An Integrated Isotope-Geochemical Approach to Characterize a Medium Enthalpy Geothermal System in India

The Manuguru geothermal area, situated in the Telangana state, is one of the least explored geothermal fields in India. In this study, the chemical characteristics of the groundwater (thermal and non-thermal waters) are investigated to elucidate the source of the solutes dissolved in the water and to determine the approximate residence time of the thermal waters. The major hydrogeochemical processes controlling the groundwater geochemistry have been deciphered using multivariate statistical analysis, conventional graphical plots and geochemical modelling (PHREEQC). Geochemically different groundwater clusters (bicarbonate type, bicarbonate–chloride type and chloride type) can clearly be identified from the chemometric analysis, i.e. PCA and HCA. Thermal waters are mostly Na–HCO₃ type having low EC and TDS compared to non-thermal groundwaters. Silicate weathering and ion exchange mainly contribute to the dissolved ion budget in the groundwater of the study area. The carbon isotopic composition of DIC (δ¹³C) points to silicate weathering with soil CO₂ coming from C₃ type of plants. Stable isotopes (δ¹⁸O, δ²H) data confirm the meteoric origin of the thermal waters with no oxygen-18 shift. The low tritium values of the thermal water samples reveal the long circulation time (>?50?years) of the recharging waters. Radiocarbon dating (¹⁴C) shows that the approximate residence time of the thermal waters ranges from 9952 to 18,663?year BP (before present).