Origin of salinity and hydrogeochemical features of porous aquifers from northeastern Guanabara Bay, Rio de Janeiro, SE - Brazil

Abstract

Study Region: Porous aquifer system of Northeastern Guanabara Bay, Rio de Janeiro, Brazil. Study Focus: The present work aimed to comprehend the geochemical processes responsible for the considerable range of salinity (48 to 5651 μS. cm−1) through chemical composition of groundwater (hydrogeochemical modeling through PHREEQC) allied to chemical ratios (Cl/Br ratio) and stable isotopes data (δ18O and δ2H). New hydrological insights for the region: The PHREEQC modeling showed that high pH and low pe values conditioning the main processes controlling the hydrogeochemical evolution of groundwater in that region. The salinity origins should be explained by 4 hypotheses: 1) a group related to recharge zones, close to the basin headboard or connected to the fractured aquifers from the basement rocks (low Cl/Br ratio and predominance of light δ18O and δ2H isotopes; 2) a group formed by groundwater with high Cl/Br ratio and predominance of heavy δ18O and δ2H isotopes, associated to dissolution processes of Tertiary brackish water environment sediments; 3) a group formed by groundwater with low Cl/Br ratio, high Cl− concentrations and low δ18O and δ2H, related to groundwater under influence of Caceribu River (high content of domestic effluents); and 4) a group composed by groundwater with high salinity, high Cl− concentrations and enrichment of δ18O and δ2H, located at a mangrove area, where the influence of seawater intrusion in the aquifer is recognized. Keywords: Chloride/bromide ratio, PHREEQC hydrochemical modeling, Guanabara Bay porous aquifer, Regional hydrogeochemistry, Tertiary/quaternary sediments