Hydrogeochemical study of spas groundwaters from southeast Brazil

•Physicochemical data in groundwaters from well-known Brazilian spas•Relationship between TDS, EC and IS in the aquifers•Enriched concentration levels of silica and fluoride in many water sources•Usefulness of the silica geothermometer in the aquifers investigated

This paper describes a hydrochemical study focusing spas groundwaters occurring at São Paulo and Minas Gerais states, Brazil, that are extensively used for drinking in public places, bottling and bathing purposes, among other. The water samples (75) for this study were taken from springs and pumped tubular wells drilled at different aquifer systems that are inserted in Paraná and Southeastern Shield hydrogeological provinces. The data acquisition for temperature, electrical conductivity (EC), pH, redox potential (Eh), dissolved gases (O2, CO2 and H2S) and alkalinity was in situ performed for avoiding losses and modification due to transportation. The total dissolved solids (TDS) concentration was evaluated by gravimetry, the major cations and anions, iron and silica by colorimetry/atomic absorption spectrophotometry (AAS), and fluoride by potentiometry. The acquired database allowed establish the principal trends among the parameters analyzed after assuring its consistence from expected relationships found in hydrogeochemical surveys. The groundwaters are reducing (from pH and Eh data), there is a direct TDS-EC relationship, implying on a significant correlation between their ionic strength (IS) and EC. The major ions justifying such trends were sodium, (bi)carbonate, chloride, sulfate and phosphate that also correlated positively with the IS. The spas groundwaters were classified according to the guidelines of the Brazilian Code of Mineral Waters (BCMW) and EU directive for mineral waters. The major hydrochemical facies were also determined, as well the main sources influencing the groundwater composition and possible subsurface temperatures.