Humex (Humic lake acidification experiment): Chemistry, hydrology, and meteorology
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Pergamon Environment International, Vol. 20, No. 3, pp. 267-276, 1994
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HUMEX (HUMIC LAKE ACIDIFICATION EXPERIMENT): CHEMISTRY, HYDROLOGY, AND METEOROLOGY
Egil T. Gjessing Norwegian Institute for Water Research, 0411 Oslo, Norway
EI 9310-213 M 'Received 15 October 1993; accepted 10 February 1994)
The Humex project is an on-going whole-catchment manipulation project that studies the relation- ship between humie substances (HS) in soil and water and the acidification processes. The project is based on a dystrophic lake, which is artificially divided into experimental and control halves. This report describes the lake and its catchment and the technical installations used for the artificial acidification of the 1.8 ha terrestrial experimental area and the corresponding 0.9 ha lake. Chemical data from the experimental lake half and the control half are presented, covering a period of 24 months before and 30 months after start of treatment with H2SO4 and NH4NO3. The results show that dividing the lake into two basins resulted in only small changes in the inorganic water chemistry. The concentration of HS, however, changed due to the difference in the ratio between lake volume and catchment area. The acidification treatment resulted in a 10% increase in ionic strength, mostly due to an increase in SO4, accompanied by Ca, Mg and Na. There was also a significant increase in the concentrations of NO~, NH4, and organic N. The increase in SO4 and organic N can not be explained only by the amount of chemicals added directly to the lake.
During the last two decades, the importance of humic substances (HS) for all chemical and biologi- cal processes in soil and water has gradually been acknowledged. Acid rain research during the 1970s and 1980s gave only minor consideration to the role of HS; this may account for the discrepancy in some results and conclusions.
The aim of the HUMEX project is to study the role of HS in the acidification of surface water and the role of acid precipitation on the chemical and biologi- cal properties of HS. The on-going project is based on an artificial acidification of a dystrophic lake and its catchment. By dividing a coloured lake into two halves (by a plastic curtain), one half is used for the
acidification experiment and the other is kept as a control. This paper describes the scientific and tech- nical background, the technical installations, the in- strumentation, and the hydrochemical results from the project start in 1988. Results from the 24-month preacidification period and from the 30-month treat- ment period are presented.
LAKE/CATCHMENT AND TECHNICAL SET-UP
Lake Skjervatjern was divided in October 1988 (Fig. 1). The maximum depth of the dividing curtain is about 4 m. The lower end of the curtain is pressed down into the soft lake sediments by sandbags. This
268 E.T. Gjessing
Fig. 1. Lake Skjervatjern and its catchment.
separates the lake into two basins (Figs. 1 and 2) The difference in the catchment/lake-volume ratio gives a theoretical retention time of 1.6 months and 4.5 months for Basin A (experimental) and Basin B (con- trol), respectively (Gjessing 1992).
Temperature, precipitation, and light Air and soil temperature, precipitation volume,
and light were recorded from May 29, 1991 (Fig. 3). The lowest air temperatures were as follows: -10.3C in 1991; -14.4C in 1992; and -19.3C in 1993. The soil temperature (10 cm depth) did not fall below freezing during the recording period (May 1991 to March 1993).
Water flow The outflowing water from the two basins has been
recorded since January 17,1991. Figure 4 illustrates the water flow from A and B during one week in November 1991 and the corresponding precipitation. The water flow was measured by an acoustic techni- que. An acoustic signal was sent into the flowing water in the pipe at a given point at time t 1. The pipe was arranged so that it is always filled with water. The same signal is received at a fixed point at time t 2. Depending on the flow rate, tl-t 2 will differ; the time difference is proportional to the water flow. The lake water level has been recorded simultaneously since August 28, 1991.
Area (m 2)
Lake half A S~ewatjern
Area (m =) Volume (m 3)
Lake half B Skjervatjern
Fig. 2. Surface/depth and volume/depth relationships.
HUMEX: chemistry, hydrology, meteorology 269
solar radiation( M Joules/m 2
precipitation (decimetre ~ m
" ' . \ / /
~ ~ < ~ ~ u. < ~ ~ ~ ~ <
; Precip (dm) Sol.rad. x Mean temp. Soil temp. i !
Fig. 3. Monthly mean air and soil temperature and radiation. The photo sensor responds through the wavelength range of 400-700 nm. The radiation is monthly mean values in Joules s t m 4 integrated over 24 h. Units: M Joules m "a.
