For water samples from mining lake ML 111 of the Lusatian Lignite District and surrounding aquifers δ34S(SO4−b), δ15N(NH4+), δD, and δ18O (H2O) values have been determined to asess the flow dynamic of the in- and outflow scenario. The high mineralization and acidification (pH > 2.6) of the lake water results from an intensive interaction with the surrounding dump sites and the inflow of extremely sulfate rich dump water (up to 4,100mg/1 SO4−). δ34S values range from −7‰ to + 7°CDT in the inflow aquifers. The lake water is considered to be a mixture of different water bodies. The evaporation signature in the outflow aquifer suggests that the groundwater consists of 60% lake water. The nitrogen transfer is explained by15N signature of ammonium. Sulfate reduction, perceptible in outflow and dump aquifer, cannot be proved within the mining lake by34S and hydrochemistry.