British Geological Survey

TECHNICAL REPORT WC/94/4 Overseas Geology Series

OVERBANK SEDIMENTS IN CONTAMINATION ASSESSMENT AND REGIONAL GEOCHEMICAL MAPPING J Ridgway, D M A Flight, B Martiny, A Gomez-Caballero, C Macias-Romo and K Greally

A Report prepared for the Overseas Development Administration under the ODA/BGS Technology Development and Research Programme, Project 9 1/16

ODA classification: Subseaor: Others Subject: Geoscience Theme: Mineral Resources Project title: Environmental Geochemical Mapping Reference number: R5547

Bibliographic reference: Ridgway J Land others 1994.0verbank sediments in contamination assessment and regional geochemical mapping. BGS Technical Report WC194l4

Keywords: Gemhemistry, geochemical mapping, environmental monitoring sediments, floodplains. Mexico

Front cover illustration: Sampling overbank sedunents at El Salitre, Mexico.

0 NERC 1994

Keyworth, Nottingham, British Geological Survey, 1994

S U MMA RY

This report describes a study carried out as part of Project No. R5547 (91/16), Environmental Geochemical Mapping, under the Overseas Development AdministrationBritish Geological Survey Technology Development and Research Programme, which forms part of the British Government programme of aid to the developing countries.

Bac kg mund

Studies of the geochemistry of overbank sediment (material deposited on a river floodplain during flood-flow conditions) are important for two main reasons, Firstly, contamination from historical mining operations and other contaminative land uses can travel tens of kilometres from its source to be stored in river floodplains. These contaminated floodplains form ‘chemical time bombs’ and are a potential environmental hazard because the metals could find their way into the food chain via crops and groundwaters or be released into river systems through a variety of human activities (e.g. aggregate extraction, trenching for irrigation systems etc.) and natural processes (e.g. bank erosion during flood flow). Secondly, it has been suggested that vertical profiles through floodplains allow the sampling of sediments deposited at different times in the past which can be combined to produce a more representative sample than would be collected using conventional geochemical techniques. Overbank samples also offer the possibility of preparing geochemical maps showing the state of the environment both before and after man’s influence. These properties have led to overbank sediment being proposed as a wide-spaced sampling medium for a geochemical map of the world that would allow regional geochemical maps of previously unsurveyed areas to be prepared in a relatively short time period with potential health and economic benefits for developing countries. Such regional geochemical maps would delineate major areas of trace element deficiencies or excesses which might affect animal productivity, crops and human health. They might also identify metallogenic provinces which would guide exploration for particular types of mineral deposit thus enhancing the prospects of making a discovery.

0 bjec tives

The project set out to determine to what extent the examination of overbank sediments would allow contamination hazards in mining regions to be identified, to evaluate their use as a regional geochemical mapping medium, and to compare the geochemistry of overbank sedirnents with that of stream sediiiients i i i order to assess their relative merits for contamination studies and regional geochemical mapping Two areas in central Mexico, with histories of metal mining going back to the 16th and 17th centuries, have been studied In each area one contaminated and one uncontaminated river catchment was investigated through the collection and chemical analysis of sediments from a total of 120 overbank profiles

Resiilts

Both overbank and stream sediments show clear geochemical differences between the base metal contamination in the Rio Puerco basin and the neighbouring uncontaminated Rio Salitre The less pronounced contamination of the Rio Guanajuato catchment can also be distinguished from the uncontaminated sediments of the Rio Lajas basin, but less easily I n all four basins, overbank sediment profiles from small areas of the floodplain show strong lateral and vertical

variations in chemistry and sampling of any one profile could give an unrepresentative view of the geochemistry of the basin. In the contaminated basins, no systematic pattern of variation in metal concentration away from the source can be discerned. Some of the most contaminated material occurs at sites farthest away from the mines. Overall, stream sediments are as effective as overbank sediments in showing that a basin is contaminated and are simpler to collect. Studies of overbank sediment geochemistry need to be backed up by fluvial geomorphology and age dating and cover the whole floodplain in order to reveal the full magnitude and extent of contamination present in a basin.

Conclusions and Recommendations

River floodplains are present throughout most of the world and are the sites of extensive agricultural, urban and industrial development. They are frequently subject to contamination as a result of man's activities, including mining. Overbank sediments are particularly useful for the detailed assessment of the magnitude and extent of contamination in a river basin and would be especially appropriate for Environmental Impact Assessment (EIA). In offering the ability to detect contamination which cannot be seen in conventional geochemical sampling media, overbank sediments provide a unique and valuable tool for contamination studies. Detailed definition of the extent of contamination will allow appropriate measures to be taken to alleviate the hazards and assist in the planning of future development. Knowledge of the location of areas of high metal content in floodplains is important for health, particularly in regions where people have a diet restricted to food of local derivation, and for planning so that environmental damage, as a result of the extraction of aggregate for new development and the digging of irrigation channels for example, can be avoided. Measures to protect contaminated segments of floodplains against natural erosion can also be taken, to ensure that downstream development, whether agricultural or urban, takes place in as safe an environment as possible and river waters are not polluted by stored heavy metals.

For regional geochemical mapping purposes, stream sediments are a more reliable and cost effective medium than overbank sediments The idea, put forward by advocates of the use of overbank sediments, that material from deep i n the floodplain will reflect the period before man's activity affected the environment, while the topmost layers show the present state of contamination, has been shown to be seriously flawed In addition, the requirement for expensive, high technology age-dating inay preclude the use of overbank sediments for regional geochemical mapping i n many developing countries The systematic geochemical mapping of unsurveyed parts of the world to aid in the search for mineral deposits and delineate areas where trace element levels may effect human, animal or crop health should, wherever possible, be based on the collection of stream sedinients, preferably from low order streams Stream sediments are simpler and more cost-effective to collect and because of this a higher density of coverage can be achieved Interpretation is thus simplified because aberrant results have less impact on large datasets l'he collection of stream sediments from high order streams (giving low density coverage) IS not recommended, especially in areas where contamination i s powble, becaure ero\ioii of contaminated overbank material can give rise to resultc. which distort the geocIit.niic~i1 patterns

The results of the research have widespread relevance and should be of particular interest to developing countries that require ( 1 ) to identify hazards related to contaminated floodplain sedinients, and ( 2 ) to prepare regional environmental geochemical baseline maps

I I

CONTENTS

SUMMARY . , . . . . . . . . , . . . . . . . .

INTRODUCTION . . , . . , . . . . . , . . .

PROJECT OBJECTIVES

POTENTIAL BENEFITS . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . .

, . . . _ . . . _ . . . . . . . . .

STUDY AREAS . . . . . . . . . . . . . . . . . . . . . . . .

2

3

4

SAMPLING AND ANALYSIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

RESULTS A N D DISCUSSION . , . . . . . , . . . , . . . . .

(a) Local and regional variability . . . . . . . . . . . . . . . . . .

Rio Guanajuato Basin . . . . . . . . , . , , . . . . . . . . . .

Rio Lajas Basin . . . . . , . . . . . . . . . . , , , . . , . . . .

Rio Puerco Basin . . . . . . . . . . . . . . . .

Rio Salitre Basin . . . . . . . . . . . . .

Discussion

(b) Availability of pristine material prc-dating major environmental

interference by man . . . . . . . . . . . . . . . 34

(c) Comparison with stream/river sediments . . . . . . . . . . . . . . . . . . . 41

d) Overbank sediments as indicators of mining contamination , . . . . . . . , . . 43

7

. 7

7

20

21

22

22

CONCLUSIONS 43

43

44

44

44

(a) Local and regional variability

(b) Availability of material pre-drltlng man's i n fuence 011 the cnvironment

(c) Re1 at I on sh i p be t ween o vc r b ;in I\ an tf \ t rcniii sed i men t s

(d) In d i cat I on s of m i n i ng con ta in I 11 ;it I o n

Summary 45

REFERENCES . . . . . . . . . . . . . . . . . . . . 46

APPENDIX 1 : Tabulated analytical data . , . . . , , . . . . . . . . , , . . . . . . , . . . . . . . . 49

APPENDIX 2: Overbank profiles: variations in chemistry with depth . . . . . . . . . . . . 73

FIGURES

FIGURE I : Location of the study areas and overbank profile groups. . . . . . . . . . . . .

FIGURE 2: Scatter plots and regression coefficients for 40 pairs of replicate

4

analyses 6

8 FIGURE 3 : Arsenic levels in overbank profiles from the Rio El Cubo floodplains, . , .

FIGURE 4: Patterns of chemical variation in profiles 27-34 from the Rio

Guanajuato. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

FIGURE 5 : Lateral and vertical chemical variation in profiles 41-46, Rio Lajas. . , . , 20

FIGURE 6: Lateral and vertical chemical variation in profiles 67-74, Rio Puerco. . . . 21

FIGURE 7: Lateral and vertical chemical variation in profiles 86-91, Rio Puerco. . . . 28

FIGURE 8 : Lateral and vertical chemical variation in profiles 92-99, Rio Puerco. , . . 34

FIGURE 9. Box and whisker plots for the major drainage basins . . . . . . . . . . , 35

FIGURE 10 Chemical variation and age relationships 10 km downstream of

Guanajuato . . . . . . . . . . . . . . . . . . . . . . . . , . . . 38

TABLES

TABLE 1 Summary statistics for the Gumjuato basin 9

TABLE 2 Summary statistics for the Lajas basin 15

'JABLb 7 Summary stati%tics for the Puerco basin 23

?JABLE 4 Summary statistics for the Salitre basin 29

TABLE 5 Summary of chief differences between major basins 39

INTRODUCTION

The investigation reported here is part of a wider study of the application of regional

geochemical mapping to environmental problems (Project R5547, 9 1/16, Environmental

Geochemical Mapping) funded by the Overseas Development Administration as part of the

U.K. programme of aid to developing countries and carried out under the ODABGS

Technology Development and Research Programme. Project collaborators in Mexico (B.

Martiny, C. Macias-Romo) were supported by the Instituto de Geologia, Universidad

Autonoma de Mexico. The Instituto de Geologia also provided laboratory facilities in Mexico

and transport in the field.

