Sulfur isotope studies to quantify sulfate components in water of flooded lignite open pits – Lake Goitsche, Germany

A system of connected lignite mining pits (part of the former Goitsche mining complex, Germany) was flooded with river water between 1999 and 2002. A considerable accumulation of acid associated with oxidized sulfides in sediments was seen as a critical point for the development of the lake water. To characterize the components contributing to the supply of dissolved lake water SO 4 hydro-chemical and isotope investigations with respect to groundwater, pore water in the sulfide bearing sediments, river water and lake water were performed. δ 34S of pore water SO 4 that was dominated by oxidized pyrites ranges around −25‰ VCDT and differs strongly from river water SO 4 with about +4.4‰. Thus, interactions between lake water and sediments were particularly pronounced during the first phase of flooding. For this period, a more quantitative estimation of the SO 4 components in the lake water was difficult because of the heterogeneous SO 4 distributions between the different sub-basins of the lake and according to the flooding process itself. Later, a component separation was attempted following mixing of the whole lake, which first occurred in spring 2002. A very heterogeneous groundwater environment with respect to highly variable SO 4 concentrations and δ 34S values and changing interaction with the forming lakes proved to be one of the most important limitations in the calculations of the mixing.