix, J. > Département de chimie appliquée du Commissariatà l’énergie atomique,
B.P. 85, Centre de Tri, F-38041 Grenoble Cedex
Olive, Ph. Université Pierre et Marie Curie,Centre de recherches géodynamiques,Avenue de Corzent, F-74203 Thonon-les-Bains
GERMANY, FEDERAL REPUBLIC OF
Geyh, M.A. Niedersächsisches Landesamt für Bodenforschung,Postfach 510153, D-3000 Hanover 51
Moser, H. Gesellschaft für Strahlen- und Umweltforschung mbH,Institut für Radiohydrometrie,Ingolstädter Landstrasse 1, D-8042 Neuherberg
Münnich, K.O. Institut für Umweltphysik der Universität Heidelberg,Im Neuenheimer Feld 366, D-6900 Heidelberg
263
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INDIA
R ao, S.M. Bhabha Atom ic Research Centre, Isotope Division, Bom bay 4 0 0 085
IRAN
Movahed, M.A.
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Iranian Ministry o f Energy,Department of Pollution and Quality Control of
Water and Radioisotopes,Teheran
Pahlavi University,Irrigation Department of the Agriculture College, Shiraz
ISRAEL
Gat, J .R .
Issar, A.
Levin, M.
The Weizmann Institute of Science, Isotope Department, PO B ox 26 , Rehovot
Ben Gurion University of the Negev,Institute for Desert Research,Kiriat Sdeh-Boqer
Ben Gurion University o f the Negev,Institute for Desert Research,Kiriat Sdeh-Boqer
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TURKEY
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UNITED KINGDOM
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Lloyd, J.W . The University o f Birmingham,Department o f Geological Sciences,PO B ox 3 6 3 , Birmingham B 1 5 2T T
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