Studies in the UK based on the collection and chemical analysis of overbank sediment

(material deposited on a river floodplain outside the drainage channel during flood-flow

conditions) have shown that contamination from historical mining activity can travel tens of

kilometres from its source and lead to high concentrations of heavy metals being stored in

river floodplains These contaminated floodplains form 'chemical time bombs' (Stigliani and

Salomons, 1993) and are a potential environmental hazard because the metals could find their

way into the food chain via crops and groundwaters or be released into river systems through

a variety of human activities (e g aggregate extraction, trenching for irrigation systems etc )

and natural processes ( e g bank erosion during flood flow)

Interest in overbank sediments has also been generated by International Geological Correlation

Programme 229 Internationat Geocheniical Mapping The use of overbank sediment as a

sampling mediuin for regional geochemical mapping was first proposed by Ottesen e / d

(1989) More recently, Bolviken cil 01 (1903) have advocated the use of overbank sediment

as a suitable medium for the preparation of a geochemical atlas of Western Europe based on

wide-spaced sampling, which would show both the present state of contamination and natural

geochemical patterns undisturbed by mail's activities, and also as a niediuni for international

geochemical mapping They suggest that drainage basins of 60-600 k m ' be used and that three

samples should be taken from each site " ( I ) one sample of overbank sediment from the upper

I S cni of the sequence to assess the influence of man (airborne and river borne pollution), (2)

one sample of pre-industrial overbank sediment at depth from present - 01 terraces of earlier -

I

flood plains to map natural conditions, and ( 3 ) one sample of active stream sediment to

provide a linkage to data sets of national surveys and to contribute to the mapping of the

present pollution of the drainage basins" Studies in the U K (e g Lewin and Macklin, 1986,

Macklin ef U / , I992a, Macklin e / ul , 1992b) have demonstrated the close association between

mining contamination and the metal content of contemporaneous overbank sediments and have

also shown that the oldest and least contaminated material is not necessarily found at depth

in the floodplain, but may be in older terraces well away from the present river channel

Overbank sediment thus has the potential to provide a sampling medium for geochemical

surveys which can yield both spatial and temporal data

PROJECT OB JECTlV Es

This report describes an investigation of overbank sediment geochemistry in two areas of sub-

tropical central Mexico, in which floodplain sediments have been contaminated as a result of

mining and mineral processing. I t complements a similar review of overbank sediments in

England and Wales by Macklin e1 al. ( in press).

The aim of the project was to assess whether the findings of research 111 the 1JK are applicable

to other parts of the world, in particular to developing countries in the tropics and sub-tropics

The spec; fi c objectives were:

1 ) to evaluate whether overbank sediments can be used to delineate the extent and

magnitude of contaniination from historical and active mining and mineral processing

i n Mexico

2) to assess the relative effectiveness of drainage sediment and overbank sampling media

for the detection and delineation of contamination and for regional geochemical

m app i n g

Since the 1050s i t has been accepted tha t drainage seciinicnts ;ire rcprcseiitntive samples of

the drainage basin upstrearn of the sample site (Plant 01 trl , I <18H) }:or mapping purposes

overbank sediment must be similarly representative and, in order to be cost effective, easily

sampled. A second consideration is that if overbank material is to be used to depict patterns

related to both the present state of the environment and a pre-contamination state, it is

necessary to be able to identify with certainty pristine sediment which pre-dates man's major

activities.

The present study therefore examines:

(a) the local and regional variability in overbank sediment profiles as a measure of how

representative they are of the upstream catchment area;

the availability and ease of recognition of pristine sediment;

the relationship between the geochemistry of overbank sediments and the well-

established regional geochemical mapping medium of stream sediments; and

the use of overbank sediments to indicate both the magnitude and extent of mining

contamination.

(b)

(c)

(d)

POTENTIAL BENEFITS

River floodplains are present throughout most of the world and are the sites of significant

agricultural, urban and industrial development The results of the research thus have

widespread relevance and should be of particular interest to developing countries, where the

identification of contaminated floodplains would allow appropriate measures to be taken to

alleviate the hazards and assist in the planning of future developnient Confirmation that

overbank sediment is a viable sampling medium for regional geochemical mapping would

provide a valuable impetus to world-wide geochemical mapping Wide-spaced ( I e low

density) geochemical sampling would allow large areas, particularly in the developing

countries, to be surveyed rapidly to provide Laluable information for metallogenic and

environniental studies The recognition of nietallogenic provinces can help to guide

exploration for mineral deposits with iniportant economic implications, while the identification

of regional variations i n trace element concentrations would allow the delineation of areas

where excesses or deficiencies could affect the health of humans, animals and crops

(Thornton, 1984, 1 ewis, 1086)

3

STUDY AREAS

Two areas with a long history of mining activity, the Guanajuato region, GUanajUatO State,

and the Angangueo-Tuxpan region, Michoacan State, both In central Mexico, were chosen for

the study (Fig I )

4747 - b 0

FIGURE 1 Imcation of the study areas and o b c r b a n h protilc groups

In the Guanajuato region the drainage basins of two major rivers, the Rio Guanajuato and the

Rio Lajas, were selected for investigation (Fig I ) Both drain southwards into the Rio Lerma

over broadly similar geology consisting of Mesozoic-Tertiary volcanic-sedimentary sequences

The Rio Guanajuato basin is heavily mineralised and has been extensively mined, firstly for

Ag and later for A u and Ag, since 1550, with the vast majority of mineral processing having

taken place locally Precise production figures for the district are not available, but i t has been

estimated that 30,S96 tons Ag and I 3 3 tons A u were produced up to 1968 (Queroi e / a / ,

I99 I ) Only minor. unworked tin inineralisation is documented in the Lajas basin

The Rio l’uerco, draining the mining area near Angangueo, and the Rio Salitre, draining

unmined ground (Fig I ), were examined in the second region The geology here also consists

of Mesozoic-l’ertiaw volcano-sedimentary sequences, but with a greater proportion of

intermediate and basic compositions than in the Guanajuato region Mining at Angaiigueo,

primarily for Ag, Pb and I n , began i n 1623 and continued u n t i l IO()I As at Guanajuato,

mineral processing took place locally, but production figures are not available

SAMPLING AND ANALYSIS

Overbank samples were collected from groups of profiles in several separate floodplain

segments within each river valley. Sample sites were determined by the presence of an

accessible vertical overbank profile. A significant proportion of samples sites were positioned

on river terraces, where it was considered, on geomorphological grounds, that the older

material would be found. Representative samples (4-5 kg) of each major distinctive horizon

were collected and sieved, after air drying where necessary, to retain the < I 50 pm fraction.

A total of 120 profiles of different depths were sampled, the number of identifiable horizons

within any one profile ranging from 1-9 and also being of variable thickness.

Active stream sediment samples were collected for comparison at each overbank profile group

location, wet-sieved on site to the same size fraction as the overbank material and air dried.

The only exception to this was in the case of one site in the Guanajuato basin where the

presence of a dam immediately upstream had disrupted normal flow and thus prevented the

collection of normal stream sediment. Each sample was a composite made up of material from

several (normally > 10) sites spread over a length of 30-50 m of river bed around the nominal

sample location. This method of compositing material is generally accepted as yielding

representative samples (Levinson, 1974), as demonstrated in several orientation surveys for

British Geological Survey regional geochemical survey programmes (e.g. Ridgway, 1983).

Thus in most cases only one stream sediment sample was collected at each overbank profile

group location, although at several sites in the Guanajuato region duplicate or triplicate

samples were taken as a further check on within-site variability Sampling in the Guanajuato

region was carried out in November 1990 and that in the Angangiieo area in April 1991.

All samples were analysed by X R F spectroinetry for As, Ca, CO, Cr, Cu, F, Fe K, Mg, Mn,

MO, Na, N I , P, Pb, Sn, U , Zn (Ca. K, Mg and N a are expressed in oxide form i n the tables

and text) Loss on ignition (LO1) was also determined A separate series of 40 replicate

samples, collected at the same time as the original samples from the Guanajuato region, were

analysed with the Angangueo samples Scatter plots and the results of regression analysis on

5

WO: R - 0 991

* t

1 2 5 4 6 6 6 45 a5 12s 165

J xn

NI R I 0 e74 I

I s ' l ( i ( JKl r 2 Scatter plots and repression coet't'isieiits t'or 40 I X I I I S ot' rcplicalc analyses

the two sets of data are shown in Fig. 2 and confirm the reliability of the sampling and

analytical methodologies for most of the elements concerned. Correlation is worst for those

elements with low concentrations, which meant that determinations were carried out near the

detection limit of the analytical method with a consequent detrimental effect on the precision

of the analyses (Thompson and Howarth, 1973). The data were particularly poor for MO and

U, largely because of the very low concentrations present, and these elenients are not shown

or discussed further. In the case of Sn, the replicate sample pairs all contained low

concentrations (max. I5 ppm) and although the precision of the deterniinations was

consequently lower this is not considered to invalidate conclusions for the main body of

results where large variations in Sn content are concerned. The full dataset is tabulated in

Appendix 1 and plotted as a series of profiles showing variation in concentration with depth

in Appendix 2.

RESULTS AND DISCUSSION

Summary statistics for the elements under discussion in the overbank profile groups examined

are given in Tables 1-4, along with approximate catchment basin areas. Values for stream

sediment samples corresponding to each profile group are also given.

(a) Local and Regional variability

Although this study has not specifically addressed the possibility of the observed chemical

variations in overbank profiles being due to post-depositional chemical mobility, this

phenomenon is not thought to be significant in the profiles examined. Similar degrees of

variation are shown by the more mobile elements (Cu, Zn, As) and the largely immobile ones

(Pb, Cr, Sn). I n addition, the variations do not indicate systematic migration of elements in

profiles which must have been subject to the same post-depositional processes. Material which

obviously had been subjected to soil forming processes was not collected and in some profiles

this meant that the youngest sediment was not sampled.

The floodplains of both the Rio Guanajiiato and its main east bank tributary El Cub0 were

sampled.

1210 I:'/ ('rrho

In the Cubo valley, where mining in the headwaters region began in the late 18th century

(Querol ci 0 1 , 1991), the maximum levels of f'b, Cu, and Zn encountered are only 70, 42 and

I67 ppm respectively, reflecting a lack o f significant Vu, Pb, Zii minerallsation in the ore

bodies, with no enrichment nearest to the iiiiiies and no systematic pattern of variation

7

downstream (profiles I - I S , Table I ) Arsenic values are also relatively low nearest the

workings (profiles 1-2, 10 km downstream) but rise in profiles 3-7 (approximately 12 5 km

downstream from El Cub0 mine) where maximum levels of 252 and 300 ppm As are found

in the lower two horizons of profile 3 in association with the highest Pb and Zn values noted

above (Fig 3 )

Y...:...: ::

... ... I# ....

... ...... ... C‘ .:::.. ... .... ... . . ..:.. .:.::: ... .: .... ...

