Biotechnological remediation of an acidic pit lake: Modelling the basic processes in a mesocosm experiment

Due to lignite mining and pyrite oxidation, there are about 100 acidic pit lakes in Germany. Several research projects are dealing with the examination of the chemical and ecological state of such pit lakes to assess the possibilities of a natural or an artificial restoration. A biotechnological remediation, which accelerates the microbial sulphate and iron reduction at the water–sediment interface (successfully tested by column-experiments), started with the addition of lime and straw in one of these lakes (enclosure tests). Parameters such as pH, redox state and TRIS (total reduced inorganic sulphur) indicate significant changes in the treated lake sediments. The development and stability of reaction products over time, and the interaction between the acidic lake water and the anoxic sediment layer have been investigated for 3 years. The progress of the remediation can be calculated from the amount of accumulated sulphur species in the sediment and the diffusive exchange between sediments and water of the lake. Hydrogeochemical processes and limnophysical characteristics of the lake are combined to understand the geochemical behaviour in greater detail and predict further development. A simple model based on the code AQUASIM allows to realise important uncertainties as far as the remediation strategy is concerned. The low sulphate reduction rates in the upper sediment, a slow diffusive exchange at the water–sediment interface, the inflow of groundwater and mineral reactions inhibit the success of remediation in the lake area of the enclosure under investigation.