Seasonal variations in the isotopes of oxygen and hydrogen in geothermal waters from Bakreswar and Tantloi, Eastern India: implications for groundwater characterization

Abstract

δ¹⁸O and δD values of non-thermal and thermal waters from the Bakreswar and Tantloi geothermal areas indicate a meteoric origin for the thermal springs. Both non-thermal and thermal waters undergo seasonal isotopic variations, being enriched in ¹⁸O and D in the winter relative to the rainy season. For non-thermal waters, this is due to a higher rate of evaporation in winter as a consequence of depleting atmospheric humidity; the effect is more pronounced for surface waters than groundwaters. The degree of variation is, in general, very low for thermal waters. The difference in issuing temperature, as well as small variations in the stable isotopic composition and tritium content of the springs, are suggestive of very low to moderate dilution of thermal waters by nearby shallow groundwater. Agnikund, the hottest spring (71 °C), is least diluted (10%) by local non-thermal waters and hence is the best representative of deep-seated thermal water. The presence of very low amounts of tritium (0.6±0.5 to 1.9±0.5 TU) in thermal waters indicates their long subsurface circulation time (>50 years). Similar δ¹⁸ O and δD values for Bakreswar and Tantloi spring waters point to their origin from the same meteoric water and therefore they are genetically related. Recharge of the springs is likely to take place somewhere in the vicinity of Gondwana basins occurring along the central part of the Chotanagpur plateau.