| Summary: | Anomalous natural concentrations of chromium, sometimes exceeding the potability limit (0.05 mg.L-1), have been detectedin the groundwater of Adamantina Aquifer in the municipality of Urânia, and in a wide region of the western part ofthe State of São Paulo. In order to identify the possible geochemical reactions that may explain the occurrence of chromiumin groundwater, chemical and mineralogical analyses were conducted in rock samples collected from deep boreholes drilledin the city of Urânia. Multilevel monitoring wells were installed in the boreholes, and stratifi ed groundwater samples werecollected for chemical analyses. The analyses of the borehole samples indicated the occurrence of a geochemical anomaly ofchromium (average concentrations of 221 ppm) and pointed to chrome-diopside containing from 1,000 to 6,000 ppm Cr as themain reactive mineral that contributes to this anomaly. Groundwater chemical analyses indicated the occurrence of a hydrochemicalstratifi cation of the aquifer: waters from the base of the aquifer are alkaline (pH higher than 10) whereas in the shallowzone of the aquifer, pH is neutral to mildly acid. Chromium has been identifi ed in its hexavalent form, and prevails in theaquifer, reaching the maximum concentration of 0.13 mg.L-1. The geochemical reactions that explain the release of chromiumfrom the solid phase to the water probably involve the dissolution of Cr3+ minerals such as diopside, followed by a redox reactionthat oxidizes Cr3+ to Cr6+ and reduces manganese oxide minerals. Additionally, adsorption reactions may take place, anddesorption and mobilization of Cr6+ may be favored in some portions of the deep aquifer where pH is anomalously alkaline.
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