In the context of the recultivation of lignite mines, several projects at field scale, laboratory scale and modelling techniques have been carried out by our workgroup since 2000 in cooperation with the AUSTRALIAN NUCLEAR SCIENCE AND TECHNOLOGY ORGANISATION (ANSTO), and the BRANDENBURGER TECHNISCHE UNIVERSITÄT COTTBUS (BTU). The aim of these studies is to enhance our understanding of processes which control the release of contaminants from waste disposals in order to assess future contaminant loads into the hydrological system. In this case the natural hazard is the release of sulphate into the surface water system. To perform the long-term assessment we applied and developed different modelling tools and verified them by field and laboratory data. Currently we are implementing a project in cooperation with the BUNDESANSTALT FÜR GEOLOGIE UND ROHSTOFFE which investigates the influence of hard pans in mining waste dumps for oxygen diffusivity. The diffusive oxygen flux into these waste dumps is a crucial factor because oxidation leads to the mobilisation of contaminants which constitute a hazard for the environment.
A further research is the assessment of sulphate release by pyrite weathering from heap sediments into the ground- and surface water. The objective of this study is the prediction of contaminant loads related to acid mine drainage from overburden spoil piles at open-pit lignite mines. An assessment of future sulphate discharge released from the unsaturated zone into the groundwater as a result of pyrite weathering was performed, based on measurements of soil-gas, pore water and sediment chemistry over a profile of 9 m in the unsaturated zone of an abandoned lignite mine. The results were interpreted using a vertical one dimensional reactive transport code, a modified version of SULFIDOX. The model was calibrated for effective diffusion using measured oxygen and sulphate concentrations in the unsaturated zone of the heap. The calibrated model showed small sensitivities to the Monod parameters and the intrinsic pyrite weathering rate, but a high sensitivity to oxygen supply. The inclusion of secondary mineral dissolution and precipitation yielded a smaller concentration peak but a retarded release of sulphate. To account for the sulphate discharge from the groundwater into the surface water the vertical reactive transport code was coupled with MT3D. Scenario calculations indicated a rising discharge of sulphate into the adjacent surface water for the next 50 years due to continued pyrite weathering.