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

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 Paraná Basin. The SGA is one of the most important groundwater reservoirs in northeastern São 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 São Paulo State (Brazil) from January to April 2013. Two main hydrochemical facies were recognized: Ca–Mg–HCO 3 related to water–rock interaction reactions in basaltic outcrop, such as mineral dissolution due to atmospheric CO 2 uptake, and alkaline Na–HCO 3 groundwater, evolved from mixing with groundwater from the underlying Guarani Aquifer System. Stable isotope (δ 18 O and δ 2 H) 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 δ 13 C vary from −21.53 to −7.11‰ VPDB, while 14 C activities vary from 1.2 pcm to more than 100 pcm, presenting a trend to enrichment and decrease in 14 C activities westward, concordant with the regional groundwater flow direction. Most recent groundwaters have δ 13 C ratio contents mostly consistent with C3 plants.