The impact of pyrite variability, dispersive transport and precipitation of secondary phases on the sulphate release due to pyrite weathering Claus Kohfahl a, * , Paul L. Brown b,1 , Claire M. Linklater b,2 , Kai Mazur c , Parviz Irannejad b,3 , Asaf Pekdeger a a Institute of Geological Sciences, Freie Universität Berlin, Department of Earth Sciences, Malteserstr 74-100, D-12249 Berlin, Germany b ANSTO Environment, Australian Nuclear Science and Technology Organization, Private Mail Bag 1, Menai, NSW 2234, Australia c Faculty of Environmental Sciences and Process Engineering Brandenburg University of Technology Cottbus, Konrad-Wachsmann-Allee 6, D-03046 Cottbus, Germany article info Article history: Received 22 May 2008 Accepted 11 October 2008 Available online 18 October 2008 Editorial handling by Dr. R. Fuge abstract 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 SO 4 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 O 2 in the gas phase and SO 4 concentrations in the liquid phase from the unsaturated zone of the heap. The results show high sensitivity to O 2 supply and initially present gypsum, but the inclu- sion of secondary mineral precipitation in equilibrium is of minor importance for the results. To account for the transport of released SO 4 from the saturated zone into the sur- face 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 SO 4 into the adjacent lake due to continued pyrite weathering for 80 a. Results are highly sensitive to dispersivity, whereas the spatial vari- ability of pyrite distribution did not show any influence on the results. The consideration of initially present gypsum shows a major effect on the modelled SO 4 release. Ó 2008 Elsevier Ltd. All rights reserved. 1.Introduction The oxidation of pyrite bearing mine waste heaps re- sults in the release of acidity and SO 4 among other weath- ering components. Oxidation of layered natural sediments starts during dewatering of sediments prior to mining. During sediment excavation and deposition oxidation is intensified and after deposition, secondary oxidation of pyrite often continues for several decades in the aerated upper part of the heaps. After decommissioning, mine dewatering often stops and the groundwater table rises and converts the pit into a lake which is frequently planned to be used for recreational purposes or water stor- age. During the flooding, which often extends over several years to decades, the acidity and other chemical compo- nents in the groundwater are flushed into the newly formed lake (Gerke et al., 1998). The transport of contam- inant load discharged into the surface water is therefore a combination of release and vertical transport in the unsat- urated zone followed by horizontal saturated transport in the groundwater. 0883-2927/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2008.10.001 * Corresponding author. Address: Freie Universität Berlin, Inst. für Geologische Wissenschaften, Fachrichtung Geochemie, Hydrogeologie, Mineralogie, Arbeitsbereich Hydrogeologie, Malteserstr. 74-100, Haus B, D-12249 Berlin, Germany. Tel./fax: +49 30 838 70 742. E-mail addresses: kohfahl@zedat.fu-berlin.de (C. Kohfahl), paul.l.- brown@riotinto.com (P.L. Brown), clinklater@srk.com.au (C.M. Linklater), mazur@tu-cottbus.de (K. Mazur), piran@ut.ac.ir (P. Irannejad). 1 Present address: Rio Tinto, 1 Research Ave. Bundoora, Vic. 3083, Australia. 2 Present address: SRK Consulting, 44 Market St., Sydney, NSW 2000, Australia. 3 Present address: Institute of Geophysics, University of Teheran, Tehran P.0. Box 14155-6466, Iran. Applied Geochemistry 23 (2008) 3783–3798 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem