Natural occurrence of hexavalent chromium in a sedimentary aquifer in Urania, State of Sao Paulo, Brazil

Anomalous concentrations of hexavalent chromium have been detected in groundwater of the Adamantina Aquifer in at least 54 municipalities located in the northwestern region of the State of Sao Paulo, southeast Brazil, occasionally exceeding the permitted limit for human consumption (0.05 mg.L-1). An investigation was conducted in the municipality of Urania, where the highest concentrations of chromium were detected regionally. It was defined that the origin of this contamination is natural, since high concentrations of chromium were detected in aquifer sandstones (average of 221 ppm) and also in pyroxenes (6000 ppm), one of the main heavy minerals found in the sediments. Besides, no other possible diffuse or point sources of contamination were observed in the study area. Stratification of groundwater quality was observed and the highest concentrations of Cr6+ were detected at the base of the aquifer (0.12 mg.L-1), where groundwater shows elevated values for redox potential (472.5 mV) and pH (8.61). The origin of Cr6+ in water may be associated with the weathering of pyroxene (augite), followed by the oxidation of Cr3+ by manganese oxides. The highest concentrations of Cr6+ are probably related to desorption reactions, due to the anomalous alkaline pH found in groundwater at the base of the aquifer.