Use of major ions to evaluate groundwater chemistry and identify hydrochemical processes in a shallow coastal aquifer in southeast Brazil

Abstract

Coastal aquifers of southeastern Brazil have a fundamental role in economic and social terms, as it is a densely populated region with intense economic activity. The present research was carried out in the coastal region of the State of Rio de Janeiro and enclosed the Brazilian Nuclear Power Plants (NPP) site. The study aimed to assess the hydrochemistry of the shallow aquifer at the NPP's site and surroundings. The hydrochemical study was carried out based on data from 12 water samples, collected in October 2018, in which parameters were measured in situ (such as electrical conductivity, total dissolved solids, temperature, and pH) and in the laboratory (such as alkalinity; Ca2+, Mg2+, Na+, K+; Cl−, SO42−, and silicon). Using Piper and Chadha diagrams, the research identified four water types: Group 1 (Ca2+–Na+–HCO3−), Group 2 (Ca2+–HCO3−), Group 3 [Ca2+ (Na+ rich)–HCO3−], and Group 4 (Na+–Cl−). From the hillsides towards the beach line, the research results suggest a hydrochemical evolution of local groundwater from Group 1 (high rainwater influence), passing through Group 3 (intermediate between Group 1 and 2) and Group 2 (rock influence), to Group 4 (seawater like). The waters of groups 1 to 3 showed potential for base cation exchange (softening or freshening process), while, in Group 4, the potential is for reverse cation exchange (hardening or salinizing process). In a general hydrogeology context, the methods and applied techniques proved to be particularly efficient in identifying potential hydrochemical processes. Noteworthy is the freshening cation exchange identified in the shallow aquifer at NPP’s site. Therefore, theoretically, in case of an accidental release of radionuclides from the NPP to the aqueous phase, the local clay lenses can capture the divalent cation Ra2+ in a natural contamination attenuation process.