Characterization of low enthalpy non-volcanic geothermal systems at Atri and Tarbalo, Eastern Ghats Province, India: An integrated isotope-geochemistry-geothermometry studies and geochemical modelling

Hot springs in the stable Indian shield are non-volcanic in origin. Atri and Tarbalo are two such hot springs in the Eastern Ghats Province (EGP), Eastern India and these are characterized as part of a low enthalpy geothermal system. Stable isotopic, geochemical and geothermometric studies were carried out on these two hot springs as well as on the groundwater of this region to understand the origin and evolution of these non-volcanic hot springs as well as subsurface water system in terms of the source of the dissolved solute in the water, mixing processes and the residence time of the thermal and non-thermal waters. Surface temperature of the slightly alkaline hot spring waters ranges from 45 to 58 °C. Temperature of the cold groundwater, collected from tube and dug well varies between 28 and 32 °C. A distinct hydro-chemical difference can be interpreted from the major ion concentrations of hot waters and non-thermal waters. Hot spring waters have higher concentrations of sodium, potassium and lower calcium, magnesium than cold water. While the hot springs waters are enriched in Cl^- and F^- and cold waters are rich in bicarbonate. The low bicarbonate concentration of thermal waters may indicate that the hot spring reservoirs have no atmospheric effect. Definite geochemical differences between these two types of water suggested that there is no mixing between hot spring water and cold groundwater. Thermodynamic calculations suggest that mineral dissolution is the predominant evolutionary mechanism for the thermal and non-thermal waters and these waters hold a partially equilibrated state with the surrounding rocks. Bivariate plots of the major ions also indicate that silicate weathering is the dominant mechanism controlling solutes concentrations in the cold water whereas evaporite dissolution more likely involves in the evolution of hot spring water. The measured stable isotope ratios (δ²H and δ¹⁸O) of all the hot and most of the cold-water samples plot along the Global Meteoric Water Line (GMWL), indicating their meteoric origin where as some cold waters show evaporation effect which suggests atmospheric influence. Tritium and ¹⁴C ages indicate that the cold waters are relatively modern, while the hot waters have a longer residence time of about 5000 years. Based on the chemical characteristics of the hot waters Na-K thermometer, Na-K-Ca thermometer and silica (quartz) thermometer were used to estimate the reservoir temperatures. Cation and silica geothermometers yield similar estimation of the reservoir temperature between 125 -150 °C for hot spring waters. Results of geochemical (numerical) modelling of water-rock interaction in this region, using PHREEQC, are consistent with hydrochemical analysis. Inverse modelling and saturation indices of minerals indicate that water chemistry in this region is controlled by the dissolution of feldspar and saturated with kaolinite, gibbsite and fluorite. This equilibrium is attained in the thermal waters, which therefore show a more restricted range of composition than the non-thermal, colder waters. The higher fluoride concentration in the thermal water may also be attributed of chemical equilibrium with the enclosing host rock.