Natural chromium contamination of groundwater at Leon Valley, Mexico

Chromium in groundwater resulting from natural sources has been detected in Leon Guanajuato Valley, Central-Mexico. The aim of this work was to assess the types of rocks and the geochemical processes responsible for the chrom ium presence in the water. Superficial and groundwater chemical analyses, X -ray diffraction and EDS determinations in rock samples from the polluted a rea were performed. Leaching experiments with various types of rocks includ ing SEM/EDS and petrographic observations were used to identify the mineral s with higher possibilities to liberate chromium. To the East of Leon valle y the chromium comes from the Sierra de Guanajuato ultramafic units and the ir alteration products. Recent and past tectonic and hydrothermal events ha ve enhanced the chromium release. Among the studied rocks, the serpentinite s showed the highest water pollution potential. Disintegration of exsolutio n borders appeared to be the main geochemical process for derived chromium. (C) 2000 Elsevier Science B.V. All rights reserved.