RESUMOTeores anômalos e naturais de cromo ocorrem nas águas subterrâneas do Aquífero Adamantina no município de Urânia (SP) e em uma ampla região do oeste do Estado de São Paulo, algumas vezes ultrapassando o limite de potabilidade (0,05 mg.L -1 ). Visando identifi car as possíveis reações geoquímicas que justifi cam a ocorrência do cromo na água subterrânea em Urânia, foram realizadas perfurações com coleta de amostras contínuas de rocha para a condução de análises mineralógicas e químicas, construídos poços de monitoramento multiníveis e realizadas coletas e análises de amostras estratifi cadas de água subterrânea. Análises das amostras de testemunhos de sondagem indicaram a ocorrência de uma anomalia geoquímica de cromo (concentrações médias de 221 ppm), sendo o diopsídio cromífero provavelmente o mineral geoquimicamente mais reativo que contribui para esta anomalia, apresentando concentrações de cromo de 1.000 a 6.000 ppm. As análises químicas de amostras de água coletadas dos poços de monitoramento indicaram uma estratifi cação hidroquímica do aquífero: águas na base do aquífero apresentam pH anomalamente alcalino (superior a 10), enquanto águas mais rasas possuem pH neutro a ligeiramente ácido. O cromo ocorre predominantemente na forma hexavalente e alcança concentrações máximas de 0,13 mg.L -1 . As reações geoquímicas que explicam a passagem do cromo da fase sólida para a água provavelmente envolvem a dissolução de minerais contendo Cr 3+ (diopsídios), seguida de uma reação redox que oxida o Cr 3+ para o Cr 6+ , provavelmente relacionada com a redução de óxidos de manganês presentes no aquífero. Adicionalmente, devem também ocorrer reações de adsorção, sendo que os ambientes de pH elevados favorecem a dessorção e mobilização do Cr 6+ para a água. ABSTRACTAnomalous natural concentrations of chromium, sometimes exceeding the potability limit (0.05 mg.L -1 ), have been detected in the groundwater of Adamantina Aquifer in the municipality of Urânia, and in a wide region of the western part of the State of São Paulo. In order to identify the possible geochemical reactions that may explain the occurrence of chromium in groundwater, chemical and mineralogical analyses were conducted in rock samples collected from deep boreholes drilled in the city of Urânia. Multilevel monitoring wells were installed in the boreholes, and stratifi ed groundwater samples were collected for chemical analyses. The analyses of the borehole samples indicated the occurrence of a geochemical anomaly of chromium (average concentrations of 221 ppm) and pointed to chrome-diopside containing from 1,000 to 6,000 ppm Cr as the main reactive mineral that contributes to this anomaly. Groundwater chemical analyses indicated the occurrence of a hydrochemical stratifi cation of the aquifer: waters from the base of the aquifer are alkaline (pH higher than 10) whereas in the shallow zone of the aquifer, pH is neutral to mildly acid. Chromium has been identifi ed in its hexavalent form, and prevails in the aquifer, reaching the maximum concentration ...
Anomalous concentrations of chromium VI, sometimes exceeding the maximum limit permitted for human consumption (0.05 mg/L), have been detected in groundwater from the Northwestern region of São Paulo state. Previous studies indicated that this ocurrence is of natural origin. The city of Urânia was selected for the study since the local aquifer has the highest and most persistent contents of chromium in this region. As part of a geochemical investigation of rock-water interaction, this article describes the chemical, mineralogical and petrographical characterization of sandstone samples taken from boreholes that reached the depth of 100 m, aiming to identify in which layers and in which manner chromium is inserted in the solid phase. Solid phase analyses were performed by X-ray fluorescence, X-ray diffraction and scanning electron microscopy (SEM). The results of this investigation indicated that: (1) the concentration of chromium in the Urânia sandstones is anomalous, since the average value obtained (221 mg/kg) is quite higher than the reported average concentration for a generic sandstone (35 mg/kg) and for soils from another regions of São Paulo state (26,3 mg/kg); (2) the highest chromium concentrations were detected in the depths of around 50 meters (336 mg/kg); (3) chromium concentrations reached values of 6.000 ppm in pyroxen (augite) crystals, which is one of the main heavy minerals of the sandstones; and (4) chromium concentrations on clay minerals and on iron hidroxydes are also relatively high (up to 650 ppm).
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