Open cast mines as river sediment and pollutant sinks. The example Mulde Reservoir (East Germany)

摘要

Since 1975, the Mulde River has flown through the abandoned open-cast lignite mining pits at Muldenstein near Bitterfeld (Germany) forming a lake named Mulde Reservoir. Ever since, the Mulde Reservoir has built up a sediment and pollutant deposit protecting the downstream water bodies and sediment quality in the lower Elbe stretch extending to the port of Hamburg area and the North Sea. Therefore, the Mulde Reservoir is an example for beneficial effects of riverine flow-through of pit lakes.Core drillings have revealed that the sediments currently deposited in the Mulde Reservoir still have to be regarded as highly polluted as to their contents of As, Cd, Cr, Hg, Ni, Pb, Zn, according to the classification of the Pollutant Sediment Management Concept (PSMC) of Elbe River. Arsenic and the mentioned metals originate from former metal mining and processing in the upper part of the catchment of the Mulde River. However, a decrease in pollutant inflow since 1990 has been documented, correlating with the long-term changes observed in the upper course of the Mulde River. As far as organic pollutants are concerned, the lake sediments do not show any excessive concentrations, apart from a few exceptions.From a geochemical and hydraulic point of view, there is presently no need to remove the contaminated sediments from the Mulde Reservoir. Main activities in sediment management should focus on further improving stabilization and efficiency of the Mulde Reservoir sedimentation zone and its transregionally vital ecosystem services. Given the present retention capacity of the Mulde Reservoir, it will continue to serve as an important sediment trap of supra-regional significance for the downstream water courses for at least another 770 years.