I~ICiIJIU< 3 : Arsenic levels in ovcrbank profiles from

the R i o El Cubo floodplains, showing within group

and downstrcam variation. (iroups arc arranged in

downstream order from top to bottom but the

arrangement of profiles within each group has no

lateral o r vertical significance for field relationships,

cscept for the division into lctt and right banks

Iilemcnt values in ppni 1’13 = plastic hag Thin

vertical lines cstending hcyond thc tops 01’ prol‘iles

indicate urisarnpled soil oI otherwise Jist~irhcJ

horizons. Sec Fig I f o r 1oc;itions o f thc prol’ilc

groups and test t‘or I’urthcr csplanation

RIGHT BANK

RO EL CUB0 j

Marked lateral and vertical variation, on both local and more regional levels makes correlation

between horizons i n different areas of the floodplain difficult A plastic bag found at a depth

of 1 5 ni in horizon 14A (16 5 km downstream of the mine. Figs I and 3 ) testifies to the

recent origin of th i s sediiiient and shows that deposition rates were sometimes high However,

this horizon shows no similarities in depth or As content to 7C where a bag was also found

(Fig. 3 )

8

TABLE 1

TABLE 1 (continued 1 )

a - - m N N N N y w w r -

a - r -

N N N N ? ?

- w m r - N N m P N N N N

w 3 W N N N N N b y w m

a * - w N N N N 9 9 - p '

N W W

N - - N r - O \

w m - m ? f l y - + - - -

m a P

- 0 0 m r - ? w m m a o w m o

- 0 0 -

o o o w wuJar- ""rnfl

w r - c a r - o o m m a P N w

m m a O N N m v a C O O N -

" W o q r - r ? z o o - P m m l O O

- 0 - r - o m ~ m

NNlO w m l f m b m

m m m m m m m o N m O m

- r - r - x c , r - r - l O c, c, e, c

U , - 5. 7 V i - f N 7.

f . " O Q c )

c, N m m m m - I n r - P r - N - C N

c cc, YI N n x - 3 - - N N

7 'I8 Q M 7 c. cc, r.I

r , r , c1

I < , 2 Y , e. - I- P r r

TABLE 1 (continued 2)

w d 0 v) VI VI VI

ffi m

N U2 ci - VI r- d

1??6 N N N E'

" 2 2 " - 0 2 2 -.N

- N - V I - - - -

- r - W O w - n - r -nVIVI

c i N N W w m m w

P ' w r - 0 m v ) v ) \ D

c VI v/ -7 'I, ,, N N ' c l V,

- m m 7 - - - -

m t . x x

c N r - N N

IS

Rio Guanajualo

In the Rio Guanajuato valley samples were collected over a total downstream distance of

approximately 25 km, the most upstream samples being 10 km below the city of Guanajuato

(pop. c. 45,000) and its mine workings. Both urban and mining contamination might be

expected in Rio Guanajuato sediments and metal values are generally higher than those

encountered in El Cubo with maximum values of 254 ppm Pb, 236 ppm Zn and 252 ppm Cu.

In contrast, the maximum As concentration is only 63 ppm and this occurs 35 km from

Guanajuato, below the confluence with the Rio El Cubo. As in the Rio El Cub0 the

downstream dispersion of Cu, Pb and Zn shows no systematic pattern with concentrations that

generally can be considered anomalous only when compared with background levels from the

Lajas basin (Fig. 1 and Tables 1 and 2). Correlation between profiles from the same group

can be difficult (Fig. 4).

Terrace

0 - k C i t U k l t 102Bt ' 3

P - POTTERY FRAr;MENTS I

Terrace

2 % Mod. FVPlain Mod. FI/Plain

Terrace

RIO GUANAJUATO

Terrace T er racs

TABLE 2

C 2 n c

$ v a v v u v v v v v 0 z r c c c c c c c c c

TABLE 2: (continued 1)

w w r r m h O N m m m r .

C N W N

- 0 0 s m r - o s - s w \ D ' f

- m o o 7 - 0 0 -

- m m 0 0 - m m o

m w m m m q N m N

O O ' r - m m 0 - N N b

3 2 % % / . - 0 0

IC).?-c N W O ' f N N N -

a m a N - 0 N N N

- x m m - o n - N N - N

3 Vi vi f -

w u x z

c, c 7

17

TABLE 2 (continued 2)

n m a c

m m o u N - N C

- w ' 4 n N N

E s a * w

E - u r n T m N N

r n N C 7 Ic 7

, " w o ^ f f i /.

Rio Lajas Basin

The overbank data for the Rio Lajas drainage basin, where no metalliferous mining is known,

show considerable differences from the Guanajuato drainage basin (Tables 1 and 2). Most

notable are the low Pb, Cu, Zn and As values recorded over the whole of the sampling area.

Substantial downstream and within profile variation is largely restricted to Sn and could relate

to natural variation from the erosion of minor tin mineralisation in the catchment, although

contamination associated with urban refuse from the town of Dolores Hidalgo cannot be ruled

out. The highest Sn levels (c. 40 ppm) occur below 2.25 m in profile 41 (Fig. 1 and Fig. 5),

but the upper horizons in this profile show only modest concentrations (< 10 ppm). Within

and between profile chemical variations can be seen in Mn and F in Fig. 5 and again

highlight the difficulties of correlating between profiles from the same small area. Variations

exist for other elements (Appendixes 1 and 2), but in almost all cases are small. For

example,the total range in concentration for Pb is 16-34 ppm and for Ni 4-16 ppm.

A 3 1 - Age dalc W A 80 BP

4n-1 . AW nnla I -.n . I(c BP

RIO LAJAS

q 44D 1

44C

U6

14A

4 3 F

I I4 lG

I I l i F 1 41C

113 I

41C

41R

Llh

Rink

P

2 0

Rio Ru?rco Basin

_ _ 67D

67C

6/8

67A

Between Angangueo and Tuxpan the Rio Puerco i s clearly heavily contaminated, with Fe

oxides coating the river bed and in suspension. Bands of metal sulphide and oxide rich

sediment can also be seen in the river banks. A s was the case at Guanajuato chemical

variation within groups of profiles is strong and correlation between profiles difficult. Patterns

of variation and absolute values vary between both profiles from the same bank of the river

and those from opposite banks. In Fig. 6, only profiles 69 and 74 can be correlated with any

confidence, both having low values of Pb, Sn and Zn and coming from left bank terraces.

Bank

I Bank

Terrace

Terrdce

LEFT BANK

m a Z n 61714 053 86 1w L 3 , f L 1

71D

7 7c

7 2 8

72A

Bank

Terrace

n8a-m RANK

I 1 F h h h

RIO PUERCO

The most polluted material does not occur in the two groups of overbank sediments nearest

to the Angangueo mine (63-66 and 57-62), which contain maximum levels ( i n ppm) of 607

As, 97 Cu, 789 Pb and I 1 5 5 Zn, and downstream variation has no consistent pattern (Table

3) The highest As, Cu and Pb values (1968, 208 and 1714 ppm respectively) occur in the

basal horizon of profile 87 and the highest Zn (3543 ppm) in horizon 88C, some 26 km from

Angangueo (Fig 1 and Table 3 ) Half n kilometre downstream i n profiles 92-99 the overall

21

level of contamination has dropped, most significantly in A s (max 374 ppm) and Zn (max.

2331 ppm).

Profile 76A ( 1 2.5 km downstream of Angangueo; Fig. I , Appendixes 1 and 2) is from a small

(0.55 m) terrace on the left bank of the Rio Puerco, but is nonetheless of recent origin, as

indicated by the occurrence of plastic bags at both the base and top. Despite this, levels of

contamination are relatively low with maximum concentrations (in ppm) of I3 1 As, 39 CU, 159 Pb and 480 Zn. Similar concentrations are found in profile 92, a further 14 km

downstream and also dateable from the presence of a plastic bag at its base (0.65 m deep).

Maximum values here are 175 As, 41 Cu, I93 Pb and 954 Zn, all increases on those found

upstream. The highest levels of contamination thus occur neither in the overbank profiles

nearest to the source of contamination nor in the highest and most recent overbank sediments

as recommended for sampling by Bolviken et al. (1993).

Rio Salitre Basin

Overbank profiles from the Rio Salitre show uniformly low levels of the elements responsible

for the contamination in the RIO Puerco (Table 4) Maximum levels of As, Cu, Pb and Zn are

I t , 22, 21 and 1 I 1 ppm respectively Other elements and oxides vary in concentration

vertically within single profiles, between profiles i n a group, and between groups distributed

along the river (Appendix 2 and Fig 1 ) Correlating horizons between profiles within a group

is difficult but the variations are too small to be of significance on anything but the local

scale

In the Rio Guanajuato (including El Cubo) and Rio Puerco basins there are strong lateral and

vertical variations in chemistry within groups of piofiles from relatively small areas of the

floodplain (Figs 3 , 4 and 6) Whatever the c;iuses of this variation i t presents ii major obstacle

to obtaining an overbank sediment sample which can tw considered to be representative of

the upstream drainage basin 'The most recent sediment is not normally the most polluted and

material of the same Seneral age from different reaches of n river does not show comparable

33

TABLE 3

V I -

r - r - -t-: M M N * * - WI .n N

- 0 - M M N P m m z r

G E E N - -

- - r - r - r -u l t T - - M

r n N O * m N o o m

m m r n

E m w - 2 r - M - r o m m

M - N O " M - - 0 0 * n m m m w

M N b

- - - v v v - - " v "

\ D N Q w . t w N * -

h

v) m

, N = = - l r . - - 3 2 4 s

N N

X N 7 3 - r - 3 7 3 I ? M N

N N -

2 3 s - - N

U-$ m w w w n 'I, wl 7

7 Z N - - r . m - N m N -

23

TABLE 3: (continued 1 )

w - m - N - ? N c !

--a w - o q - - - - - a y l ? ? - - - w - -

Y ? ?

. 5 : ; : : ; : : E , , ,

-n r - " 0 9 - - - m o o

T - 7 - I -

r i m - - - !? - - -

2 % P P E 0 0 0

N w w 0 0 0 ? ? 0 9 $ a F

0 - 0

W W N

0 0 0 ? ? U !

N - O

0 - 0 ? P E

m - 2 9 " .

0 - w 0 0 0 ? - ? ?

m t - N 0 0 0 ? ' ? %

a" w - - 0 0 0

0 0 0 222 w - - 0 0 0 Y S - ?

N - q t - w w 0 0 0

- m t -

0 0 0 ? Y E

w m *

0 - 0 0 9 - ? 0 9

0 0 -

- - 0 1 - 0 9

m m m a!-- - - r i c i

0 0 0 ? ? ?

v)n l f lwP- 2-22 w v t - 0 0 m a w - 0 0 0 -

P m m - a m t - w 0 0 0 -

w o o m . n m 0 0 0

z z z 0 0 0

P a r -

0 0 0 o s ? ? N N W

N N ? - - - e r - ? 0 0 -

w a o S ? " - - - v ) v ) m w w - - - - 0 0 . 3

m m o - - - - - a N w m - - - O a N O N - N 9 - - - -

r n m m N O 0 - - - 7 0 0

- 0 - m m v ) 0 w -

- 3 - - - - rnr-4- N P ' t - - 0 0

0 5 0 m

- 0 0 m m o ?

