Origin of the chemical composition of São Francisco ultra-fresh thermal water (Itabira region, Minas Gerais, Brazil)

Geochemical studies documented the composition of ultra-fresh thermal water in the Fazenda Giráu, near Itabira (Minas Gerais state, Brazil) for the first time. The studied water is characterized by temperature of 35 °C, very low electric conductivity (approx. 20 μS/cm), uncommon Mg–Ca–HCO3–NO3 hydrochemical type, and an elevated radon concentration (about 500 Bq/L). The presence of tritium suggests the modern age of studied water, contained in highly resistant quartzite-dominated aquifer rocks. Application of a geothermometric approach gave an estimation of the equilibrium temperature in the geothermal system at about 42 °C. Inverse geochemical modelling quantified water-rock reactions responsible for the observed groundwater chemistry. Decomposition of soil organic matter and decay of primary (feldspars, biotites, muscovite) and secondary (montmorillonites, illite) aluminosilicate minerals accompanied by kaolinite and gibbsite formation are the dominant processes forming chemical composition of water. The investigated water has balneotherapeutic potential due its elevated temperature and high concentrations of radon and silicic acid.