Isotope-geochemical assessment of thermal waters and their impact on surrounding potable water resources in the Tapi valley geothermal area, Maharashtra, India

Geochemical and isotopic investigations were carried out in the Tapi Valley geothermal area in India to determine the principal factors that are responsible for the geochemical evolution of the thermal and non-thermal waters in the study area. The impact of thermal water discharges on the surrounding potable water resources had also been assessed. It was found that thermal water did not contain any toxic elements (mercury, antimony, arsenic) which are detrimental to human consumption. Thermal waters were found to be less mineralized compared to the non-thermal ground waters. The water types of the thermal and non-thermal waters were found to be mostly Na–Cl–HCO₃ and Ca–Mg–HCO_3, respectively. Silicate weathering and ion-exchange processes were found to be main factors that determined the dissolved solute concentration in the study area. Stable isotopic (δ¹⁸O, δ²H) analysis unequivocally established the meteoric origin of the thermal waters. Thermal waters in Unapdeo, Anekdev, Kundwa and Ratanpura were found to have distinct isotopic compositions signifying that separate reservoirs were feeding these hot springs. The application of chemical geothermometers showed a wide variation in the estimation of reservoir temperature. The quartz geothermometer produced the most reliable reservoir temperature estimate of 110 ± 12 °C. Multicomponent geothermometry was found to better constrain the reservoir temperature. This modelling result indicated that the thermal water in this region had attained simultaneous equilibrium with respect to quartz, aragonite, stilbite, calcite, enstatite, heulandite, diopside minerals in the temperature range of 120 ± 5 °C which could be taken as the subsurface reservoir temperature.