Water flow B. / ..... ~k /" / , l - -m
~ i ~ i~ ~
// / Water flow A
j , ' - . 4 I-i /' i .... , . . . . i ~1 i
s a - - i - -a - -m~i ~ :
Precl itati , " rm! I4~ ~ P ' ( ) l i ' / r ~ i 'i i
1991 Nov.4. Nov.5. Nov.6 Nov. 7. Nov. 8. Nov. 9. Nov. 10.
Fig 4. Example of water flow measurements. Precipitation and water flow from the experimental half (A) and from the control (B) during one week in November 1991.
The watering system was activated manually after setting the period of operation. The experimental catchment was divided into five subareas which are treated in sequence automatically, according to the following pattern: 1) 2 mm clean precipitation, 2) x mm acid precipitation (x = 10% of the natural precipitation the previous week); and 3) 2 mm of
clean precipitation in order to wash off residual acid from vegetation (Figs. 5 and 6).
Since the division of the lake in October 1988, water samples were collected weekly from the two outlets and from several depths in both basins (3 to 10 times a year).
270 E.T. Gjessing
Border between sub areas
..... Main pipe . ~ ~ ~ 1 Sprink,ers (Dist__~ribution radius 15m)
Area ) " Area 3
Distribution device "Water from main pump (automatic) (acidified)
Fig. 5. Distribution of sprinklers in the five terrestrial subareas.
To distribution system
Stock solution 10% H2SO 4 and 10,3% NH4NO3
JL From Lake Asvatn (pipe insulated and heated during winter)
Fig. 6. Pumping, dosing, and distribution system.
HUMEX: chemistry, hydrology, meteorology 271
RESULTS AND DISCUSSIONS
General chemical composition of Lake Skjervatjern (Humex lake)
Table la summarises the chemical nature of the lake water. It appears that the water is low in salts and with a pH as low as 4.6, due to the presence of natural organic matter. The TOC content is about 6 mg C/L. The relatively high content of sea salts, which is due to the short distance from the North Sea, should be emphasized.
Chemical consequences of dividing the lake
Dividing the lake resulted in only small differen- ces in chemical composition. The lower two lines in Table lb show the difference in the mean concentra- tion between A and B. The ionic strength decreased in Basin A compared to Basin B (4%) during the two years prior to start of treatment. Similarly, the mean content of humic substances (represented by TOC, colour, and UV-abs.), was also lower in A compared to B (10%) as a result of the division.
These changes in water chemistry may have been caused by changes in the catchment size/lake volume ratio and by differences in retention time for the water in the lake halves. The ratios for the ex- perimental half (A) were 3 m2/m3; the control half (B) ratios were 1 m2/m 3. The corresponding differen- ces in retention time were 1.6 months and 4.5 months for A and B, respectively.
Both the lake halves stratify during early spring. From Fig. 7, it can also be seen that oxygen saturation decreased to
272 E.T. Oj~ssing
0 ~ ~.
' ; - oo
N ~ 4o
~,,N N . - - :o
HUMEX: chemistry, hydrology, meteorology 273
0 4 I I I I I
C mg C/L mg SO 4/L
8 12 I I I I I I I I I
May 5. 1993
% O= saturation
Fig. 7. An example of a vertical profile of oxygen saturation, temperature, and TOC, and S04 concentration in the two basins during summer.
J_. 4- ;= ;>
start applying acid
1989 ~ 1990 ~ 1991 o
Fig. 8. Monthly mean difference in H* concentration between A and B [A-B] for period October 1988 to April 1993 (curve). Amount of acid added directly to the experimental lake half, divided by the lake volume (upper 2 m =7600 m j) [bars].
274 E.T. Gjessing
1'2 a) I ImgSO4/L ~ ~ ~
= A-B 0.8
0.6 Mean "conc" diff. in application period ~ I O o~ Mean "conc." dose ~ T
. 0.4 l~ , ,~,1 l t I l ~ll[ l l t~l II 0.2 o . . . . . . . . . . . . . . . . . . . . . . . . . . n . . . . . . . . , , , ,, ,
-0.2 ~ ~ r ~ "~ '~~o~ ~ ~ ~o o~
-0.4 1990 - F e ~
.< 0"40 I 06-Oct / ~ / ~pt'~'" B I 0.20 n . n \ 0.00
Fig. 9. a) Monthly mean difference SO, concentration between A and B [A-B] for period October 1988 to April 1993 (curve). Amount of sulphate added directly to the experimental lake half, divided by the lake volume (upper 2 m = 7600 m ~) [bars]. b) Detail
of the added SO, and the effect on lake water concentration during initial application period (Sept. 30. 1990 to Jan. 20. 1991).
in autumn was 3 to 4 months after the summer tempera- ture maximum and the late spring TOC maximum was 3 to 4 months after the temperature minimum.