C O O " = m m / . . m m a

M V - - w m 7 - -

3 N c* 3 N N w - -

(c, - . D g m 5.D"

N W M - - - - r, 7 VI

- - a r n m - - - N

a- rap ' m 7 n m

: c c. 2 PI N - r- - 7 - T a - - ? ? -

c - m m - - - P N - N v i N -

2 5

TABLE 3: (continued 2)

v) 0, m - N m n z 2 ul

N N -

! 4 w w m Z N N N m - I - m e m N W W m m m m m - m m m W N N N m m m V N N m m w n n W O W N m m N O N

w w m N N m

N O - m m m m m o N N m w m m

N N N m o w N N N

V W W N - N

V o m N N N

0 0 0

N N N v v v

08- 8 8 8 N N N N N N S N S N N N

v v v v v v v v v v v v v

/ m 7 m - ~ w m m

~ o o m m o p ' m m m r - -0 . m N P 4 - 0 0 N u l m m w w o u l m m w r - r - r - r - m m m m m m m w w

5 - a - - - m - / . w 7 w

patterns of variation. Horizons 7C and 14A from the Rio El Cubo, approximately 4 km apart,

both contain plastic bags and must have been deposited within the last 50 years but contain

very different As levels (Fig. 3) . In the Puerco basin, near Tuxpan, profile groups 86-91 and

92-99 are separated by only I km but show different patterns of variation and absolute levels

of contaminants (Figs. 7-8 and Table 3) . Profiles 86-89 all come from the same left bank

terrace but show different patterns of variation and absolute element levels despite each being

separated from its neighbour by no more than 50 m. In group 92-99 the lowest element

concentrations are found in the river bank sections while the terrace of 93B-D has

substantially higher values. Both these groups could be taken as being representative of an

upstream catchment of just over 200 km2, but could yield very different results, particularly

if only a single profile was sampled. Sampling of present river bank profiles in group 92-99

might lead to the erroneous conclusion that the Rio Puerco basin is uncontaminated, although

the minimum concentrations are above the background levels found in the Rio Salitre

The uncontaminated basins of the Rio Lajas and Rio Salitre show less variation within and

between groups of profiles However, in the Lajas basin there is a diversity of Sn values

which casts doubt on how representative any particular profile or part of a profile really is

(Table 2 , Appendix 2 and Fig 5 )

Gross regional variations are illustrated i i i the box and whisker plots of Fig 9, i n which the

28

TABLE 4

...

w

TABLE 4: (continued 1 )

c m m v , * - a V N V v m \ D ' - m m m r - ? O O-?? NChN B o o 0 0 0 0 0 0 0 - 0 -

3 1

c = m 3 7

, - - a m

i

O ' O W O A P

O I O N n m n

m m m 2 m r - N -

r n c x N 7 N

I ? v , - f r . N N N N

m 0 1 n N - -

- - - 7 -

0 0 P

m - - 2 5 . z

I F I N Y ) m * m

m P P m M m m U

- 0 m m m m

0 0 0 0 0 0 p! N. N

.r VI N N N N

-201.0 N N - N

0 2

9 N 2 m moo

09" w v j m

w m m P P P

0 N - Y - P P - Y P

0 0 0 0 0 0 N N N v v v

-,a E m , "

data are grouped by drainage basin. Differences in geochemistry can be seen between the two

study areas, the contaminated and uncontaminated basins within each study area, the two

contaminated basins and the two uncontaminated basins. These differences are summarised

in Table 5. There are also notable differences in the degree of within basin variation,

particularly in base metals, between the contaminated and uncontaminated catchments, the

former having the much greater spread of concentrations.

I I

L. , 1 1 1 I

Terrace I &nu

Ttvrau,

Tnrrwco

(b) Availability of pristine m a t e d pie-dating major enviionmental interfeience by man

I t is clear from the above and Tables 1-4 that sampling of the upper 15 cm of overbank

sequences, as suggested by Bolviken ct (11 ( I 993). would not provide a reliable guide to the

levels of contamination present in a basin Sampling of the deepest layers o f overbank

sediment I S similarly not a guarantee that the niaterial is pristine, because in some profiles the

deepest material contains the highest levels of a particular metal (e g As, Cu and Pb in profile

87 from the Rio Puerco basin, Table 3 and Fig 7)

34

FIGURE 9

1

I

I

' 1

I

A.. . . . . . . . . . . . . . . . . , : : - - ; n .

.... .. . r l ............ - T , . 7 .',

I

I 11

-t T1

I

+ { I t + * * 0 -

+H

*t

FIGURE 9 (continued)

1 - - - 7 - 7 -7 T 7 7 .-7 - , I

I

I

, . . , . . , . . , . . , , '7 r - - - - r = ' - - r - - - 7

m -a-

- -+I l l

--m

...,

n .

37

WOHT 8ANK LEFT BANK

i,U%

UpperTerrace UpperTerracm

FIGIJRE 10: Chemical variation and age relationships 10 km downstream of Guanajuato showing possible pre-

mining age alluvium in profiles 16 and 17, mining age contamination in profile 18 and recent sediment in

profiles 19 and 20. See Figure 2 and 3 captions and test (or I‘urther explanation

It is sometimes possible to establish relative ages of profiles from their position in terraces

For example, at the site nearest to Guanajuato (profiles 16-20, Fig I O ) , the oldest sediment

occurs in a terrace approximately 2 m above the present river level (16 and 17), an

intermediate terrace ( 1 8) provides younger material and the present floodplain ( 1 9 and 20) the

youngest Levels of Cu, Pb and Zn are low in the oldest sediments, rise to a maximum in the

upper part of the intermediate terrace and fall again in the modern floodplain The upper

terrace may thus represent pre-mining sediment and the intermediate terrace material from the

18- 19th centuries, when mineral processing plants were situated along the river just south of

Guanajuato (Brading, 1971) However, Cr is also high (max 198 ppm) in this group of

profiles and since the Guanajuato ores are iiot rich in Cr, the contamination could stem from

industrial processes other than mining The fall in concentrations in the most recent sediments

probably reflects improved mining (or industrial) practices in modern times 7 he pattern of

variation here, and also 111 some of the Rio El Cub0 and Rio Puerco profiles already

described, is clearly more complex than the simple one of surficial contamination and pristine

3 8

Guanajuato Region Angangueo Region OB K, Na I Mg Ni,Mn,Fe,Cr ss Na,K As, Fe, Ni

Guanajuato Ca, Mg , Na ,Pb, Ni ,

Zn,Cu,Fe,As As, Ni, Mg , Ca

OB

ss

Puerco OB K,Pb,Zn,Mn,Cu,As ss As,Zn,Fe,Pb,Cu,Mn,Mg,K

Guanajuato OB Mg ,Cat Na, K ss Mg,Ca,Na,K

Lajas K

Fe,Na,K

Salitre Ca,Na,Ni Ca, Na, Ni

Puerco Ni,Pb,Mn,As,Zn,Cu As,fn,Fe,Pb,Mn,Ni

Sali t re Ni

Fe, Mn, Ni

TABLE 5: Major geochemical differences between drainage basins and regions. For each pair, elements with higher concentrations than in the counterpart are shown, for overbank (OB) and stream sediments (SS).

material at depth suggested by Bolviken e[ al. (1993). This view is further exemplified in

profiles 27-34 (Fig. 4), in which the highest Cu concentrations occur at a depth of

approximately 3.5 m in horizon 27A and decrease upwards to 27E, while in profiles 30 and

34 an increase upwards is observed. Plastic and rubber refuse in profile 3 I attests to the recent

origin of the relatively low levels of Cu and Zn i n horizon 3 I B and dating of wood fragments

in horizon 33D indicates that the highest concentrations of these elements stem from the 19th

century (Fig. 4).

Terraces do not, however, necessarily contain the oldest sediment as demonstrated by horizon

76A in the Rio Puerco, described above, neither do they always contain the lowest

concentrations of contaminant elements In profile group 75-81, the lowest As, Pb and Zn

values are found in the deeper levels of profile 80 arid not i n the terraces of 75, 76, 77 and

79 (see Appendix 2)

Some of the profiles sampled contain overbank sediment with very low concentrations of ore

related elements and which could, therefore, reflect periods when man's influence on the

environment was absent or minimal. The chief difficulty in using overbank sediment as a

pristine medium for regional geochemical mapping is in knowing the location of these profiles

before sampling. It is not cost effective to sample several profiles in each of several localities

and then to analyse them all in order to find material which is assumed to pre-date man's

activities because of the low concentrations of certain elements. Low concentrations may not

in any case be a sufficient indication of the pristine nature of the sediment; the high Sn values

found in the deeper levels of profiles in the Lajas basin could be the result of natural

influences, since no mining is recorded in this area. Macklin et al. (1992a) describe a

floodplain unit in the Tyne basin of N.E. England, which they relate to a period of

deforestation of an unmineralised part of the basin in the late Iron Age, in which Pb and Zn

concentrations are lower than in an older uni t which derived its sediment from both

mineralised and unmineralised parts of the system.

It can be concluded from the above that the only reliable way of determining that overbank

material pre-dates man's influence is to obtain an age date. Common dating techniques such

as pollen analysis and palaeomagnetism are not practicable in the Mexican environment

because of insufficiently established chronostratigraphic frameworks and the situation is likely

to be similar in many developing countries. Other methods (e.g. thermoluminescence) are too

expensive for routine use in a geochemical survey. Even with suitable age dating techniques,

the identification of pristine material might still necessitate the collection of a large number

of samples.

Indications of age are given by included artifacts, which can only place a maximum age on

the enclosing sediment. Plastic bags arid other modern items, such as rubber-soled shoes, give

a clear indication of the very recent age (c. 50 years B.P.) of some of the profiles.

Radiometric I4C dating was used on wood and charcoal fragments from several localities but

must be interpreted with caution: in profile 43 from the Rio Lajas floodplain a wood fragment

from horizon C gave a date of 350 +/- 80 years R.P. while a similar fragment from slightly

deeper, at the junction between B and C, yielded an age of 170 +/- 60 years E3 P.. The oldest

date obtained by ''C methods was 420 R . P from a charcoal fragment i n horizon 87C in the

40

Rio Puerco. The highest metal values recorded from the Puerco basin occur more than 20 cm

below this level in horizon 87A, and since mining at Angangueo commenced in 1632 this

method of dating is again shown to be unreliable

In no case has it been possible to state with certainty that part of an overbank profile

contained pristine or even pre-mining sediment. Modern artifacts (e.g. plastics) were fo.und

at depths of more than 1 m suggesting that sedimentation rates may be as high as 2 m per 100

years. If sedimentation rates were comparable in the past, pre-mining sediments could be

deeply buried below the surface and not easily accessible.

