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The objective of this study was to investigate the impact of flow, transport and geochemical parameters in the unsaturated and saturated zones on the release of SO4 from overburden lignite spoil piles into the adjacent lake. A vertical one-dimensional model was set up using the reactive transport simulator SULFIDOX in order to account for the unsaturated zone. The SULFIDOX model was calibrated for effective diffusion using measured O2 in the gas phase and SO4 concentrations in the liquid phase from the unsaturated zone of the heap. The results show high sensitivity to O2 supply and initially present gypsum, but the inclusion of secondary mineral precipitation in equilibrium is of minor importance for the results. To account for the transport of released SO4 from the saturated zone into the surface water, scenarios were performed by using SULFIDOX results as input concentration for a two-dimensional vertical model set up with PROCESSING MODFLOW and MT3D. These scenarios indicate a rising discharge of SO4 into the adjacent lake due to continued pyrite weathering for 80 a. Results are highly sensitive to dispersivity, whereas the spatial variability of pyrite distribution did not show any influence on the results. The consideration of initially present gypsum shows a major effect on the modelled SO4 release.