Table 2. Secchi depth transparency in Lake Skjervatjem. A = ex- perimental half; B = control half.
A B number obs.
Pre-acidification 1.85 1.98 5
Post-acidification 2.22 2.26 7 to April 1993
Inorganic nitrogen was added together with the sulphuric acid since the beginning of October 1990. During these first 30 months, 28.3 kg, as NH4NO 3, were applied to the lake surface (correspond- ing to approximately 3 mg N/L) and 2.5 g N/m 2 to the terrestrial area. There was apparently an immedi- ate response to the application (Figs. 1 la and Fig 1 lb). Comparing the concentration increase in Basin A relative to B with the calculated concentration in- crease, due to inorganic N added directly to the lake,
HUMEX: chemistry, hydrology, meteorology 275
Soil temp. ~ TOC
- - I - - ] I I I I I I I I I I i I I I ] I - ~ ~ ~ t"q ~ t"q t"q t 'q t"q t 'q t"q ~ 'q ~ t"q r~ '~ t '~
Fig. 10. Monthly mean temperature in soil and monthly mean TOC in outflowing water from A and B.
indicates this increase was the residual N that was not assimilated by the water organisms.
the peaks suggest that the catchment may be involved during particular events (Fig. 1 lb).
Organic N was calculated as the difference be- tween total N and sum of NO 3 and NH 4. There was apparently an immediate utilisation of the added in- organic N by the organisms (Figs. 12 and lla). Or- ganic N from the catchment may contribute to the higher concentration of organic N in Basin A. This is in agreement with the findings of Malcolm and Gjess- ing (1993). Their results showed an increase in the content of N (organic) in the XAD isolates. Some of
FURTHER PLANS FOR THE HUMEX
The treatment of the experimental half will tenta- tively be stopped in October 1994 and some recovery studies will be continued through 1995. Thereby, the total duration of the project will be 7 y: a 2-y preacidification period; 4-y treatment period, and 15-month recovery studies. The routine collection of data related to chemistry, hydrology, and meteorology will continue through 1995.
300 a) I ]/Jg NIL
250 = A-B 17
Z~ 150 Mean theoretical conc. based r7 ~ 1
Mean "conc" diff.[A-B] ~ '
period October 1990 - February 19~ Fig. 1 la. Monthly mean difference in concentration of inorganic N between A and B [A-B] for period October 1988 to April 1993
(curve). Amount of nitrogen added directly to the experimental lake half, divided by the lake volume (upper 2 m = 7600 m' ) [bars].
276 lg.T. Gjessing
T b) w |
ct, Nov. ' ," -20 ' ' ,~
,,,'" "',,, ~ ~ rlorg. N
Table 1 lb. Detail of the added (NHd~O)-N) and the response on lake water concentration during the initial application period (Sept. 30, 1990 to Jan. 20, 1991). The response in inorganic N and organic N is illustrated.
Mean conc "dose"
Mean "cone" diff (A-B)
.F ! I I .. II
-100 : " ~ (see Fig. 9b)
Fig 12. Curve: monthly mean difference in concentration of organic N between A and B [A-B] for period October 1988 to April 1993. Bars: amount of nitrogen (NH,NO)-N) added directly to the experimental lake half, divided by the lake volume (upper 2 m = 7600 m ' ). Detail of the added SO~ and the response on lake water concentration during initial application period (Sept. 30, 1990 to Jan. 20, 1991).
Acknowledgment - - This work was sponsored by The Norwegian Institute for Water Research, by the Commission of European Communities (STEP-CT90-0112 and NV 5V-CT92-0142) and by the Norwegian Research Council (NTNF). In addition there have been considerable contributions from a number of institutions in Canada, Europe, and USA. The author would like to mention the technical assistance from Tor Holsen and Oddleiv Hjellum and all their help in collecting samples.
Gjessing, E.T. The HUMEX Project: experimental acidification of a catchment and its humic lake. Environ. Int. 18: 535-544; 1992.
Malcolm, R.L.; Gjessing, E.T. Definite changes in the nature of the natural organic solutes in Lake Skjervatjern after acidifica- tion. HUMOR/HUMEX Newsletter 1/1993; HIVA Report, Nor- wegian Institute for Water Research, Oslo.