Estimating the effect of water table oscillations on acidification using a hydraulic model

Abstract

During and after the process of artificial recharge by flooding of former pit mines, oscillations of the water level, caused by seasonal effects or management strategies, are often inevitable. In the study area LOHSA in the new eastern federal states of Germany, the annual water oscillations due to management strategies are predicted to range between 5–8 m. This leads to oxygen input by freshwater and air, which causes pyrite oxidation in the surrounding sediments and leads to additional acidification of both groundwater and the flooded lakes. To quantify the contribution of water oscillations to the acidification processes in the flooded lignite mine LOHSA II, a two-dimensional vertical model was built up using Processing Modflow. The strong lateral migration of the shoreline due to the low morphologic gradient was simulated by the reservoir package. The modelled transient water fluxes were used to estimate the discharge of acid builders from the water-saturated part of the heap into the lake. The contribution of acidification due to the continuous release of weathering products in the unsaturated zone was calculated separately. The modelled scenarios indicate that the contribution to acidification by release of weathering products from the unsaturated zone is small in comparison to the mass input by acid mine drainage from the saturated zone of the heap.