Assessment of water recharge source of geothermal systems in Garhwal Himalaya (India)

The geothermal systems in the Himalaya are complex, and their genesis, circulation pattern and processes of sustenance are largely unknown. The present study aims to systematically analyse the characteristics of oxygen and hydrogen isotopes, major ion data and strontium ratio (87Sr/86Sr) of geothermal systems, river waters and rainwater to understand the movement of groundwater and the mechanisms for the formation of geothermal systems in the region. Further, the strontium isotope was also used to understand the fluid source of geothermal systems. Field observations show that the geothermal waters have a higher temperature, hydraulic pressure and elevated δD and δ18O values than river waters. Thus, large river systems are not the principal recharge source of geothermal systems. Meteoric water (rain and snowmelt) in high mountains can infiltrate and circulate deep down the active tectonic belts or sutures and recharge geothermal systems. The cold surface water evolves into high-temperature thermal water after deep circulation and is discharged as a geothermal spring at the surface, under a high water-head difference. Therefore, the large-scale geothermal systems in the Garhwal Himalaya develop and are maintained by rapid groundwater circulation and interaction with a heat source. Further, the water temperatures of these systems in Garhwal Himalaya have remained the same over the period 1975–1994 (Geological Survey of India) and 2010–2016 (this study) with an error of < 5%.