(c) Comp;uison with s t r e d r i v e r sediments

Stream sediments are the normal sampling medium for regional geochemical mapping in most

parts of the world (Plant et al., 1988). It is unusual for streams of the high order sampled here

to be used in survey programmes but there has been much interest in recent years in wide-

spaced sampling for the production of a geochemical atlas of the world (Darnley, 1990). A

comparison between high-order stream or river sediments and overbank sediments is therefore

appropriate.

At all three overbank group sites in the Cubo basin, the most upstream site on the Rio

Guanajuato, and at one site on the Rio Lajas, stream sediment samples were collected in

duplicate or triplicate The analytical results are shown in Tables 1 and 2 In the Cub0 and

Guanajuato basins the ranges of trace metal values i n the stream sediment data are, in the

great majority of cases, smaller than the total range shown by the overbank sediments In

most instances this 15 also true for comparisons w i t h the ranges shown i n the topmost and

basal overbank horizons The situation is similar in the Rio Lajas with the notable exceptions

of Sn and Zn, where, for group 47-5 I , the range shown in the two stream sediment samples

is much greater than the range in overbank sediments This large variation in Sn and Zn

values may reflect their occurrence In heavv mineral accumulations, which are known to be

erratically distributed over even small areas of a stream bed and can introduce considerable

variability into geochemical data (Saxby and Fletcher, 1986)

Stream sediment element concentrations might be expected to compare most closely with

those from the youngest horizons of overbank profiles, although i t must be remembered that

some overbank horizons which might have been affected by soil forming processes were not

sampled.. In the unmined Rio Lajas and Rio Salitre basins, where the overbank sediments in

general show much less variability than in the mined drainage basins (Tables 2 and 4; Fig.

9), the comparison is generally good, although there is an overall tendency for the stream

sediments to have higher trace metal values than the topmost overbank horizons. The possible

presence of unworked Sn mineralisation in the Lajas basin shows up much better in the

stream sediment data than in those from the overbank sediments. The situation is more

complex in the two mined basins. In the Rio Guanajuato and Rio El Cubo catchments stream

sediment values are lower overall than the overbank sediments whereas in the Rio Puerco

basin the opposite is generally true with stream sediment values often very much higher than

in the corresponding overbank material (Tables 1 and 3; Fig. 9). The reasons for this are

beyond the scope of the present study but could relate to the presence of sulphide rich ores

from the Angangueo mine in the floodplain of the Rio Puerco. This would lead to low pH

groundwaters circulating in the contaminated floodplains and consequent high solution metal

concentrations. The pH would be higher in the fluvial environment due to the contribution of

surface waters from unmineralised parts of the basin, thus causing precipitation of Fe/Mn

oxides when ground and surface waters met, with associated scavenging of metals along with

adsorption of metals on clay minerals. This conclusion is supported by the high Fe levels

found in the Rio Puerco stream sediments (Table 3 and Fig. 9). The relatively low-sulphide

ores of the Guanajuato district (Querol c/ U / . , 1991) would give rise to higher pH

groundwaters and lower concentrations of metals in solution.

Stream sediments give a good indication of the contamination in the Rio Puerco, but are not

so useful in the Rio Guanajuato catchment, although As levels indicate that contamination is

present, particularly when compared with concentrations in the Rio Lajas. Neither stream nor

overbank sediments show well-defined downstream decay patterns from the site of mining,

although in some cases (e.g. As in the Puerco catchment) both sediment concentration and

mean concentration for the topmost overbank hori~ons show slightly erratic downstream decay

(Tables 1 and 3 ) . The erratic nature of the downstream decay patterns i n the stream sediments

is probably related to bank erosion of contaniinatcd overbank material from the floodplain

42

(Macklin, 1992). The variable patterns of distribution of metals in the floodplains themselves

result from the complex interplay of fluvial processes and intermittent mining activity which

may have lead to mining waste moving downstream in discrete parcels rather than

continuously (Lewin and Macklin, 1987).

Overall, stream sediments are as effective as overbank sediments in showing gross regional

variations in geochemistry (Fig. 9 and Table 6) .

d) Ovehank sediments as indicators of mining contamination

In both the Guanajuato and Angangueo regions the full magnitude of mining related

contamination is not revealed by examination of the topmost overbank horizons (Tables 1 and

3, Figs. 3, 4, 6-8) as advocated by Bolviken cl al. (1993). The most contaminated material is

frequently buried in the floodplain and only a detailed examination of floodplain segments

using many overbank profiles will allow the potential impact on the environment of past and

present mining to be properly assessed. Such assessment may require drilling or trenching of

floodplains in addition to investigation of exposed overbank profiles.

CONCLUSIONS

(a) Local and mgional vaiiability

Vertical profiles through overbank sediments i n river floodplains retain a record of changes

in the chemical composition of fluvial sedinients with time Ihese changes are most

pronounced in drainage basins which have been affected by contaminative activities such as

mining However, correlation between overbank profiles from the same relatively small area

of floodplain is difficult Lateral and vertical variations within floodplain segments and lateral

variations between segments can be large, with the highest levels of contaminants often buried

within the floodplain Samples from a single overbank profile are unlihely to be representative

of the upstream catchment area and i t is necessary to examine multiple profiles in order to

gain a suite of samples which can be said with confidence to reflect the overall chemistry of

the drainage basin

(b) Availability o f mateiial pm-dating man's influence on the envimnment

Detailed studies of the fluvial geomorphology and geochemistry of a floodplain, combined

with age-dating, are necessary to establish that deeply buried overbank sediment or terrace

materials pre-date man's interference in the environment. Artifacts included in overbank

profiles indicate that deposition rates are high, but their use for dating can only provide a

maximum age for the enclosing sediment. This may be misleading, as the examples described

earlier in this paper have shown. The high deposition rates suggest that, without drilling or

trenching, sediments which record the state of the environment before human activities may

be difficult to find in Mexico. Dating methods based on the sediment itself (e.g.

thermoluminescence) are the only reliable means of identifying the age of an overbank

horizon and these may be too expensive for regional geochemical mapping programmes,

especially in developing countries.

(c) Relationship between ovehank and stmarn sediments

Although in general terms the sampling and chemical analysis of overbank sediments,

particularly bulk sampling of many whole profiles, would allow recognition of the presence

of contamination and the discrimination between contaminated and uncontaminated drainage

basins of different geology, stream or river sediments offer a more cost effective alternative.

In the Rio Lajas basin, river sediments give a clearer indication of the possible presence of

Sn mineralisation than the overbank sediments. However, i n the Guanajuato catchment neither

river sediments nor overbank samples collected using the Bolviken e / al. (1993) strategy

would reveal the full magnitude of contamination, particularly the relatively high As levels

found in profiles 3 - 7 . In the Rio Puerco basin, river sediments overall are better indicators

than overbank sediments of the level of contamination present.

(d) Indications o f mining contamination

Contaminated layers of sediment present at depth in tloodplains will be detectable neither in

the uppermost horizons of overbank sediment (and thus not in surface soil samples) nor,

unless such contaminated horizons are undergoing active erosion, in stream sediments These

contaminants could have a potentially hazardous impact on the environment through a variety

of anthropogenic activities, such as extraction of construction materials or digging of irrigation

ditches, and natural processes like bank erosion. High metal concentrations do not appear to

tail off systematically downstream in either overbank,samples or stream sediments, making

the prediction of hazards related to point sources of contamination complex. Detailed studies

of overbank profiles within individual floodplain segments throughout a drainage basin may

be the only way to gain a full appreciation of the contamination present in a catchment.

Summary

The variations in chemistry between profiles taken in relatively limited areas of floodplains

create practical difficulties in developing sampling strategies for regional geochemical

mapping and in the general use of overbank sediments for examining historical contamination.

Without means of assigning either absolute or relative ages to profiles, an understanding of

the record of chemical variation and correlation between profiles is difficult. A full

appreciation of the geochemical record contained within overbank profiles requires detailed

study involving age dating and fluvial geomorphology in addition to chemistry. Nevertheless,

in offering the ability to detect contamination which cannot be seen in conventional

geochemical sampling media, overbank sediments provide a unique and valuable tool for

contamination studies. However, the sampling strategy suggested by Bolviken et al. (1993)

for regional geochemical mapping, described earlier in this paper, would probably not be

viable in the areas studied or in other parts of the world (Macklin C I al. i n press). In contrast,

geochemical mapping based on the systematic collection of low order streani sediment

samples has been shown to be effective in discriminating between contaminated, mineralised

and background regions (e.g. British Geological Survey, 1992) without the sampling

difficulties associated with overbank material

The results of this project and similar studies in the U K show that overbank sediments in

different climatic regimes have similar degrees of chemical variation and the conclusions may,

therefore, be applicable to many parts of the world ‘The detailed investigations necessary for

a full understanding of the record contained in floodplain sediment sequences are probably

beyond the scope of many developing countries, but the examination of overbank profiles can

45

nonetheless provide valuable information in areas of historical and present mining activity.

Floodplains throughout the world are sites of urban, industrial and agricultural development.

Knowledge of the location of areas of high metal content in floodplains is important for

health, particularly in regions where people have a diet restricted to food of local derivation.

It is also important to recognise the location of contamination so that environmental damage,

as a result of the extraction of aggregate for new development and the digging of irrigation

channels for example, can be avoided. Measures to protect contaminated segments of

floodplains against natural erosion can also be taken, to ensure that downstream development,

whether agricultural or urban, takes place in as safe an environment as possible and river

waters are not polluted by stored heavy metals.

Geochemical mapping can indicate areas of mineral potential and can also delineate areas of

trace element excesses or deficiencies which have implications for human, animal and crop

health. Overbank sediments, however, do not offer a reliable alternative to stream sediments

for the geochemical mapping of large unsurveyed regions in developing countries. Stream

sediments are simpler and more cost-effective to collect and because of this a higher density

of coverage can be achieved. Interpretation is thus simplified because aberrant results have

less impact on large datasets. The collection of stream sediments from high order streams

(giving low density coverage) is not recommended, especially in areas where contamination

is possible, because erosion of contaminated overbank material can give rise to results which

distort the geochemical patterns.

REFERENCES

BRITISH GEOLOGICAL SURVEY I992 I(egrona1 geochenii.$/ry o j /he / ,oke llistnct and

cd/uccn/ areas (Keyworth, Nottingham British Geological Survey)

BOLVIKEN B, BOGEN J DEMETRIADES A, DE VOS W, EBBING J, HINDEL R,

OTTESEN R T, SALMINEN R, SCHERMANN 0 AND SWENNEN R (1993) Final Report

of the Working Group on Regional Geochemical Mapping 1986-93 Forum of European

Geological Surveys (FOREGS) (icologrcal . Y i r r c . q . of Noi-~vay Opcrr l.Ik I<epot~ 93-092, I 8p ,

6 Appendices

36

BRADING D.A. 1971. Miners and merchants in Roirrbon Mexico: 1763-1810. Cambridge

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DARNLEY A.G. 1990. International geochemical mapping: a new global project. J Geochem.

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LEWIN J. and MACKLIN M.G. 1987. Metal mining and floodplain sedimentation in Britain.

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LEWIS G. 1986. Geochemistry and animal health. In Applied Geochemistry in the 1980s

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in Holocene alluvial sequences: the Tyne basin, northern England. In Archaeology (Jnder

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MACKLIN M.G., RIDGWAY J . , PASSMORE D G . and RUMSBY B.T. in press. The use

of overbank sediment for geochemical mapping and contamination assessment: results from

selected English and Welsh floodplains. Applied Geochemistty.

MACKLIN M.G., RUMSBY B.T. and NEWSON M.D. 1992b. Historical floods and vertical

accretion of fine-grained alluvium in the lower Tyne valley, north east England. In Dynamics

of (;ravel-HeJ Rivers (eds P. BILLI, R. HEY, P. TACCONI AND C. THORNE), 573-589.

Wiley, London.

OTTESEN R.T., BOGEN J. , BOLVIKEN B and VOLDEN 'I, 1989. Overbank sediment: a

representative sample medium for regional geochemical mapping. ./. (;cocherrr. 1:'xplor. 32,

2 5 7-277.

QUEROL F., LOWTHER G.K. and NAVARRO E. I99 I . Mineral deposits of the Guanajuato

Mining District, Guanajuato. I n Economic (;cology. Mcxicw (ed. G.P. SALAS), 403-4 14. The

Geology of North America, Vol. P-3, Geological Society of America, Boulder.

PLANT J . A . , HALE M. and RIDGWAY J I988 Developments i n regional geochemistry for

mineral exploration. 7'trin.s. Insln. hlitr hlctdl ( ,SCL.I I ) Apiil. cat?/] s s i , ) , 97, B I 16-B 140.

KIDGWAY J . 1983. IGS Zimbabwe Project Geochemical Exploration - results of an

orientation survey and the organisation of the sampling programme Insli~rr/c of (;eologic-cil

37

Sciences Repori No. 83/14 (open file). Institute of Geological Sciences, Nottingham.

SAXBY D. and FLETCHER K. 1986. The geometric mean concentration ratio (GMCR) as

an estimator of hydraulic effects in geochemical data for elements dispersed as heavy

minerals. J Geochem. Explor, 26, 223-230.

STIGLIANI W. and SALOMONS W. 1993. Our fathers' toxic sins, New Scientisf, 1903, 38-

42.

THOMPSON M. and HOWARTH R.J. 1973. The rapid estimation and control of precision

by duplicate determinations. Analyst, 98, 153-60.

THORNTON I . (ed.). 1984. Applied I~nvimnmen~ul Geochemistty. Academic Press, London.

48

APPENDIX 1 : Tabulated analytical data

APPENDIX 1: Page I Overbank liorizons

Number

OB-00 I A

OB-00 I 13

OB-00 1 c OB-00 1 D OROO2A

OMO2R

OB-002C

OB-003A

OB-003B

OB-003C

OR004A

OB-004B

CM3-004C

OB-O05A

OB-0O5H

OB-006A

OB-006R

OB-006C

0 8 0 0 6 D

OB-007A

OR-00713

OI3-007C

013-008A

Ol3-00X13

OI3-(JO9A

013-00913

()13-009C

013-OIOA

0 1 3 - 0 1 0 1 3

( ) I 3-0 I oc ( ) I 3 4 101)

0 1 3 - 0 l O l r

OIHJI I / \

0 1 3 - 0 1 1 1 3

( ) I 1 4 I2:\

013-I)l213

( ) I 1 - 0 13. \

013-01713

0 1 3 - 0 1 4/\

( ) I 3 4 1413

013-0I5i\

O I ~ - O l 5 l 3

0 1 3 - 0 I6,\

( ) I 3 - 0 I61 3

Ol3-0l6C

0 1 3 - 0 161)

ol3-olol~: ( ) I I - ( J I 7 . \

Fe

64543

5 1478

23233

22468

48356

41301

31 196

37270

35857

3 1745

42894

42381

37387

34634

39403

51784

37423

33518

37432

36253

38233

45386

53755

47222

37x19

3 3 5 0 0

74697

7 0 5 3 1

3421x

42570

36406

347x0

47204

3201 5

1470h

17')IX

3x422

74427

52 1 o x 70570

54OIO

5060T

760 X 2

72876

720x4

721x2

74 142

X002')

Mn 496

44 I

318

248

736

527

3 79

318

51 1

736

674

682

682

643

666

829

728

XI3

674

658

612

6x2

I046

604

449

44 I 472

X O S

627

720

635

6 5 X

674

0 5 x

O S X

705

774

7 76

922

7.1 1

0 XO

77.1

1 7 10

1 2 1 1

I 309

1038

9 0 x 99 I

As

I I

1 5

9

8

9

9

7

252

300

58

62

45

53

89

49

10

60

87

60

69

33

100

X

9

I4

17

20

6 X

17

.. $ 5

4 3

5 0

17

0 7

57

i l 1

07

31)

I 'I

2 ' )

I 3

I I

<)

I \

I I >

I (>

I:

I \

Cd

n.d.

n.d.

I1 d

I 1 . d

n.d.

n.d

n.d.

n.d.

n.d.

n.d

a d .

n.d

I1 d.

1l.d.

I1.d

n.d.

11.d.

n.d.

n.d.

I1 d

II d

I1 d.

1l.d

II d

I1 tl

11 d

II d

I1 d

I1 d

I1 d

II d

11 d

II d

II J

11 ti

I1 ( I I I d

II <I

II (I

I1 d

I1 <I

I1 d

11 (I

I 1 d

I I 11

II d

II '1

I I <i

CU

i n 17

21

12

I5

12

22

33

30

26

22

20

29

20

26

i n

21

25

26

37

25

Jx

16

16

12

'1

12

26

25

29

3 1

70

3 1

4 2

2 5

2 3

I X

1 5

22

I X

22

20

42

( 1

17

40

(> 7

3'1

Pb

14

14

26

27

16

19

35

76

58

34

28

29

37

33

37

16

40

49

37

53

31

45

18

25

2 8

27

26

56

24

41

45

41

ZX

5 0

1.4

7 0

3 0

21

24

I 5

2 0

1 5

I I

I 0

1 0

I2

I ')

I >

Sn 6

5

4

6

<2

2

9

5

4

3

I I

6

5

4

8

7

3

5

4

5 x 6

4

4

8

2 0

15 $

X

2

5

2

3

i 1

$

7

i

') I0

3

4

2

2

2

3

i

i

>

zn MgO 99 4.03

74 3.10

64 0.85

63 0.58

73 2.91

64 2.12

55 1.32

167 1.79

I24 1.86

92 2.59

82 2.58

87 3.12

89 2.76

82 2.87

89 2.73

76 3.39

100 2.42

102 2.28

101 2.88

101 2.51

97 3.15

120 2.81

81 3.42

70 2.35

60 2 I5

61 I 8 3

65 I .93

I 2 5 2.22

7 2 2.56

101 3.20

99 2 70

I03 2.6 I

70 2 97

1 2 2 2 43

74 2.56

0 7 2 75

9 2 .3 117

XJ 2.02

102 3 7 2

I 2 4 2 66

XO 3 77

X O 3 71

"1 5 5 5

') I 5 s v 90 5.76

x7 5 01

I00 5 7x

') 2 5 70

APPENDIX 1 Page 2

O\c.rbailk horizons

CaO

I 78

1 5 5

0 87

o no I 72

I 46

1 2 3

0 74

3 29

6 30

I no 3 05

5 29

7 90

4 18

2 17

4 42

6 1 1

4 71

5 no 4 42

4 81

I 7 3

1 4 4

I 0 5

0 9x

I 0 7

5 47

7 x6

1 00

J 5 2

5 23

1 90

5 X J

1 'P) J I0

5 75

5 5 1

1 7')

6 73

I X2

2 10

364

3 3 0

\ Of,

3 4 0

2 ')X

1 X2

Na20

1.12

I 3 1

I XJ

I 0 5

1.42

1.37

I .46

0.85

0.87

1.34

I 2 3

1.49

I 3 3

I 38

I 49

1.32

1.27

1 1 3

1.38

I 2 5

I 5 1

I 35

I 2 4

0 69

1 40

I S(J

I 0 2

0 0 . 3

1.46

1 4 5

1 . 4 '

I 2 7

I 9 7

I I I

1 3 ' )

I .4J

I 5 3

1 6 2

1 X ?

I 00

I .I4

I JI

1 0 2

I < ) . J

2 '11

2.41

2 I 5

2 I X

h 2 0

2 67

2 no 3 no 4 01

2 68

2 no 3 10

2 20

2 07

2 5s

2 45

2 40

2 ox 2 26

2 17

2 61

2 42

2 I9

2 19

2 II

2 42

2 04

2 5 1

2 32

142

3 02

3 61

2 07

2 17

2 41

2 1s

2 0 5

2 x 3

I XI)

2 2 0

2 2 4

2 70

? 7')

2 X 6

I 90

2 0 5

2 71

I 52

I ' 0

I 4 5

I 44

I $ 4

I 4 0

I'

339

2 79

2 79

502

232

180

202

202

249

279

296

3 i x

339

262

309

326

275

2 79

305

305

339

339

2 1 5

245

2 12

240

245

2 1 5

270

33'9

.I.$()

. 3 0 9

2")

2 5 1

2'12

322

3 i 2

2 9 0

3 J X

2 5 1

2 70

1.4x

472

40 3

.10 1

3 0 ' )

112

.4 I 2

('0

0

X

5

I)

I I

7

5

1

5 7

6

7

I 0

5

X

I 5

6

s 1

1

9

7

13

I3

7

It

7

0

X

X

J

X

I '

7

7

x I I

I 5

4

I 3

I I

2 5

2 0

2.4

2 1

I (

2 ')

Cr

75

64

2 1

25

54

53

42

44

47

5x

82

65

61

5 2

5 2

60

50

52

53

46

51

63

GO

J 0

40

JI

4 7

44

52

60

.. 5 5

53

5 0

40

.1 1

5 3

71,

0 4

70

52

71

0 X

1.4')

I 4 2

I X I

1 5 3

145

1 x 1

1,'

i n n 1053

546

595

76 I

517

917

946

702

985

556

663

x49

732

76 I

683

839

517

1034

819

x39

I765

61.1

l25X

x29

I024

91 J

6.14

X I 0

w s 120')

I500

x 5 x

1 0 3 1

XJ')

1471

' ) 5 0

X . 3 0

7'91)

O h 3

007

570

940

I002

'150

X ' ) l

I 0 0 5

12'11

NI

16

14

9

9

I 6

13

7

1 1

9

10

I2

13

n 10

X

I5

9

13

10

1 1

9

13

20

I 0

I0

I It

0

X

I 2

I2

I 1

X

I I

I 2

5

I0

I 4

I ?

