Seasonal water quality variations in a river affected by acid mine drainage: the Odiel River (South West Spain) M. Olı ´as a, * , J.M. Nieto b , A.M. Sarmiento b , J.C. Cero ´n a , C.R. Ca ´novas a a Department of Geodynamics and Palaeontology, University of Huelva, Campus ‘El Carmen’, 21071, Huelva, Spain b Department of Geology, University of Huelva, Campus ‘El Carmen’, 21071, Huelva, Spain Received 5 January 2004; received in revised form 4 May 2004; accepted 13 May 2004 Abstract This paper intends to analyse seasonal variations of the quality of the water of the Odiel River. This river, together with the Tinto River, drains the Iberian Pyrite Belt (IPB), a region containing an abundance of massive sulphide deposits. Because of mining activity dating back to prehistoric times, these two rivers are heavily contaminated. The Odiel and Tinto Rivers drain into a shared estuary known as the Rı ´a of Huelva. This work studies dissolved contaminant data in water of the Odiel River collected by various organisations, between October 1980 and October 2002, close to the rivers entry into the estuary. Flow data for this location were also obtained. The most abundant metals in the water, in order of abundance, are zinc (Zn), iron (Fe), manganese (Mn) and copper (Cu). Arsenic (As), cadmium (Cd) and lead (Pb) are also present but in much lower quantities. The quality of the river water is linked to precipitation; the maximum sulphate, Fe, Zn, Mn, Cd and Pb concentrations occur during the autumn rains, which dissolve the Fe hydroxysulphates that were precipitated during the summer months. In winter, the intense rains cause an increase in the river flow, producing a dilution of the contaminants and a slight increase in the pH. During spring and summer, the sulphate and metal concentration (except Fe) recover and once again increase. The Fe concentration pattern displays a low value during summer due to increased precipitation of ferric oxyhydroxides. The arsenic concentration displays a different evolution, with maximum values in winter, and minimum in spring and summer as they are strongly adsorbed and/or coprecipitated by the ferric oxyhydroxides. Mn and sulphates are the most conservative species in the water. Relative to sulphate, Mn, Zn and Cd, Copper displays greater values in winter and lower ones in summer, probably due to its coprecipitation with hydroxysulphates during the spring and summer months. Cd and Zn also appear to be affected by the same process, although to a lower degree than Cu, experiencing a slight reduction in summer with respect to Mn and sulphates. D 2004 Elsevier B.V. All rights reserved. Keywords: Odiel River; Acid mine drainage; Water pollution; Heavy metal; Rı ´a of Huelva 1. Introduction Both the Tinto River and the Odiel River are located in the south west of Spain and drain one of the most fa- mous sulphide-mining regions in the world: The Ibe- rian Pyrite Belt (IPB) which contained original reserves of the order of 1700 Mt divided up in more than 50 massive sulphide deposits; one of these being the Rio Tinto mining district described as the largest massive sulphide deposit in the world (Sa ´ez et al., 1999). The mineralogical composition of these deposits is dominated by pyrite (FeS 2 ), with lesser amounts of 0048-9697/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2004.05.012 * Corresponding author. Tel.: +34-95-901-9864; fax: +34-95- 901-9440. E-mail address: manuel.olias@dgyp.uhu.es (M. Olı ´as). www.elsevier.com/locate/scitotenv Science of the Total Environment 333 (2004) 267– 281