Groundwater Onalily: Natural and Enhanced Restoration of Groundwater Pollution (Proceedings o f die Groundwater Quality 2001 Conference held at Sheffield. UK, June 2001). IAHS Publ. no. 275. 2002.
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Groundwater flow into mining lakes
E. B O Z A U & G. S T R A U C H Department of Hydrogeology, UFZ Centre for Environmental Theodor-Lieser-Strafie 4, D-06120 Halle/Saale, Germany e-mail:
[email protected]
Research
Leipzig-Halle,
Abstract The drainage basin of mining lakes mainly consists of dumped sediments. These sediments vary greatly in terms of grain size, hydraulic conductivity and chemical composition. Groundwater flow assessment for this heterogeneous material is problematic, particularly at certain locations, but seepage measurements can help to explain the hydrodynamic and geochemical processes in more detail. Two closely investigated mining lakes were chosen to evaluate seepage meter measurements in order to enhance understanding of the groundwater flow into these lakes. Key words acidification; isotopic signature; mining lake; seepage; water balance
INTRODUCTION The groundwater in- and outflow of lakes can be estimated by water balance and isotopic studies as well as by groundwater modelling. Seepage meter measurements are also a helpful tool for obtaining flow data. A seepage meter consists of an open bottomed container connected to a plastic bag (Fig. 1(a)). The work of seepage meters has been tested in several studies (Lee, 1977; Bélanger et al., 1992; Knoll et al., 1999; Hirsch & Randazzo, 2000). The water inflow into the bag is equal to the water inflow for a specific area during a certain time period. If bags filled with water are used in an outflow area, the groundwater outflow can be calculated by the loss of the water in the sampling bags. The main problem when generalizing the data collected at the indivi dual sampling points for the whole lake is estimating the in- and outflow area of the lake. In- and outflow can change at an individual sampling point with time. Another problem is the time-dependent measuring data. A measuring campaign normally lasts one year. If the hydrological data of this year are out of the ordinary (e.g. high precipi tation), caution needs to be applied when drawing conclusions for longer periods. The heterogeneity of the surrounding and lake sediments can also cause uncertainties.
STUDY A R E A S Mining lakes require investigation in order to predict the further development of their water quality following acidification caused by pyrite oxidation in the surrounding sedi ments, as well as the use of old opencast mines to dump domestic and industrial waste. Since most of these mining lakes are used for recreation activities, bad water quality could impair not only the ongoing natural processes of the lake but even human health. The two mining lakes (Fig. 1 (b) and Fig. 2) are located in eastern Germany: Lake Hufeisensee near the city of Halle (central German coal fields) and Lake RL 111
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F i g . 1 (a) Seepage meter, (b) Location of the lakes.
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Hufeisensee
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111
Quaternary Aquifer
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Lignite mining
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Waste
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6 P 0 S l t
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Groundwater inflow
&
400 m
Seepages ainpling lo c ation
Fig. 2 Diagrams of the lakes investigated showing the seepage sampling locations
near the town of Lauchhammer (Lusatian mining district). Lignite was mined in both locations. The water table of the lakes is controlled by the in- and outflow of groundwater, the surface runoff from the banks of the lakes, as well as by the precipitation and evaporation rate. Lake Hufeisensee has a drain to the nearby Reidebach stream.
Acidic mining lake R L 1 1 1 The acidic mining lake R L 111 attained stable hydrological conditions 30 years ago. The surface area of the lake covers about 100 000 n r . The average pH value of the lake water is about 2.6. The lake is situated on the base of the former Plessa mining pit between the Lusatian ice marginal valley in the south and a disturbed moraine in the north. The lake is about 900 m long and consists of three basins, each of which is 120 - 1 4 0 m wide. The point of maximum depth (10.2 m) lies in the middle basin. Lignite mining took place between 1923 and 1958. The world's first overburden conveyor bridge was erected in the mine at Plessa in 1924. The use of this conveyor
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495
bridge accounts for the heterogeneous structures of the dumps on the eastern and southern lake shores. The sedimentary layers of the western shore, consisting of Quaternary sand, clay, Tertiary lignite and sand, are not disturbed by mining activities. The groundwater of the Quaternary aquifer and the dump passes through the lake from south-southwest to north-northeast.