1 1

I 3

I S

I 4

47

4 X

.I 2

4 I

.4 7

47

1 . 0 1

4.5

4.3

2.3

2.3

4.5

4. I

3.5

1.7

1.7

1.9

I .6

2.4

2.0

I .9

2 2

5 7

I .9

1 6

2. I

1.9

I .9

2 1

5 1

6 4

3 7

1 7

3 0

J.0

I 9

3 X

1 ' )

I X

1 5

1 ')

2 I)

2 1

: I

2 .1

31

2 .1

J I)

4 7

5 0

s s 4 3

.4 3

5 0

J 0

5 2

APPENDIX I . Page 3

Overbank horizons

Number

OR0 17B

08-0 17C

0B-o 18A

OB4188

OK019A

OB419B

OB42OA

OB4208

OB420C

OB42 1 A

O B 4 2 1 B

OB-021c

OB42 1 D 0B-o22A

0 B 4 2 2 B

OR022C

0 B 4 2 2 D

OB-022E

0B423A

0 B 4 2 3 B

OB-023C

OB-024A

OR-02413

013-024C

OI3425A

013-02 5 I3

Ol3-025C

013-02 5 I>

OI3-026A

013-026 I 3

013-026C

( ) I 3 - 0 2 6 I )

013-0261:

013-027A

011-02711

013-027C

013-0271>

013-0271.

() t3-02 X A

( )I 3-02 x I3

( ) I 1-02xc'

()13-O2XI>

Ol1-02XI

()l3-020/\

013-02913

013-029c

011-0291)

( ) I 3-070/\

Fe 79462

81 I36

836 I o 70527

71832

70392

72552

70293

61061

39637

38008

41265

7942 1

43281

45764

45045

39385

61295

37747

39673

70054

40527

40599

45 I 89

44 1 72

45440

45269

47xxx

4164 I

44 I72

37639

44163

55914

52855

54x97

49575

429x4

47670

57 I8X

5 I00 I

47717

46XOX

49205

I5570

16310

27327

70x44

4070')

Mn 937

1425

1084

1603

1262

I239

1541

1332

I200

759

852

720

666

960

937

774

720

968

457

527

527

5 1 1

573

5n I

72n

914

1 I77

1069

006

1061

9x4

1076

I 107

666

x29

945

945

I06 I

x9 I

1022

900

045

1022

xx 1

9 17

457

70x

U 0 I

As

14

21

13

24

24

27

23

23

20

9

7

14

n 5

3

n 6

3

5

4

9

6

X

U 16

23

26

i n

27

21

23

21

22

I 1

1 0

I 9

24

10

I4

1 5

26

2 0

2 x

2 6

27

?,

5

9

Cd

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

n d

I1 d

n d

I1 d

I1 d

n d

n d

I1 d

I1 d

I1 tl

I1 d

11 d

I1 d

I1 d

I1 d

n t i

I1 d

11 11

11 tl

I1 d

I1 d

I1 d I1 C I

I1 11

I1 11

I1 d

s 3

CU 41

106

44 202

64

70

77

73

57

16

13

17

14

16

13

16

I2

19

12

10

10

I I 14

I4

nx 102

67

66

I I9

57

5 1

63

7x

252

I x4

I 4 2

61

5 9

9 4

0 1

I57

76

57

40

4 0

I T

15

2')

Ph 14

40

1 1

254

83

90

59

66

54

22

22

25

25

21

25

21

20

24

19

20

20

i n

21

21

54

60

85

64

53

64

71

Xh

71

15')

I 5x

9')

57

66

0 4

5 0

72

5 X

ss sx 6 5

I X

I 0

23

Sn <2

5

<2

5

6

7

6

3

<2

I 1 5

10

8

9

10

5

6

18

n 8

6

X

x 6

10

10

x 3

7

7

3

1,

I0 5

0

7

f,

7

2

4

2

4

6

2

' I

5

5

5

Zn 84

1 I9

86

I97

I I4 12s

I20

I21

1 I9

83

no 87

78

n7

88

71

76

109

82

84

54

n5

n6

no 95

I I9

200

I47

119

I66

I no 209

I49

205

202

161

I 5 2

I50

1 1 5

9x

I30

132

I I 0

I70

203

6 1

60

xo

MgO 6 0 9

6 04

5 86

6 18

6 28

S 92

6 51

6 4 0

5 96

2 80

2 76

3 30

3 21

346 2 85

3 78

3 40

2 66

3 27

3 26

2 01

7 25

3 22

3 44

3 45

3 87

4 29

4 65

3 91

4 18

3 46

4 20

4 x2

4 29

4 45

3 95

7 87

4 14

4 5 5

4 29

4 5 5

4 I I 4 43

1 I!, 1 62

2 5 1

2 92

3 5 1

APPENDIS 1 Pagc 4

Overbank horimtis

CaO

3 65

3 S6

2 73

5 n i

4 9n

4 69

5 ss S 07

5 81

2 18

2 37

2 64

3 2s

190

I91

2 78

3 so 2 84

I n4

19u

2 06

I 85

1 xx 2 01

2 67

7 3 1

4 7x

4 64

3 x7

5 4 s

7 1 5

4 3 1

s 3n

2 54

1 2 1

1 10

1 9 5

1 xh

1 0 2

1 00

1 X I )

1 00

4 4 4 5 9 7

5 41

12 6 2

1 9 5

2 8 2

Na20

I no 1 6 3

1 U9

I 5 0

I 5n

I 9 0

I 4 3

I 6 5

I no 123

I 1 7

I I5

I 4 7

1 0 1

I 5 1

157

45

95

2 n

27

91

2 9

3 0

h20

I41

I59

I 4 1

I 6 4

I6X

I 79

1 6 7

I 77

I 9 3

2 56

2 44

2 46

2 5 5

2 44

2 69

2 46

2 54

2 59

2 61

2 59

2 U 0

2 67

2 69

2 47

2 00

2 5 5

2 23

2 I X

2 77

2 23

2 10

2 14

I x7

2 1)s

I ' )O

2 2 1

2 51)

2 I0

2 27

2 21

2 17

2 5 0

2 ( 4

2 1 R

2 1s

1 7 8

2 70

2 2')

I'

472

644

305

7X I

622

622

592

532

463

2n7

352

275

272

215

I U5

257

249

202

94

107

I us 90

I 1 2

2 12

3 I X

320

3 9 5

4 I 0

109

122

11x

472

442

1 5 2

3 9 ' )

100

425

.I C)

413

Jx')

5 9 2

620

622

2x7

I 1 . r

2x7

1 h 2

16')

Cl>

24

2 x

I 9

2 5

24

22

2x

25

I9

6

X

6

6

I I

I0

9

6

1x

0

X

3

7

0

h

I I

X

1 2

I0

I 0

X

'1

I I

I 1

1 5

I7

I 4

I0

'1

I4

I 3

I 4

I 4

I I

x S

I1

I0

I1

Cr

I96

I O U

I S4

I 5 1

I tx 1 ns I67

160

I 33

33

42

45

37

30

<x 6 3

41

U3

35

36

7')

27

30

.1 4

74

xo 9.1

xx X1

7x

72

x 2

I S X

X Y

101

X I

7 2

ss

X 5

70

xo 12

x4

i1.I

VI

I 5

-I 1

7x

I: ion3

995

11s1

1131

I063

888

100s

1053

I102

1492

100s

I512

1414

1609

12sx

1 3 x 5

I 580

I590

1521

IS21

9x5

1473

I so2

ID46

790

90 5

XOO

no0

633

0 x 5

7q I

907

90 5

X S X

Oh0

1 0 5 1

'107

120')

9 9 5

722

XOX

122')

I I 0 1

7 1 I

82')

9 16

XJO

I 0 0 1

N I

49

5 2

5 s

44

4s

42

5 1

47

40

I 1 14

13

14

IS 12

19

14

I3

I I

10

10

I 2

12

12

I 0

20

21

2x

22

23

I X

24

2 0

2x

2 0

2 0

I 9

24

2x

2 6

27

24

2x

I 0

I X

x I >

17

1,Ol

4.6

5 . I

S.O

3.9

3.8

5.6

4.9

5.2

4.0

6.2

7.0

6.4

5.4

7.1

4.9

S. I

4.9

4.2

6.0

6.9

4.5

0.3

5.3

x.2

3.x

4 , s

4.2

3.x

3.X

3 X

3 1

4 5

3 0

s 2

4 9

5 X

s 0

5 1

5 6

0 x 4 1

h ') 7 0

.I J

.I x I 0 ')

7

i) ?,

54

APPENDIX I Page 5

Owrbank 11or17ons

1.S

46340

43272

35696

40680

46943

50254

53143

48086

52099

401 14

4085 1

47744

46043

46844

3x1 16

4x878

40572

38962

3x683

33086

33392

3ox I o 3x035

IS 1 5 0

4903 I

33413

519x2

306x4

3'160 I

40572

3 1 'I07

42 I 0 2

4.1550

5 0 I04

1JOOJ

JJJOO

I X l I 2

30514

35210

3 1742

2'1235

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APPENDIS I : Page 7

Overbank horizons

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57

APPENDIX 1 ' Page 8

Overbank horizons CaO

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N'PINDIS I Pagc 9

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OB-11x3 OB-lI7A

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OB- 1 1 RA

OB-I im OB-I 19A

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0.67 0.95

0.82

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1.57

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0.89

1.61

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1.94

1.61

1.81

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1.27

1 3 9

APPENDIX 1 : Page 16

Overbank horizons

Number OB-I 14A

OB-11413

OB- I 14C

OB- I 141)

OB-IISA

O B l l S A

OB- I 17A

OB-ll7B

OB- I 17C

OB- 1 1 8A

OB-I 18B OB1 19A

OB-120A

OB- I 20r3

OB-1 2 I A

OB1 2 1 D OB-122A

OB-122B

OB-1 23A

OB12313

CaO

I 78

I no 162

I 90

2 60

1 81

3 96

3 53

4 I2

3 sx 3 64

2 17

3 91

4 52

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7 87

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21

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20

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8 7

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6 8

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53

12 x x 4

7 1

APPENDIX 1 I’agc 17 Strsani sedinicnts

Number

I ss zss 3ss 5ss 6SS XSS

loss 16SS I9SS 35ss 36SS 4 5 s 47ss soss 54ss 62SS 64SS 70SS 72SS 78SS 82SS

90ss 9 7 s 103x3

I loss 116SS

I24SS

Fe 44325

26 I04 30036 32294 33689 37792 36784 92608 95227 44x47 36793 46574

47645 77033 47798 100577 8295 1

74978 72040 66025 60989 59940

64347 63088 600 10

61x29

58052

hlii

689

798 790 743 689 852

798 I464 21 14 635 945 999 844 1410 743 7745 7202

2 1530 19594 2966 I I626 9836 20988 3872 4105 1162

20 14

,L\ 2 0

27 23

20 I8

25

I9 I5

21

I I

I I

3 9 6

2

1803 1224 50 I x3.1

5 5 2

23

2 1 x

112

1

I 0

0

I4

Cd

1i.d

n d

II d

n.d.

n.d

n d

1l.d

11.d.

n.d

11.d

I1.d.

n.d.

