As a part of the research project "Remediation of organically and inorganically polluted ground water and surface water" the acidic mining lake 111 (situated in Eastern Germany) has been investigated to assess the possibilities of a successful biotechnological remediation. These investigations include a detailed hydrogeological study of the whole catchment area. Ground, seepage and lake water, as well as the surrounding sediments have been examined to model the hydrodynamic processes and the geochemical development of the lake. The isotope signatures of the various waters have also been analysed.