Influence of acid sand pit lakes in surrounding groundwater chemistry, Sepetiba sedimentary basin, Rio de Janeiro, Brazil

Abstract

Seropédica–Itaguaí sand mining district has an important role in the Rio de Janeiro Metropolitan Region, supplying approximately 70% of the sand for civil construction. The sand extraction process removes surface sedimentary layers (with quartz–feldspatic composition), increasing water table in the mined pit. However, this activity causes some chemical and physico-chemical changes in the water, such as the oxidation of reduced sediments lowering pH values (reaching values < 3.5) and increasing SO 4 concentration (reaching more than 90 mg.l − 1 ). The relatively low pH values of those waters promote an increase of weathering rate, especially the silicate minerals and, as a consequence, high Al concentrations (> 10 mg.l − 1 ) are obtained. Four sand pit lakes and four wells located close to each other were sampled from November 2005 to March 2007. The groundwater physico-chemical parameters and the dissolved species in the sand pit lakes are controlled by sand extraction process as well as the rainfall regime. During dry season the reduced sediments become exposed by the water table drawdown, promoting oxidation which releases H + and other ions present in those lithologies. The increase of water table during wet season induces lithologies lixiviation, increasing the concentration of dissolved species in groundwater. However it also promotes a decreasing of pH values due to the diluting factor induced by rainwater infiltration. During dry season, the sand pit lakes have high pH values due to buffering action caused by the dissolved species present, leading to the ion shielding for H +, caused by evapoconcentration process (high ionic strength for this season). The evapoconcentration conditions along that season promote the precipitation of some complex salts of Na +, Mg 2 + , Cl − and sulfate, decreasing the concentrations of major dissolved species. The opposite occurs during wet season, where pH values increase due the rainwater input, breaking the ion shielding (lower ionic strength) and dissolving some precipitated compounds (increasing the concentrations of major dissolved constituents). PHREEQC and WATEQ4F modeling were used for sand pit lakes and groundwater samples in order to show the Al behavior in these specific environments, which could be a limiting factor for an aquaculture activity towards the end of mining activities. The sand pit lakes showed Al 3 + as the main dissolved species in both dry and wet season, while groundwater presented Al(OH) 2 + and Al(OH) 2 +, which are the most toxic species for aquatic life. The dissolved aluminum concentration in the studied environments was considered by the minerals formation, which the SO 4 2 − salts complex are the main Al scavengers, followed by the silicate minerals in both seasons. The information provided by this study gives an insight to the conditions of aquaculture in sand pit lakes, having peculiar characteristics. On the other hand, the dissolved aluminum should be considered in groundwater due to the possible water contamination, considering that wells used for water consumption are prevalent in the region.