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

• Published in: Environmental earth sciences 2017-02, Vol.76 (4), p.1-16, Article 150
• Main Authors: Gastmans, Didier, Menegário, Amauri Antônio, Hutcheon, Ian
• Format: Article
• Language: English
• Subjects: Agriculture; Aquifer systems; Aquifers; Basalt; Biogeosciences; Carbon 14; Carbon dioxide; Carbon dioxide atmospheric concentrations; Composition; Convergence zones; Cretaceous; Earth and Environmental Science; Earth Sciences; Environmental Science and Engineering; Evolution; Geochemistry; Geology; Groundwater; Groundwater flow; Groundwater reservoirs; Hydrochemicals; Hydrology/Water Resources; Isotope ratios; Isotopes; Original Article; Outcrops; Rain; Ratios; Residence time; Sedimentary facies; South Atlantic Convergence Zone (SACZ); Stable isotopes; Temporal variations; Terrestrial Pollution; Uptake; Water analysis; Water sampling; Water supply
• ISSN: 1866-6280; 1866-6299
• DOI: 10.1007/s12665-017-6468-1

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.