Stable isotopes (H, O, S) signatures evidencing evolutionary trends of Brazilian spas groundwaters

Abstract

This paper reports a stable isotope (H, O, S) study of groundwater samples (25) in 10 spas from southeast Brazil. They are cold (25 °C, 36%), hypothermal (25–33 °C, 60%) and mesothermal (33–36 °C, 4%) waters; and are predominantly composed of bicarbonate, carbonate and sodium. Dissolved sulfate is the dominant anion in one water sample and its concentration increased with increase in pH, electrical conductivity (EC) and total dissolved solids (TDS) values in the samples. 2H and 18O data reported in the literature for circa 700 rainwater samples collected in São Paulo State and Brasília airport, Brazil, were used to construct the following regional meteoric waterline (RMWL): δ2H V-SMOW (‰) = 8.06 δ18Owater V-SMOW (‰) + 12.85. The HO isotopes of the studied groundwater samples show a δ18Owater variation of −14.1 to −5.3‰ V-SMOW, whereas the δ2H range was −66.5 to −31.7‰ V-SMOW, which was plotted on or near the RMWL, suggesting a meteoric origin for them. However, one hypothermal water sample (29 °C) plotted away from this RMWL, and is from a deep fractured aquifer which mainly occurs in fenitization aureole rocks surrounding the carbonatite complex at Araxá spa, Minas Gerais State, and a magmatic hydrothermal water may contribute to this water sample. The use of both δ34Ssulfate and δ18Osulfate values in the selected sites permitted the plotting of a mixture line of the dissolved sulfate in the different aquifer systems of this study, considering two major sources as endmembers: sulfide oxidation, SOX (δ34Ssulfate = +1‰ and δ18Osulfate = +1‰) and sulfates possessing δ34Ssulfate = +9‰ and δ18Osulfate = +21‰ (OSW). This is an unusual isotopic composition opposite from that of sea water. The δ34Ssulfate and δ18Osulfate values tend to increase with increase in dissolved sulfate concentration. The OS isotopes signatures that deviated from the general mixture line could be due to several processes, including atmospheric deposition, SO2 oxidation in the atmosphere, soil-derived sulfate, sulfide oxidation, evaporites dissolution and the presence of dissimilatory sulfate-reducing bacteria. Three sulfate isotope composition of spring waters from Águas da Prata spa in the Poços de Caldas alkaline massif (PCAM) represented endmember sources that defined a triangle around the remaining PCAM samples, as well as the samples from the crystalline basement, São Francisco craton, and Alto Paranaíba igneous province (APIP). Such triangle allows for estimation of the relative isotopic contribution in the different spas groundwaters, which is a new approach that focuses on the use of δ34Ssulfate-δ18Osulfate pair in environmental studies, and shows its usefulness in addition to other conventional hydrogeochemical fingerprints.