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

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  • 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 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.

    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 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~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 (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.

    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 , " 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.

    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

    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.

  • 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

    .r.

    0 ~ ~.

    = i0

    ' o

    ' ; - oo

    m

    N ~ 4o

    t"-I

    e4~

    ('-1 ~

    t..i

    t~

    NN~, dd

    d d

    (",1 ~

    4~4

    ~,,N N . - - :o

    <

    e-

    .

  • 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

    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 ~

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