Geochemical processes in a highly acidic pit lake of the Iberian Pyrite Belt (SW Spain) C.R. Cánovas a,b, ⁎, S. Peiffer c , F. Macías d , M. Olías b , J.M. Nieto d a Department of Biology, Faculty of Marine and Environmental Sciences, University of Cádiz, Pol. Rio San Pedro s/n, 11510 Puerto Real, Cadiz, Spain b Department of Geodynamics and Palaeontology, Faculty of Experimental Sciences, University of Huelva, Campus el Carmen s/n, 21071 Huelva, Spain c Department of Hydrology, Bayreuth Center of Ecology and Environmental Sciences—BayCEER, University of Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany d Department of Geology, Faculty of Experimental Sciences, University of Huelva, Campus el Carmen s/n, 21071 Huelva, Spain abstract article info Article history: Received 18 June 2014 Received in revised form 10 December 2014 Accepted 11 December 2014 Available online 20 December 2014 Editor: Carla M Koretsky Keywords: San Telmo pit lake Schwertmannite transformation pH buffering Iron and sulfate reduction Alkalinity generation Compared with pit lakes originated by coal mining, little is known about in-lake neutralization processes in pit lakes from sulfide ore mining in hard rock substrates, which are typically very deep and acidic and receive low carbon (C) inputs and groundwater flows. Physicochemical processes in water and sediments from a pit lake (San Telmo, 130 m deep) in the Iberian Pyrite Belt were investigated. San Telmo is a meromictic and highly acidic (pH 2.8) pit lake due to pH buffering by precipitation of Fe(III) minerals (schwertmannite and jarosite). The sed- iments have a low abundance of C (below 0.60%) and iron minerals (below 0.12%) compared to most coal-mining pit lakes. In San Telmo sediments, iron reduction and sulfide oxidation may be thermodynamically favored due to low pH values in pore waters (below 3.8) and the presence of reactive iron. Although schwertmannite is the main ferric mineral precipitating in the water column, mineralogical analyses reveal a low abundance of schwertmannite in the sediment. This may be due to the preferential use of this mineral in reduction reactions mediated by bacteria, together with a low replenishment rate of the schwertmannite pool in the sediment. The transformation of reactive iron (schwertmannite and jarosite) into goethite may limit sulfate reduction, promoting acidic conditions in the sediment. As long as the acid mine drainage continues to discharge into the lake, the pH buffering exerted by ferric minerals in the sediments will limit the neutralization of the pH by sulfate reduction. © 2014 Elsevier B.V. All rights reserved. 1. Introduction and scope The Iberian Pyrite Belt (IPB) is one of the most important polymetallic sulfide-mining regions in the world. Mining activity in the IPB dates back to prehistoric times, specifically to the third millenni- um B.C., and developed to such a magnitude that it provoked an unprec- edented impact on the environment (Nocete et al., 2005). The long history of metalliferous mining in the region has left a legacy of derelict mines and an enormous amount of mining wastes, including sulfide- bearing waste rock piles, tailings, and so on. The oxidation of sulfides and consequent production of acid mine drainage (AMD), has provoked intense pollution of the main fluvial systems draining the IPB: the Tinto and Odiel rivers (e.g. Sánchez España et al., 2005; Olías et al., 2006; Cánovas et al., 2007; Sarmiento et al., 2009a). The enormous pollution caused by mining wastes deposited over the centuries remains active and will continue to generate acid mine drainage for a long time to come (Younger et al., 2002). From the second half of the nineteenth century, underground workings and opencast mining in the IPB were used simultaneously in order to exploit the mineral deposits more efficiently. In the IPB, the existence of more than 30 mine pit lakes has been recorded, many of them flooded during the second half of the twentieth centu- ry, concomitant with the decline of mining activities in the region (Sánchez-España et al., 2008). During exploitation, the water table is suppressed to avoid the flooding of active mines. However, when the mining activity ceases, the water table recovers its original posi- tion, flooding the open pits and giving rise to mine pit lakes. The rise of the water table promotes the oxidative dissolution of sulfides contained in the pit banks, interconnected underground galleries, and shafts, which may cause water acidification and the subsequent dissolution of gangue minerals from the host rocks. In many cases, water stored in pit lakes is of poor quality and may contain concen- trations of metals that greatly exceed water quality standards. In this respect, the Water Framework Directive (WFD) of the European Union (EC, 2000) considers mining lakes as artificial sur- face water bodies which have to be monitored and managed to reach an acceptable water quality status. Achieving this goal requires the implementation of remediation strategies that need to be sup- ported by a deep knowledge of the physicochemical processes and interactions between the water column and lake sediments. Most Chemical Geology 395 (2015) 144–153 ⁎ Corresponding author at: Department of Geodynamics and Palaeontology, Faculty of Experimental Sciences, University of Huelva, Campus el Carmen s/n, 21071 Huelva, Spain. Tel.: +34 959219870; fax: +34 959 219834. E-mail address: carlos.ruiz@dgeo.uhu.es (C.R. Cánovas). http://dx.doi.org/10.1016/j.chemgeo.2014.12.007 0009-2541/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo