Article ID Journal Published Year Pages File Type
4457775 Journal of Geochemical Exploration 2012 16 Pages PDF
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 SO4 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+, Mg2 +, 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 Al3 + 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 SO42 − 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.

► This study shows water acidification by sand mining. ► The acidification process can influence directly the surround groundwater. ► The high concentration of Al in those waters is the great concern in this region. ► Assessment of Al dissolved species and its reactive minerals formation. ► Water components behavior has great influence by the studied region pluviosity.

Related Topics
Physical Sciences and Engineering Earth and Planetary Sciences Economic Geology
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