Reginaldo Bertolo

Natural occurrence of hexavalent chromium in a sedimentary aquifer in Urânia, State of São Paulo, Brazil

Anomalous concentrations of hexavalent chromium have been detected in ground-water of the Adamantina Aquifer inat least 54 municipalities located in the northwestern region of the State of Sao Paulo, southeast Brazil, occasionallyexceeding 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 originof this contamination is natural, since high concentrations of chromium were detected in aquifer sandstones (averageof 221 ppm) and also in pyroxenes (6000 ppm), one of the main heavy minerals found in the sediments. Besides, noother possible diffuse or point sources of contamination were observed in the study area. Stratification of ground-waterquality was observed and the highest concentrations of Cr6+ were detected at the base of the aquifer (0.12 mg.L-1),where ground-water 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 pHfound in ground-water at the base of the aquifer.

Biogeochemical processes and the diversity of Nhecolândia lakes, Brazil

The Pantanal of Nhecolândia, the worlds largest and most diversified field of tropical lakes, comprises approximately 10,000 lakes, which cover an area of 24,000 km(2) and vary greatly in salinity, pH, alkalinity, colour, physiography and biological activity. The hyposaline lakes have variable pHs, low alkalinity, macrophytes and low phytoplankton densities. The saline lakes have pHs above 9 or 10, high alkalinity, a high density of phytoplankton and sand beaches. The cause of the diversity of these lakes has been an open question, which we have addressed in our research. Here we propose a hybrid process, both geochemical and biological, as the main cause, including (1) a climate with an important water deficit and poverty in Ca(2+) in both superficial and phreatic waters; and (2) an elevation of pH during cyanobacteria blooms. These two aspects destabilise the general tendency of Earths surface waters towards a neutral pH. This imbalance results in an increase in the pH and dissolution of previously precipitated amorphous silica and quartzose sand. During extreme droughts, amorphous silica precipitates in the inter-granular spaces of the lake bottom sediment, increasing the isolation of the lake from the phreatic level. This paper discusses this biogeochemical problem in the light of physicochemical, chemical, altimetric and phytoplankton data.

Relações Água-Rocha e a Hidrogeoquímica do Cromo na Água Subterrânea de Poços de Monitoramento Multiníveis de Urânia, SP, Brasil

Anomalous 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 Sao 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 stratified 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 stratification 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 identified in its hexavalent form, and prevails in the aquifer, reaching the maximum concentration of 0.13 mg.L-1. The geochemical reactions that explain the release of chromium from the solid phase to the water probably involve the dissolution of Cr3+ minerals such as diopside, followed by a redox reaction that oxidizes Cr3+ to Cr6+ and reduces manganese oxide minerals. Additionally, adsorption reactions may take place, and desorption and mobilization of Cr6+ may be favored in some portions of the deep aquifer where pH is anomalously alkaline