Natural chromium contamination of groundwater at Leo ´n Valley, Me ´xico J. Robles-Camacho a,b , M.A. Armienta c, * a Instituto Nacional de Antropologı ´a e Historia, Laboratorio de Rayos-X, Subdireccio ´ n de Laboratorios y Apoyo Acade ´mico, Moneda 16, Centro 06060 Mexico, DF, Mexico b Universidad Nacional Auto ´noma de Me ´xico, Posgrado en Ciencias de la Tierra, Instituto de Geofı ´sica, Circuito Universitario, Mexico c Instituto de Geofı ´sica, UNAM, Ciudad Universitaria, Mexico 04510 DF, Mexico Received 9 February 1999; received in revised form 8 September 1999; accepted 22 September 1999 Abstract Chromium in groundwater resulting from natural sources has been detected in Leo ´n Guanajuato Valley, Central-Me ´xico. The aim of this work was to assess the types of rocks and the geochemical processes responsible for the chromium presence in the water. Superficial and groundwater chemical analyses, X-ray diffraction and EDS determinations in rock samples from the polluted area were performed. Leaching experiments with various types of rocks including SEM/EDS and petrographic observations were used to identify the minerals with higher possibilities to liberate chromium. To the East of Leo ´n valley the chromium comes from the Sierra de Guanajuato ultramafic units and their alteration products. Recent and past tectonic and hydrothermal events have enhanced the chromium release. Among the studied rocks, the serpentinites showed the highest water pollution potential. Disintegration of exsolution borders appeared to be the main geochemical process for derived chromium.  2000 Elsevier Science B.V. All rights reserved. Keywords: serpentinites; pyroxenites; chromium; chromite; Me ´xico; Leo ´n Guanajuato 1. Introduction The water chemistry, environmental mobility and the toxicity of chromium have been widely studied (Mertz et al., 1974; Baes and Mesmer, 1976; Burrell, 1977; Barrett et al., 1985; Calder, 1988; Alexander et al., 1990; Ringstad et al., 1990; Gauglhofer and Bian- chi, 1991). Chromium in the environment exists mainly as Cr(III) and Cr(VI). The oxidation states of Cr develop different biological activity. It is assumed that hexavalent chromium is about 100–1000 times more toxic than trivalent chromium. Ingestion of water with high concentrations of Cr(VI) may produce intestinal, gastric and liver diseases. Cr(VI) is geno- toxic and cytotoxic for bacteria and eukaryotic cells. On the other hand, under the same conditions Cr(III) is either inactive or active at much higher concen- trations (Aitio and Tomatis, 1991; Gauglhofer and Bianchi, 1991). The US Environmental Protection Agency has set the maximum contaminant level of total chromium in drinking water at 0.1 mg/l, a maxi- mum concentration of 0.05 mg/l has been adopted in Me ´xico (Carson et al., 1986; DOF, 1996). High levels of chromium in the water resulting from natural sources are not common. Nevertheless, groundwater might be polluted by this source (Guthrie Journal of Geochemical Exploration 68 (2000) 167–181 0375-6742/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0375-6742(99)00083-7 www.elsevier.nl/locate/jgeoexp * Corresponding author. E-mail address: victoria@tontaiuh.igeofcu.unam.mx (M.A. Armienta).