The development of the geothermal resources in India has not progressed mainly due to inadequate under-standing of the deeper thermal regime and uncertainty over the sustainability of heat source. In this study, isotopes, hydrochemical techniques and integrated geothermometric models have been applied to a low-medium enthalpy geothermal system belonging to shear zones of Mahanadi graben, Eastern Ghats Mobile Belt (Eastern India) to gain better insights. Isotope results indicate that thermal waters are meteoric in nature and absence of delta O-18-shift suggests that the subsurface temperatures are below 200 ?. Distinct isotopic and hydrochemical signatures of thermal waters clearly suggest their origin from different geothermal reservoirs. Thermal waters at Atri/Tarabalo sites are derived from the southern part of the graben while at Athmalik, from northern part. Away from graben in southern direction (Taptapani site), the thermal waters are derived from rain occurring at elevated regions. Seasonal variations in chemical and isotope data are minimum in thermal waters. The estimated reservoir temperatures range between 92 and 124 ? with reservoir depths 1.6 - 2.4 km. Tritium data suggests that thermal waters are old and have 12-63% mixing with non-thermal water. The modelled radio-carbon age of thermal waters (13.3 - 20.7 Ka) suggests paleo-recharge of the reservoirs during Last Glacial Maximum (LGM), characterized by arid climate and low rainfall. A conceptual isotope model illustrating the genesis and circulation patterns of thermal and non-thermal waters is presented. The study highlights the potential of isotope-based studies in expanding the geothermal energy utilization in India.