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-chemic al interactions between chromium and the aquifer solid matrix in the m ost 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 s ilt and clay layers is significant in the saturated and unsaturated zo nes. In the saturated zone hexavalent chromium distributes preferentia lly in the water phase. oxidation of chromium by manganese was not det ected. A correlation between chromium, manganese and iron concentratio ns was observed for the aquifer solid matrix in this area, when the so urce of chromium was not anthropogenic.