Quantification of Acidity Deposition in the Sediment of a former lignite mining lake in the Wackersdorf Mining District (Eastern Bavaria)

摘要

From 1982 to 2001 acidic conditions prevailed at former lignite mining Lake Knappensee. Since 2001 the acidity of the lake water continuously decreased reaching circum-neutral pH conditions in 2010.Based on acidity time series of the lake water in combination with water budgets derived from a regional groundwater model the cumulative acidity loss in the lake for the time span from 2001 to 2015 can be estimated to be 8400 kmol.Internal neutralization by biogenic sulfate reduction with consecutive deposition of reduced sulfur in the lake sediment was supposed to be the crucial process leading to the observed decrease in lake water acidity. In order to test this hypothesis elaborate quantitative field and laboratory investigations were launched in December 2014.15 sediment cores distributed over the lake's area were sampled and analyzed for reduced sulfur species at the hydrological laboratory of the University of Bayreuth. The measured sulfur contents were converted into deposited proton equivalents and finally the results were extrapolated for the total lake sediment.The amount of at least 6400 kmol of acidity deposited and fixed in the lake sediments estimated with this procedure compares well to the acidity-loss of 8400 kmol which was calculated for the lake water as mentioned above.Measurements and data evaluation allowed for identification of naturally occurring internal neutralization to be the major process for the observed acidity decrease in the lake water. The internal neutralization processes are not only beneficial to the lake water quality, but are also of great practical importance for the assessment of the hydrochemical state of the lake.The methodology applied to Lake Knappensee consisting of field scale sediment investigations combined with water budgets gained from regional groundwater modeling could also be used for the assessment of the hydrochemical state of other mining lakes.