humex (humic lake acidification experiment): chemistry, hydrology, and meteorology

Pergamon Environment International, Vol. 20, No. 3, pp. 267-276, 1994

Copyright ©I 994 Elscvior Scicncc Ltd Pdntcd in the USA. All rights rmezved

0160-4120t94 $6.00 +.00

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 relationship 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.

INTRODUCTION

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 technical 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 treatment period are presented.

LAKE/CATCHMENT AND TECHNICAL SET-UP

Lake~catchment

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

267

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 (control), 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: -10.3°C in 1991; -14.4°C in 1992; and -19.3°C 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.

10000 I

Area (m 2)

5000 1000

Volume (m~

5000

Lake half A S~ewatjern

10000 i

Area (m =) Volume (m 3)

20000

Lake half B Skjervatjern

Fig. 2. Surface/depth and volume/depth relationships.

HUMEX: chemistry, hydrology, meteorology 269

3O

25

2 2O

'~ 15

o/)

solar radiation( M Joules/m 2

precipitation (decimetre ~ m

" ' . \ / /

- /

temp.

m

0

~ ~ < ~ ~ 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.

4

3

& 2

5 :

i

l

/

/ '

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 , " r m ! I 4 ~ ~ 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.

Sprinkling device

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 fol lowing pattern: 1) 2 mm clean precipi tat ion, 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).

Water sampling

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 1

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

Dosing pump

Recorder

pH-meter

_ll ()Pump

JL From Lake Asvatn (pipe insulated and heated during winter)

Fig. 6. Pumping, dosing, and distribution system.


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 differences in chemical composition. The lower two lines in Table lb show the difference in the mean concentration 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 experimental half (A) were 3 m2/m3; the control half (B) ratios were 1 m2/m 3. The corresponding differences 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 <50% near the bottom. Apparently, there was no vertical gradient in the chemical composition (SO 4 and TOC).

pH

The mean pH decrease in Basin A, after 30 months of application of acid, was 0.06 units. The corresponding mean difference in B was 0.00. (Table lb). On a weekly basis, the lowest measured pH in Basin A during the acidification period was 4.33 (22 January 1993) and the highest was 4.97 (11 March 1991). The corresponding pH values in the B Basin on these dates were pH 4.48 and pH 4.71, respectively.

The monthly mean H + difference between A and B (Fig. 8 curve), indicates that the division of the lake decreased the H + concentration in Basin A relative to Basin B by about the same amount as the increase in difference after the acidification (in the range of 5 geq/L). Some of the decrease may be ascribed to acid added directly to Basin A. The bars in Fig. 8 illustrate the amount of H + added directly to the lake,

divided by the volume of the water in the Basin A. The results suggest that the observed pH decrease in Basin A may have been a result of the acid applied directly to the lake, and also that most of this was neutralized.

Sulphate

The dividing of Lake Skjervatjern generally resulted in a slight decrease in SO 4 in Basin A compared to Basin B (5% lower in A, Table lb). Figure 9a shows that the sulphate levels had already increased only a month after starting the application of acid.

It is important to estimate the effect of the acid that was applied directly to the lake relative to the total increase of SO 4 in Basin A compared to the control basin. The bars shown in Fig. 9 illustrate the amount of the sulphate added during each application event, divided by the volume (upper 2 m) of the experimental basin. It appears from the mean difference in the application period and the mean concentration dose (see the two horizontal lines in Fig. 9a), that the observed SO 4 difference between A and B was not only due to the SO 4 applied directly to the lake. The results , shown in more detail in Fig 9b, support the statement that some of the SO 4 applied to the catchment, leaches into the lake (Fig. 9b). The results suggest an immediate response of the applied acid (see arrow 1, sample taken 1 day after application), but also a long-term response in which the terrestrial applied SO 4 probably also contributes to the SO 4 concentration increase in the lake (see arrow 2, Fig. 9b), at least during winter.

