Humex (Humic lake acidification experiment): Chemistry, hydrology, and meteorology

Environment International,Vol.20, No. 3, pp. 267-276, 1994 Copyright©I 994 Elscvior Scicncc Ltd Pdntcd in the USA.All rightsrmezved

Pergamon

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 biological 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 biological 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 instrumentation, 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).

5000

Area (m=)

Volume (m~

Area (m2) 10000

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 technique. 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.

1000

5000

10000

Volume (m3)

20000

i

I

Lake half A S~ewatjern

Lake half B Skjervatjern

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

HUMEX: chemistry, hydrology, meteorology

269

3O solar radiation( M Joules/m 2 25

2

2O

'~

15

precipitation (decimetre

~

"'.\

// -

o/)

temp.

m

m

/

0 ~

~

<

~

~

Precip (dm)

;

•

u.

<

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 st m 4 integrated over 24 h. Units: M Joules m "a.

/ /'

5 :

Water flow B.

/ .....~ k

/"

/,l--m ~ i ~

i~ ~

4 i

l

//

3

&

j

,'-.

i

2 s

.... ,. . . . a--i

,"rm!I4~ /

- • -a--m~i

~

PreclP itati'

(

)

Water flow A

•

4 I-i /' i ~1i ~

r ~

/

:

li

'

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 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).

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

.....

Borderbetween sub areas Sprink,ers Mainpipe(Dist__~ributionradius 15m)

.

~

~

~

1

Area 1

Area

)"

Distributiondevice (automatic)

Area 3

"Water from main pump (acidified)

Fig. 5. Distribution of sprinklers in the five terrestrial subareas.

To distributionsystem

Dosingpump

_ll Recorder

()Pump

Stocksolution 10% H2SO4 and 10,3%NH4NO3

pH-meter

JL

From LakeAsvatn (pipe insulatedand heatedduringwinter) Fig. 6. Pumping, dosing, and distribution system.

HUMEX: chemistry, hydrology, meteorology

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,

271

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

the Basin A. pH decrease of the acid that most of

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 pretreatment 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 obviously 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

t~

c5o

.r.

~.m.

r.

"'~ I' ~

'~ ~1

Ct,

" ~ t"-I I:~

e4~

¢-I

~z

('-1 0

=

~

~

t'N

0

t'-,I

~

0

t'-I

=

-

~.

i0

O'~i

t..i ¸

'~

I~oa,

'

t",l i

~i~.=

0

O,~

0

od p~

~,

'o

';-

oo

,

,Z

c,i,c~ ~

oi c,i

NN~, dd

0

0 ¢',1 ~

d d l .~,

d d ~ 0

(",1

(",1 '~ 0-,

z ~

-o ~

=

~i~

~

4~4

~/~.i

~,

~,,N .--:o N ~c5

m

N

~

i

p.,

4o

e-

<

~

e-

.<<

.<~

!ram

HUMEX: chemistry, hydrology, meteorology

273

°C mg C/L mg SO4/L 0

4

I

I

I

I

I

I

I

8 I

I

I

I

12 I

I

I

SO4

B s,nA

May 5. 1993 m

SOd

I TOC Basin B t

100%

50% % 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.

30 25

17

20

start applying acid

15

J_. 4-

;=

10

;>

5 =L

0

r..-

-5 -10

1989

~

1990

~

1991

o

1992

-15 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

O o~

0.6 Mean"conc"diff.in applicationperiod Mean"conc."dose

~T

.

0.2 0.4

l ~ , ,~,1

o

....

. . . . . . . .

-0.2 ~

.......

~

.

r

.

~

.

.

.

.

.

I l t I l ~ll[ n ........

,

~ "~'~~o~

-0.4

l l t ~ l II

,

,

,,

,

~ ~ ~o o~

1990 - F e ~

b)

0.80

0.60 .< 0"40I 06-Oct 0.20

\

/ n

B~

I .

n

/

~"p't

0.00 -0.20 -0.40 -0.60 Fig. 9. a) Monthly mean d i f f e r e n c e 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 autumnwas3to4 monthsafterthe summertempera- Inorganic N ture maximumand the late spring TOC maximum Inorganic nitrogen was added together with the was 3 to 4 monthsafterthe temperatureminimum. sulphuric acid since the beginning of October 1990. During these first 30 months, 28.3 kg, as Table 2. Secchi depth transparency in Lake Skjervatjem. A = exNH4NO3, wereappliedto the lakesurface(correspondperimental half; B = control half. ing to approximately3 mg N/L) and 2.5 g N/m2 to the terrestrialarea.Therewas apparentlyan immediA B number obs. ate responseto the application(Figs. 1la andFig 1lb). Pre-acidification 1.85 1.98 5 Comparing the concentration increase in Basin A Post-acidification 2.22 2.26 7 relative to B with the calculatedconcentrationinto April 1993 crease,due to inorganicN addeddirectlyto the lake,

HUMEX: chemistry, hydrology, meteorology

275

16 14

Soil t e m p .

~

TOC

12 10 8 6

~0

4 2 0

--

I--

]

I

I

I

I

~

I

I

~

t"q

I ~

I

I

I

i

I

I

t"q

t"q

t'q

t"q

t'q

t"q

I ~

] ¢'q

I - ~ ~

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

FURTHER PLANS FOR THE HUMEX

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 lla). Organic 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 Gjessing (1993). Their results showed an increase in the content of N (organic) in the XAD isolates. Some of

The treatment of the experimental half will tentatively 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 250

=

]/Jg NIL A-B

17

200 Z~

150

[[ Mean theoretical conc. based

Mean "conc" diff.[A-B]

r7

~

~

1

'

-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

100

T

80 60

Org. N

b)

•

•

1

i i

i i

w

L 1

|

m

40

i

,,,'" "',,,

i

| • i

i

x

, i

20 :3.

i

1

w

i

i

i

w

i

~

~

rlorg. N

i i

0

"x ct,

-20

'

Nov.

' ,"

'

Dec.

Jan.

,~

LI

?4 -40 -60 Table 1lb. 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. 350 300 250 organic N

200

..

150 ¢xo --i

II

Mean conc "dose"

100 Mean "cone" diff (A-B)

50

.F

II

!

0 -50 -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). REFERENCE

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.

Acknowledgment

--

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, Norwegian Institute for Water Research, Oslo.