Impact of erosion-transported overburden dump materials on water quality in Lake Cospuden evolved from a former open cast lignite mine south of Leipzig, Germany

Acidification is the most common water quality problem in lakes created from previous open cast lignite mines. Aeration of aquifers and dump materials from mining activities causes pyrite oxidation. Pyrite oxidation products are stored in pore water, minerals and at the exchange complexes of the aquifers and dump sediments. Rainfall runoff transports sediments on the dump slope into the lakes. Elutriation of these sediments whithin the lakes releases either acid-producing or acidneutralizing agents. At a test site south of Leipzig, the annual erosion rates were quantified by water erosion models (RUSLE, EROSION 2D, PEPP) and field measurements. They ranged from 300 up to 900 tons per hectare. Hydrogen ion equivalent release or binding at the sediment elutriation was computed from laboratory analysis of the pore-water quality, ion exchange complex and mineral composition of the sediment. Two of the three investigated sediments contained 3 mmol leg) acidity per 100 g dry sediment and revealed saturation with respect to jarosite, jurbanite and gypsum. In the third sediment, 6 mmol leg) alkalinity per 100 g dry sediment was obtained. The annual net acidity influx was calculated to be about 0.5 million mol leg) for the lake of the test site.