Groundwater chemical characterization of a Rio de Janeiro coastal aquifer, SE – Brazil

Although the inhabitants of the studied area have been complaining of increasing salinity of the water and lowered water table, indicating reduction of groundwater quality, no scientific studies had been carried out to understand the degradation processes. In the present work, a geochemical characterization of the coastal aquifer from the Oceanic Region of Niterói, Rio de Janeiro, Southeast Brazil, was carried out, using hydrogeochemical and isotopic (δ18O) data, correlation matrices and factor analysis. Based on the hydrochemistry, the groundwater was classified into three types: Group 1 (53%) belongs to Na–Ca–Cl–HCO3 facies, Group 2 (20%) belongs to Na–Ca–Cl–SO4 facies and Group 3 (27%) belongs to an intermediate Na–Mg–Ca–HCO3–Cl–SO4 facies. The water geochemical evolution, from crystalline (bicarbonate facies) to a sedimentary aquifer (sulfate facies) seem to be controlled by environmental characteristics. The descriptive statistics and isotopic tracers indicate that the increasing salinity in the shallow coastal aquifer is caused by domestic wastewater and not by marine water encroachment.