TOC, co/our, and UV-abs

Humic substances (TOC, colour, and UV-abs.), increased in both basins, when the post- and pretreat- ment periods are compared. The increase was, however, higher in the control than in the treated basin (Table 1). These numbers suggest a reduction of organic matter in A. As the division of the lake obvious- ly affected the water chemistry, it is not possible, with respect to HS, to distinguish between the effect of treatment and the effect of the division.

Secchi disk transparency was measured at five occasions before the start of treatment and seven times during the treatment. The mean Secchi depths do not suggest significant changes due to the treatment (Table 2).

There were seasonal variations in TOC. From the results illustrated in Fig. 10, there was an apparent relationship between TOC and soil temperature. Minimum temperature and minimum TOC seemed to be well correlated. However, the TOC maximum

272 E.T. Oj~ssing

.r.

0 ~ ~ .

= i0

' o

' ; - o o

m

N ~ 4 o

t"-I

e 4 ~

('-1 ~

t..i ̧

t ~

NN~, d d

d d

(",1 ~

4 ~ 4

~,,N N . - - : o

<

e-

. < <

c 5 o

~ . m . r .

O ' ~ i '

' ~ t",l i

~i~.= 0 0

0 0

~ c 5

"'~ ' ~ Ct, I ' ~ ~ 1

I :~ ¢-I

t'N 0

t'-,I ~

0 t'-I O, ~

o d ~,

p ~ p . ,

¢',1 ~

~ (",1

0 ' ~ 0-,

~ / ~ . i ~,

e-

" ~

~z = -

, Z c,i ,c~ ~ oi c,i

z ~ - o ~

= ~ ~ i ~

. < ~ ! r a m

I ~ o a ,

, i

d d l .~,


0 4 I I I I I

°C mg C/L mg SO 4/L

8 1 2 I I I I I I I I I

SO4

m

SOd

I TOC

t

B s,nA

May 5. 1993

100% 50%

% O= saturation

Basin B

Fig. 7. An example of a vertical profile of oxygen saturation, temperature, and TOC, and S04 concentration in the two basins during summer.

30

J_. 4- ;= ;>

=L

25

20

15

10

5

0

-5

-10

-15

start applying acid

17

1989 ~ 1990 ~ 1991 o

r..-

1992

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

1.4

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 ~

0.80 b)

0.60

.< 0"40 I 06-Oct / ~ / ~pt'~'" B I 0.20 n . n \

0.00

-0.20

-0.40

-0.60

Fig. 9. a) Monthly mean d i f fe rence 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 temperature 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 = experimental 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 N

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 (corresponding to approximately 3 mg N/L) and 2.5 g N/m 2 to the terrestrial area. There was apparently an immediate 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 increase, due to inorganic N added directly to the lake,


~0

16

14

12

10

8

6

4

2

0

Soil t e m p . ~ 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

Organic N was calculated as the difference between total N and sum of NO 3 and NH 4. There was apparently an immediate utilisation of the added inorganic N by the organisms (Figs. 12 and l la) . 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.

350

300 a) I ]/Jg NIL

250 = A-B 17

200 [[

Z~ 150 Mean theoretical conc. based r7 ~ 1

Mean "conc" diff.[A-B] ~ '

-50

period October 1990 - February 1 9 ~ 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

:3.

100

80

60

40

20

0

T b)

w |

m

i i

|

• i

i ,

i

i 1

w i

i i

w i

x

• 1

• i

i

L i

1 i

i

i

ct, Nov. ' ," -20 ' ' ,~

L I

?4 -40

-60

Org. N

,,,'" "',,, ~ ~ rlorg. N

"x

Dec. Jan.

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.

¢xo --i

350

300

250

200

150

100

50

0

-50

Mean conc "dose"

Mean "cone" diff (A-B)

organic N

.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).

A c k n o w l e d g m e n t - - 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.

R E F E R E N C E

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 acidification. HUMOR/HUMEX Newsletter 1/1993; HIVA Report, Nor- wegian Institute for Water Research, Oslo.

humex (humic lake acidification experiment): chemistry, hydrology, and meteorology

Documents