Geochemistry of Sediments in Acid Mining Lakes

Kurt Friese UFZ - Centre for Environmental Research Leipzig-Halle, Department for Inland Water Research, Gouvernementsberg 1, D-39104 Magdeburg, Germany


Acidification of lakes by acid mine drainage is a new field of research in Germany. This topic rised actuality after
reunification of Germany and subsequent shut down of the most coal mine facilities in the former GDR. Aeration of the strata and host rocks of the lignite horizons in the heaps and dumps leads to weathering and oxidation of pyrite and marcasite which are common accessory minerals. With the rising water table and by surface water drainage the oxidation products mainly
sulfate, iron, manganese, aluminum and acidity are incorporated into the newly formed mining lakes.

This paper reports first results of (hydro)geochemical data from five mining lakes in the Lusatian district near Cottbus
concerning lake water and lake sediment chemistry.

Results and Discussion

The investigated lakes cover a range from pH 2 to 8. Three of the lakes are strongly acidified with pH below 3, one of the lakes has a pH of nearly 4 and one lake is neutral with a pH around 8. Four lakes exhibit stratification and oxygen depletion during summer and full mixing during winter time. The ionic composition of all lakes are dominated by calcium for the cations and sulfate for the anions. In the very aicdic lakes high concentrations of iron, aluminum and manganese are observed for the water composition, with maximum values of 400 mg/l for Fe, 40 mg/l for Al and 8 mg/l for Mn, respectively. Sometimes Zn and Ni concentrations are also elevated. In general, electric conductivity is high in the acidic lakes with values about 2.5 mS/cm. In the neutral lake the concentrations of all dissolved compounds are less but the element ratios indicate pyrite/marcasite
oxidation in the surrounding soil in a less extent or associated with buffering of acidity.

The sediments of the lakes represent the compositions of the lake water. The sediments of the acidic lakes show very high contents of iron with maximum values of 50 % weight. Aluminum concentrations varies between 1 and 40 g/kg in the acidic lakes whereas manganese exhibits values in the range of hundreds of mg/kg. Cu, Pb, Ni and Cr concentrations are in the order of 10 to 40 mg/kg. In the neutral lake only the iron content is much lower, about one order of magnitude. All other elements show similar concentration levels as described for the acidic lakes.

Sediment profiles which were taken for the sediment surface layer up to 10 cm depth give a different picture for one of the acidic lakes and the neutral lake, respectively. In the neutral lake all elements decrease with depth in the same manner. In the acidic lakes only iron show a similar behaviour. Al, Cu, Pb and Cr increase with depth whereas manganese shows no variation. Zn and Ni exhibit a sharp increase between 4 and 7 cm depth which can be related to a deep black anoxic zone and associated sulfide precipitation.