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 SO4 hydro-chemical and isotope investigations with respect to groundwater, pore water in the sulfide bearing sediments, river water and lake water were performed. δ34S of pore water SO4 that was dominated by oxidized pyrites ranges around −25‰ VCDT and differs strongly from river water SO4 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 SO4 components in the lake water was difficult because of the heterogeneous SO4 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 SO4 concentrations and δ34S values and changing interaction with the forming lakes proved to be one of the most important limitations in the calculations of the mixing.