Distribution, origin and fate of chromium in soils in Guanajuato, Mexico

Total, hexavalent and trivalent chromium were determined in surface and 30-cm depth soil samples from a highly chromium-polluted area in Guanajuato state, central Mexico. Four samples were also analyzed by a sequential extraction procedure. Nearly 0.9 km(2) out of the 8 km(2) area sampled was polluted with chromium, at concentrations up to 12 960 mg kg(-1), mostly as Cr(III). Concentrations of Cr(VI) were lower than 0.5 mg kg(-1) in most sampled points, with the exception of one, where the concentration was found to be 65.14 mg kg(-1). Chromium-containing dust from a chromate factory accounted for most of the contamination. The highest concentrations of hexavalent chromium in soil, were in the bottom sediments of an abandoned water reservoir used to store polluted water from a well, before use of the water in the factory process. Tannery wastes, dust from a sanitary landfill of chromate compounds and the transport of chromium products are the sources of chromium at other sites. Chromium is fixed preferentially in the hydrous Fe and Mn oxides in the more polluted soils. Less polluted soils have a high proportion of chromium associated with the sulfide and organic fraction. Cr(III) is retained preferentially in the superficial soil layer. Variations in the physical characteristics of the soil, relative abundance of the various soil components and characteristics of the contaminant source, give rise to differences in chromium soil concentrations with depth.