GROUNDWATER POLLUTION WITH CHROMIUM IN LEON VALLEY, MEXICO

Groundwater contamination by chromium has been detected in the Leon valley central Mexico. In order to determine the contaminant concentration levels and to find its source, a sampling and analysis program was developed. The analytical determinations of chromium and physico-chemical parameters, in a known hydrogeological framework, allowed the identification of three sources of chromium, two of them anthropogenic and one natural. The anthropogenic sources are: the inadequate solid wastes disposal by a chromate factory, which produces high localized concentrations (up to 50mg/l) in groundwaters to the southwest of the valley; and residual ashes produced in brick manufacturing which generates much lower concentrations (lower than 0.05 mg/l of hexavalent chromium) over a much wider area (about 180 km2) south of Leon city. The natural source is weathering of ultramafic rocks which produces detectable amounts (between 0.004 and 0.0 15 mg/l) of Cr (VI) in groundwaters to the Northeast of the valley. Although the city of Leon is one of the main shoe-manufacturing centers of the country, surprisingly tanneries did not add much chromium to groundwaters.