Lake Hufeisensee Lake Hufeisensee is an important recreation facility for Halle. It developed from an old lignite mine with both opencast and underground mining. After the end of lignite mining in 1941, gravel was extracted until 1964. The middle part of the pit was used to dump industrial and domestic waste (about 3.3 x 10 t until 1984). The waste deposit was covered by an impermeable layer in 1997. The surface area of the meromictic lake is about 700 000 m . The deepest point is situated in the eastern part of the lake at a depth of 29 m. Anaerobic conditions start at a depth of about 27 m, in the monimolimnion. Water from the monimolimnion is not involved in the annual mixing of the lake water. Groundwater mainly flows from the northwestern Quaternary aquifer into the lake. Furthermore, groundwater also flows from Tertiary, Triassic and Permian sedimentary rocks as well as from the old waste deposit, especially into the deeper parts of the lake. 6
2
LAKE AND GROUNDWATER CHEMICAL AND ISOTOPIC DATA The lake water chemical data are listed in Table 1. Lake RL 111 is acidified due to the pyrite oxidation in the surrounding and washed-out dump sediments. Lake Hufeisensee is influenced by groundwater of relatively high salinity. The surrounding sediments are rich in carbonate so that pyrite oxidation products are well buffered and no acidification was discovered. The 5 0 and the S H values of the groundwater represent the isotopic signature of the precipitation. Lake water is isotopically enriched due to evaporation which is normally higher than the precipitation rate (Table 2). I 8
2
Table 1 Chemical characteristics of the lake water. RL 111
Hufeisensee
pH-value Conductivity (mS cm" )
2.6 2.5
Ca(mg 1"') Mgimgl- ) K (mg l" ) Na (mg 1"') N H ( m g l" ) S 0 ( m g I"') N0 (mgl-') CI (mgl"') Fe (mg 1"') Al (mg I" )
225 27 2.5 6.5 4.0 1350 1.5 11 160 35
7.3 2.5 340 110 11 230 0.1 1400 1.2 240 0.1 4
1
1
1
1
4
4
3
1
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Table 2 Mean isotopic characteristics of lake water and groundwater inflow into the lakes. 8 O ( % o ) SMOW
5 H (%o) SMOW
ls
2
Lake water
RL 111 Hufeisensee
-3.4 -3.0
-36 -32
Groundwater
RL 111 Hufeisensee
-9.0 -9.0
-66 -65
SEEPAGE MEASUREMENTS Acidic mining lake R L 111 Twelve seepage meters have been installed on the lake bottom to estimate the groundwater flow into the lake. The flow rates range from 3.00 -0.18 E/3.5 m 2.50 1.25
0.89 5.82 8.34
0.57 0.71 1.65 2.43 0 >7.40 1.88 >2.00 0 2.95 0.01 0
> Bag volume filled; - loss of prefilled water.
2000 29.02. 29.03. 27.04. 10.05. 24.05. 14.06. 16.06. 17.06. 0 0.12 1.07 0.18 0.20 0.38 0.13 0.35 6.25 >1.20 4.42 >3.00 0.58 >1.30 3.20 5.49 1.58 1.21 19.20 4.49 >11.5 >5.70 15.60 20.00 -0.92 0.09 -2.53 -0.03 0.71 0.18 -0.72 -1.27 0 -0.03 0 0.10 0.08 -0.01 0.33 0.21
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497
Table 4 Flow rates, estimated inflow area and calculated annual groundwater inflow. Minimum Average (1 m" day') (1 m' day )
Maximum Area (1 m" day') (m )
0.6 0
3.0 0.5
6.3 1.3
W (Quaternary Aquifer) 0.5 m -0.1 1.5 m 0 2.5 m 0.1
1.3 7.5 3.1
4.9 25 9.9
Deepest point 7.6 m 9.7 m
-0.2 -0.9
1.5 0.2
8.2 3.0
Eastern shore (dump) 1.5 m 2.5 m
0 -0.1
0.8 0.1
2.7 3.0
-0.1
2.0
7.1
2
Southern shore (dump) 2.5 m 3.0 m
Average of all measurements
2
1
2
Groundwater flow (m') Min. - Average - Max.
2
100x300 =30000 (a) (a) 3300-19200-42700 100 x 100 = 10000(b) (b) 1 1 0 0 - 6 4 0 0 - 14200 6x900 = 5400
0 - 7800 - 26000
2 x 100 = 200
(-40) - 60 - 400
500x4 = 2000
(-40) - 330 - 2000
(a) 37600 (b) 17600
(a) 3220-27930-71100 (b) 1020- 14590-42600
Table 5 Average data of the annual groundwater flow of the acidic mining lake RL 111. Method
Isotopic investigations (Knôller, 2000)
Groundwater inflow Groundwater outflow
23 700 m 15 700 m
3
Seepage measurements 21 300 m 13 100 m
3
3
3
Groundwater model - MODFLOW (Bozau & Strauch, 2002) 31 200 m 18 600 m
3
3
Table 6 Isotopic signatures of lake, seepage and groundwater ofthe acidic mining lake RL 111. S S ( % o ) , CDT 34
S O ( % o ) , SMOW
5 Ff ( % o ) , SMOW
,s
2
Seepage (Western shore)
1.3 -3.5
-9.0 -9.2
-66 -65
Seepage (Eastern shore)
3.7 3.1
-4.3 -4.1
-41 -42
8.2 7.3 -10.5
-9.4 -9.2 -8.9
-66 -65 -65
3.6
-3.4
-36
Groundwater Well 4 (East, dump) Well 5 (South, dump) Well 6 (West, shore) Lake water
1 8
2
groundwater. The 5 0 and § H values of the seepage water from the eastern shore are explained by a mixture of groundwater and lake water. The seepage water of the eastern j4
shore has nearly the same S S values as the lake water (Table 6). This supports the hypothesis of higher groundwater inflow from the western shore and closer interaction between the lake water and the groundwater of the dump aquifer on the eastern shore.
