Caracterização hidrogeoquímica de águas subterrâneas em aquíferos nos Municípios de Caçapava do Sul e Vila Nova do Sul, RS

Abstract

The groundwater chemical composition is strongly affected by the composition of the rocks that host the aquifer system. The rocks compositional variation increases in the proportion as solubilized ions are incorporated to its molecule by means of organic and inorganic assets. In this sense, the present work performs a hydrogeochemical characterization of groundwater collected in crystalline aquifers located in the region of Caçapava do Sul and Vila Nova do Sul, in the Vacacaí Metamorphic Sequence, Cambaí Complex, and in the Granitic Suite Caçapava do Sul. For the total rock analysis was employed Plasma Source Mass Spectrometry (ICP-MS) at ACTLABs in Canada. The same technique was performed at SGS Geosol Laboratórios LTDA. In addition, atomic absorption spectrometry was performed at the Chemical Analysis of the Federal University of Santa Maria (LAQ-UFSM). Based on the geochemical results and bibliographic data, it was possible to investigate and establish correlations between the chemical signatures of the groundwater and the associated rocks. It was observed that the chemical composition of granitic rocks of the Granitic Suite Caçapava do Sul, exerts considerable influence on the sodium and silicon concentration, probably derived from alkali feldspars. As well, it provides elements typicals from the associated hydrothermal sulphides mineralizations. The marbles of the Cambaí Complex and the Vacacaí Metamorphic Sequence confer a high concentration of calcium and magnesium elements. However, the results show that sometimes the magnesium element may show higher volume in the waters due the influence of the calcitic marbles. Comparisons related to seasonal variations of calcium and magnesium suggest a reduction in the concentrations of these elements during the winter. Only the waters influenced by the calcitic marbles showed increase of calcium in that season.