Open pit mining characterizes the area of Lusatia (Germany), which corresponds to dramatic changes of the natural ecosystem. Landscapes with huge waste heaps and acid mine lakes are created by former mining activities. The studied waste heap is about 70 years old and is situated between two lakes (Restsee 107 and Restsee 108) formed ten years later. Samples taken by drilling drills showed hints of active sulfate reduction activities within the heap and the ground water. Black precipitations were visible on the sediment cores and an intensive H2S odor was noticed. Investigations of the hydrochemistry of the ground water within the heap were started with the drilling of twelve wells along two parallel lines (I, II). Various analytical parameters were measured (see Storch et al. IBS 2007, short lecture topic environmental problems). The bacterial communities of two wells are investigated by molecular methods. The water samples differ in pH (MPL1: 2.4 and MPL11: 4.4) and were therefore chosen to investigate into more detail. Creating a 16S rDNA library, the microbial diversity in the water was characterized. The 200 clones of sample MPL11 were highly diverse: a restriction analysis using the enzyme RsaI revealed 120 different restriction types. Numbers of sequences were related to sulfate reducing bacteria and to uncultivated bacteria from acidic environmental sites. The obtained sequences will be used to study the remaining wells by T-RFLP and to discuss these results in respect to sulfate reduction under acidic conditions.