L a k e Hufeisensee Water inflow from the waste deposit occurred into the eastern part of the lake (Strauch et al., 1996). The waste deposit was covered in 1997 to reduce recharge in this area
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E. Bozau & G. Strauch
and to protect the lake. After covering, four seepage meters were installed near the adjacent bank of the waste dump (an inflow area already investigated). The four sampling points are distributed over an area of about 6 n r . The inflow rates range between 0 and 35 1 m" day" . Figure 3 shows the inflow data at the four sampling points for September-December 2000. All sampling points seem to have their own inflow characteristics, indicating the presence of distinct flow paths from the waste deposit into the lake (finger flow). 2
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Fig. 3 Inflow rates (1 m" day ') from the waste deposit to Lake Hufeisensee measured by four seepage meters (September to December 2000). 2
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The seepage water chemical and isotopic data are nearly the same as for the lake water. Sulphate reduction processes of the type found within the waste deposit and in the monimolimnion of the lake (Asmussen & Strauch, 1998) could not be investigated in the shallow lake sediments. The buffering capacity of the surrounding sediments as well as the dump sediments prevent acidification of the lake. After covering the waste deposit, the discontinued recharge in this area is compensated by lake water flowing to the waste dump, and so the isotopic signatures of the lake water are found in the seepage water at the waste deposit outflow.
CONCLUSIONS Seepage meters can be used to assess groundwater flow for complete lakes as well as to investigate special inflow areas of a lake. Estimating the in- and outflow area seems to be the main problem for the generalization of data collected at individual points. If seepage meter measurements are planned, an overview of the hydrogeological situation is necessary to establish where and how many seepage meters need to be installed. The installation of seepage meters and the sampling are quite expensive and should be done only for longer periods due to changing meteorological and hydrogeological conditions in the catchment area of the lake. Chemical analyses of seepage water have to be interpreted carefully due to the mineral content, the buffering capacity and the surrounding conditions of the lake sediments. Isotopic signatures of the collected seepage water, especially 5 0 and 8 H values, are a useful tool for explaining the origin of groundwater inflow. I 8
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Groundwater flow into mining lakes
499
A c k n o w l e d g e m e n t s W e would like to thank our colleagues from the Department of Hydrogeology, without whom it would have been impossible to obtain all the data for this study. Special thanks are due to G. Schâfer and S. Leider who assisted in the field work, as well as to the laboratory staff.
REFERENCES Asmussen, G. & Strauch, G. (1998) Sulfate reduction in a lake and the groundwater of a former lignite mining area studied by stable sulfur and carbon isotopes. Wat. Air Soil Pollitt. 108, 271 - 2 8 1 . Bélanger, T. V. & Montgomery, M. T. (1992) Seepage meter errors. L'unnol. Oceanogr. 37, 1787-1 795. Bozau, E. & Strauch, G. (2002) Hydrogeological basis for biotechnological remediation of the acidic mining lake "RL 111", Lusatia (Germany). Wat. Air Soil Polltit. (in press). Hirsch, J. D. & Randazzo, A. F. (2000) Hydraulic seepage within an astatic karstic lake, North-Central Florida. In: Groundwater: Past Achievements and Future Challenges (ed. by O. Sililo), 159-164. A. A. Balkema, Rotterdam, The Netherlands. Knoll, D., Weber, L. & Schâfer, W. (1999) Grundwasseranbindung von alten Tagebau-Restseen im Niederlausitzer Braunkohletagebaugebiet. Grundwasser 2, 55-61. Knôller, K. (2000) Anwendung stabiler Umweltisotope zur Bewertung hydrochemischer Zustiinde und Prozesse in Folgelandschaften des Braunkohlebergbaus. PhD Thesis, UniverstiUit Leipzig, Leipzig, Germany. UFZ-Berichl 3312000. Lee, D. R. (1977) A device for measuring seepage flux in lakes and estuaries. Limnol. Oceanogr. 22, 140-147. Strauch, G., Birger, J., Christoph, G., Dermietzel, .1., Gliiser, H. R., GlaBer, W., Haendel, D., llanschmann, G., Kowski, P., Krauss, G., Miihle, K., Nizsche, H. M., Richter, W. & Wieser, T. (1996) Untersuchungen an einer anaeroben Deponie zum Schadstofftransport, Stoffumsetzungen und Wechselbeziehungen zwischen Deponieinhaltsstoffen und Grund- bzw. Oberflachenwassern - Modellfall ehemaliger Braunkohletagebau Bruckdorf-Nord/Deponie HallcKanena. Bericht des UFZ - Umweltforschungszentruins Leipzig-Halle, Sektion Hydrogeologie. Leipzig-Halle, Germany.
