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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 São Paulo, southeast Brazil, occasionally exceeding the permitted limit for human consumption (0.05 mg.L -1 ). An investigation was conducted in the municipality of Urânia, 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 Cr 6+ 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 Cr 6+ in water may be associated with the weathering of pyroxene (augite), followed by the oxidation of Cr 3+ by manganese oxides. The highest concentrations of Cr 6+ are probably related to desorption reactions, due to the anomalous alkaline pH found in groundwater at the base of the aquifer.

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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 ...

the reviewed and integrated geoheritage inventory of the northern coast of the state of São Paulo, Brazil, resulted in 43 geosites distributed in eight geological frameworks. the selected geosites are representative of a geological history from the Neoproterozoic (Brasiliano-Pan African Cycle) to the recent, covering a minimum 700-Ma timespan. Considering the dynamic character of geoheritage inventories, the results presented in this work were based on previous local, town-based surveys, which were reviewed under the light of new geological data and geoconservation methods. Both qualitative and quantitative geosites assessments were used to discuss their geological representativeness and relevance, as well as legal aspects regarding protection and perspectives for future use. this resulted in a detailed inventory that reflects the current geological knowledge in the northern coast of the state of São Paulo, and may be used to make a general diagnosis of the scientific value, state of conservation, risk of degradation and potential use of the geoheritage. Moreover, it reinforces the importance of locally-and regionally-relevant sites for sustainable land management using geoheritage and to direct the decisions related to the conservation and use of this abiotic part of nature in the region.

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