Hydrogeochemical behavior of chromium in the unsaturated zone and in the aquifer of leon valley,Mexico

The ground water of Leon valley, Central Mexico, is contaminated with chromium. A study was carried out to determine the main physico-chemical interactions between chromium and the aquifer solid matrix in the most polluted area. Five boreholes, 30 m deep, were drilled and used as piezometers. Unaltered solid cores were analyzed for Cr(VI), total Cr, Fe and Mn. Cr(VI) was determined in the water from the piezometers. We conclude that reduction of Cr(VI) by iron is important, adsorption of Cr(VI) by sand and gravel layers is negligible, and adsorption by silt and clay layers is significant in the saturated and unsaturated zones. In the saturated zone hexavalent chromium distributes preferentially in the water phase oxidation of chromium by manganese was not detected. A correlation between chromium, manganese and iron concentrations was observed for the aquifer solid matrix in this area, when the source of chromium was not anthropogenic.