Predictive modelling of dump water impact on the surroundings of the lignite dump site Jänschwalde (Eastern Germany)

Open cast mining of lignite leads to dumps containing highly mineralised pore waters. To predict the impact of the dump waters on the undisturbed aquifers two steps are necessary. (1) The prediction of dump water quality at times when steady state flow conditions will be established. (2) The simulation of the dump ground water migration with a model that is able to handle the complexity of the homogeneous and heterogeneous interactions of the migration process.For the investigated site Jänschwalde, which is still dewatered, a chemical mass balance was performed. The predicted alkalinity potentials exceed acidity potentials for the dump as a whole. The distribution of these parameters show high alkalinity potentials for the northern part.In order to model the migration process the transport code PCGEOFIM^® [Anwenderdokumentaion, IBGW Leipzig, (in German)] was coupled with the geochemical equilibrium code PHREEQC [USGS, Water-Resources Investigations Report]. This was done to simulate redox reactions, mineral dissolution and precipitation, and cation exchange in the ground water zone. The model is verified by a column flow test. The results of the simulations show a small effect of the migrating dump waters on the quartenary aquifer with respect to acidity changes. This results from calcite buffering and cation exchange. The impact on the quartenary aquifer by sulphate is much higher.