Technical Article Modelling the Long-term Release of Sulphate from Dump Sediments of an Abandoned Open Pit Lignite Mine C. Kohfahl and A. Pekdeger Inst of Geological Sciences, Free Univ of Berlin, Dept of Geological Sciences, Malteser Str 74-100, D-12249 Berlin, Germany; communicating author’s e-mail: kohfahl@zedat.fu-berlin.de Abstract. The generation of acid drainage from overburden spoil piles at open-pit lignite mines impacts water quality in large parts of the Lusatian mining area in Germany. The Lohsa Mine was exploited until the early 1990s and is to be flooded by 2005. It will then be used as a reservoir basin for the river Spree. Future acidity and sulphate concentrations in the surface water are of great interest because considerable amounts of the bank filtrate of the river are used to supply drinking water to communal water plants downstream. In our study, the input of sulphate from the unsaturated zone of the heap into the groundwater was calculated using the one dimensional reactive transport code SAPY. The SAPY program, which had been calibrated for effective diffusion and tortuosity using oxygen breakthrough curves of a column experiment with original heap sediments, was scaled up to field conditions and verified by measuring the oxygen and sulphate profile of the heap. Scenarios for a period of 80 years were simulated for different distances of the groundwater level to the subsurface, and the mass input of sulphate from the unsaturated zone into the groundwater was calculated in terms of specific fluxes for different times. Plans are to use the calculated source terms in a regional three-dimensional model to predict the evolution of the ground- and surface water in the area. Key words: Lignite mines; Lohsa; pyrite weathering; reactive transport modelling; Spree; sulphate generation Introduction Large areas in the new federal states of Germany have been subjected to open cast lignite mining. Acid generation occurs when pyrite in the dump sediments oxidize. Oxidation starts when the original overburden is dewatered, and then intensifies during the mining (excavation, mixing, and dumping activities) process. After deposition, pyrite oxidation continues in the aerated upper part of the spoils, sometimes for several decades. After a mine is decommissioned, the standard procedure is to allow the water table to rise and to convert the pit into a lake for recreational purposes. During flooding, which often extends over several years to decades, the acidity and other contaminants in the groundwater are flushed into the newly formed lake (Gerke 1998). To reduce acid input during inundation, surface water is sometimes used to supplement the slow natural rise of groundwater. The study area is the Lohsa lignite mine near Hoyerswerda in Germany. This mine was exploited from 1970 to 1990, when its groundwater level was lowered artificially to a maximum depth of 50 m. The pumped water was discharged into the Spree during that period. Pumping of groundwater stopped after the mine was decommissioned, so the water level of the Spree has declined since 1990, causing water quality problems downstream. The former lignite mine is to be flooded, mainly with surface water, by 2005 and will afterwards be used as a reservoir basin for the river Spree. To equilibrate the hydraulic head of the river, present management strategies are predicting annual surface water oscillations between 5 and 8 m (LMBV 1996). Ongoing weathering in the unsaturated zone of the surrounding 40 m high heaps will continue to produce acidity, which will exfiltrate into the newly formed lake and into the Spree. The bank filtrate of the Spree is used for drinking water in urban areas downstream of Lohsa, so the long-term release of sulphate and acidity from the unsaturated zone of the heaps has to be estimated. Several models have already been developed to allow long term predictions of the hydrogeochemical evolution in abandoned and flooded lignite mine environments (Strömberg and Banwart 1994; Foos 1997). These models usually consider equilibrium reactions and additional kinetic processes. Some models also include O 2 diffusion and transport reactions (Prein and Mull 1995; Wunderly et al. 1996; Hecht et al., in press). Mayer incorporated kinetically controlled reactions in variably saturated porous media in a multi-component reactive transport model (Mayer 2002). The aim of this study was to investigate the extent to which pyrite weathering processes, which were studied in the laboratory, could be scaled up to field conditions using a model. Our objective is to provide quantitative long term estimates concerning the release of weathering products from the aerated zone of a heap into the groundwater. To provide a first