I1.d

I1.d

n.d

29 1 5

17 i n

19 I

2 0

39

I

I

I

1

CU

14 13

I2 14 12

I8

I8

48 55 2 1

26 16

14 18

19 345 193

216 I17 I24 58

x7 1 0 I

17 I6

2.3

22

1’1)

20 13 18

19 16 21

22

17 22 29 41

29 32 29 26

572 513

614

42 I 414 473 233

242 15

17 I 3

2 0

SI1

6

<2 3 <2

7 5 7 c.2

3 I 0 8

74

52 I34

21

243 141 5x

22 I 87 21 6 5 0 J

I 3

2

1

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94

66 70 75 74 I16

88 82 106 62 70 9G

95 I55 79

5581

2660

2955 30x5 3010 699

3 1 5 X

5252 I07 I04

9 6

91

MSO 3.29 2.70 2.71 2.97 3.09

3.58 2.92 9.73 9.48 2.0 I 2.59 1.30 I .43 1 .05 2.48 1.42 1.61 1.17 1.51

1.35 1.64 1.39 I .40 I .06 0.95

1 . 1 5

1.01

6 7

APPENDIX’ I Pagc 18

Stream sediments

Number

1 ss 2 s s

3ss 5 s s

6 S S

8SS

l o s s

16SS

1 9 s

35ss

36SS

45ss

47ss

50ss

54ss

62SS

64SS

70SS

72SS

78SS

n2ss

90ss

9lSS 103ss

1 1 oss I l6SS

124SS

CaO

4 21

x 16

7 16

7 30

6 26

8 67

7 34

1 4 1

6 47

3 67

5 5 5

I09

I 82

131

4 07

1 0 7

I 14

0 97

1 1 8

I19

1 6 5

163

1 1 7

2 61

2 3 0

2 24

1 U6

N a 2 0

1 46

I 06

I 16

I 2 1

I 2x

I05

I 2 7

I no I 9 7

I 8 0

1 4 8

I 6 4

I 4 4

1 1 1

I 5 5 0 66

0 66

0 50

0 75

0 60

0 72

0 79

0 xo 0 x 1

0 X I

0 x3

0 x4

K20

2.61

2 . 1 ~ 2 19

2.16

2 76

2.90

2.69

0.70

0.87

2.37

2.15

3.45

3 21

3.13

2.13

1.69

I .62

I 49

1.73

1.57

I .37

I .40

I 3 2 0 x 9

0 xx 0 xx 0 . 0 5

I’

416

296

3 i x

348

330

48 I

3 99

699

927

236

442

309

360

300

905

698

69x

960

61 I 698

7x6

1x6

916

I004

‘ M J

1004

060

CO

14

9

9

U

U

14

9

33

3x

I3

I 2

I I

12

2 )

17

2 5

29

29

33

27

2 1

21

2 0

1 1

27

26

24

Cr

60

5 5

56

68

55

61

61

302

263

I43

86

42

44

I09

13

91

89

13

97

101

I 1 2

I03

I30

I z x I Z I

I54

116

F 805

409

943

73 I

510

667

722

970

556

713

924

84 1

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81’3

514

200

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441

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200

200

277

206

N I

1 0

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13

17

1 1

75

87

13

I5

1 1

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13

13

31

33

27

34

35 41

39

4s

43

17

4X

44

1/01

11 d

11 d

11 d

nd

nd

nd

n d

n d

nd

nd

n d

nd

n d

I1 d

I1 d

nd

n d

12 3

7 9

nd

I1 d

I1 d

I 1 d

I 1 d

I1 d

I1 d

I1 d

6 8

AI’1’ENDI.Y I I’agc I9

Replicate analyses

I..c

5 1343

53449

468 I 7

323 I 2

3798 I

55167

8 1865

69753

44424

2783 I

39763

29865

I5720

06x92

42777

46403

35606

480 I4

32 123

38206

52072

38836

31610

35363

333x3

30x37

.392xo

150x4

3 I JJX

14.lO.l

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12312

160x2

1 2 7 0 6

20100

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I 6 0 5 x 152 1‘)

39547

3714.4

hfn

860

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65 I 666

666

720

1402

I557

728

581

519

503

743

945

937

906

914

767

1053

1123

914

953

697

643

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5 0 5

5 5 0

960

551)

9 5 1

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527

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71

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22

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11 d

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11 d

I1 d

11 d

n d

n d

11 d

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I1 d

I1 d

I1 (I

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I I d

I1 d

I1 d

II l l

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I I d

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11 ,I

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I I <I

I I cI

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1 1 il

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63

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160

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52

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244

16

42

19

18

20

70

5 8

57

67

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93

80

53 69 37

27

26

24

21

21

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2 7

2 1

2 1

2 3

2 5

2 1

I X

2 0

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2 0

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8

3

7

6

6

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1 I1

77

83

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1 I6

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3.52

3.50

3.02

2.47

2.43

3.83

5.86

6.12

3.66

2.83

3.22

1.88

1.82

3.26

3.60

3.88

3.20

3.83

3.40

3.88

4.09

3.82

2 71

2.13

1.46

I .26

2.08

1.79

1.41

1.59

1.98

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101

1.81

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WPENDIS I . Page 20

CaO

2 I5 1 7 5

2 02 6 06

7 x0

1 80

3 62

5 80

2 70

2X 36

2 00

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7 47

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3 81

4 78

3 70

4 66

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7 90

5 x 5

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178

2 x s

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I 32

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1 0 4

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2 61

2 57

2 x3

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2 14

2 65

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2 51

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2 60

2 x7

7 61

2 26

2 49

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1 4 7

2 ( I

2 x s 1 1')

2 xx 3 00

1 1 5

2 70

1 1 x

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2 9 5

2 h6

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I' 352

232

305

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554

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240

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287

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595

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722

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759

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14

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26

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1.01

n.d. 5.4

3.6

n.d.

2. I

4.2

4.8

3.8

4.7

10.0 6.4

4.4

1.9

3.0

5.0

6.8

3.9

5.4

3.x

4.2

6. I

4.6

3 . X

5 7 5 1

4.6

7.9

6.X

4 7

3.1

2.x

2.5

5.0

Z X

2 4

4 7

2 (1

3 4

(1.5

1l.d

70

APPENDIX I : Pagr. 2 1

Standards

Nuinber Fc

OSP-I 27137

MA-N 6015

NIM G 14157

I'CC- I 69802

ST-0 2658

ST-6 33852

ST-I5 8953

ST- I6 23291

ST-17 73929

ST- I8 79944

ST-19 49999

ST-20 37189

Mn 310

3x7

1 5 5

1394

' 10

310

77

387

387

620

542

SI0

Metalliferous Iiori/ons and t:ulmgs

OB459M 59171 24085

OB-06 I M 5966 1 8054

O B - O ~ ~ M 43869 X ~ O S S

OB-068M 41685 51346

OR-085M 118132 7899

ISS(2) 31331 1239

A\

I 13

I4

I 6

257

7

3

343

656

320

213

315

3x9

2 70

2x4

5063

26

CU

31

I42

IS I I

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I I1

X

5

245

689

216

72

Pb

53

32

39

9 5

23 <: 2

28

18

22

I I

I I

SI1

7

100%

3

I

%- 2

246

6

7

5x7

558

I94

85

%I1

I02

23 I 48

43

,'3

46

<3

101

87

266

I02

70

MgO 0.94

0.07

0.04

43.22

0.03

0.86

0.26

0.42

1.47

3.94

2.23

1.14

52 2x0 94 95 I 1.50 I04 527 48 3007 I .48 61 3% 84 1812 I .04 72 319 39 2663 1.14

94 I I xxz 4x6 23697 1.19 21 2 5 6 I04 3.67

71

Standards

Nun1bc.r

GSP- I

MA-N

NIM <i PCC- I

ST-0 ST-6

ST-I 5

ST- 16

CaO N.120

I 9 7 2 x4

0 62 5 75

0 7x 3 41

0 5x ‘ 0 01

0 12 .001

0 12 0 04

0 I ? 0 29

0 ? 5 5 3x

ST-17 0 19 0 17

ST-IR 0 71 001

ST-19 0 12 0 02

ST-20 0 25 0 32

Metalliferous hori7ons and ta111np

OB459M I 1 8 0 92

OB06 I M 0 93 0 56

OR-063M 0 XI 0 47

OfM68M 0 97 0 U2

OB-085M 1 2 5 0 87

ISS(2) 9 72 0 73

h2()

5 43

3 21

4 95

. 0 0 1

CO 01

2 40

3 95

4 16

4 69

6 03

3 71

2 44

I’

I222

576 I

87

U?

22

655

U7

349

436

17x9

524

262

c 0

(1

2

4

110

1

7

2

3

21

56

16

6

Cr

I1 d

11 d

II d

I1 d

1l.d.

n.d.

n.d.

11 d

1l.d.

n.d

n d

f1.d.

I .66 480 I5 I I X

1.52 742 20 76

1.60 567 I 1 I25

I .7x 349 24 7 0

I .75 436 2 x X I

3 62 ,330 I0 0 2

I N I

3510 I0

I6946 3

440 1 9

‘ 200 2859

I200 4

2768 21

1314 3

2460 14

453 I 60

7097 94

5282 28

268 I 21

I,OI

I1 d

n d

I1 d

11 d

n d

11 d

n d

n d 11 d n d

n d

n d

32 6 3

33 12 3

21 7.6

26 6 0

3 0 I1 d.

23 1l.d

72

APPENDIX 2: Overbank profiles: variations in chemistry with depth

73

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