Stable isotopes, carbon-14 and hydrochemical composition from a basaltic aquifer in São Paulo State, Brazil

Abstract

The Cretaceous Serra Geral Aquifer (SGA) is contained within one of the largest continental flood basalts in the world, reaching a thickness up to 1700 m in the center of the Parana Basin. The SGA is one of the most important groundwater reservoirs in northeastern Sao Paulo State (Brazil), responsible for water supply to cities and agriculture. In order to evaluate the geochemical and isotopic evolution of SGA, as well as to determine the mean residence time, a groundwater sampling campaign was carried out over the SGA in Sao Paulo State (Brazil) from January to April 2013. Two main hydrochemical facies were recognized: Ca–Mg–HCO3 related to water–rock interaction reactions in basaltic outcrop, such as mineral dissolution due to atmospheric CO2 uptake, and alkaline Na–HCO3 groundwater, evolved from mixing with groundwater from the underlying Guarani Aquifer System. Stable isotope (δ18O and δ2H) ratios range from −8.87 to −5.32‰ VSMOW and −61.31 to −31.64‰ VSMOW, respectively, closely following the GMWL. Spatial and temporal variations in isotope ratios are associated with the South Atlantic convergence zone activities and the type of rain responsible for recharge. Values for δ13C vary from −21.53 to −7.11‰ VPDB, while 14C activities vary from 1.2 pcm to more than 100 pcm, presenting a trend to enrichment and decrease in 14C activities westward, concordant with the regional groundwater flow direction. Most recent groundwaters have δ13C ratio contents mostly consistent with C3 plants.