Mixing hydrology and chemical equilibria in Bakreswar geothermal area, Eastern India

A group of alkaline thermal springs of Bakreswar, having varying temperatures (45–71 °C) and identical compositions emerges through a nearly N–S trending fault in the gneissic basement. The thermal springs are of Na–Cl–SO4 type and are rich in SiO2 and F, contrary to the local groundwaters which are mostly of Ca–HCO3 type. The springs undergo mild to moderate dilution by local groundwater, which does not significantly affect their original compositions. Agnikund, the main spring, with the highest temperature, mild dilution, consistent composition and negligible tritium, discharges almost pure deep-seated thermal water. Original thermal water is at equilibrium with quartz in the deep reservoir and, attains surface temperature largely by conductive cooling with no significant loss of dissolved SiO2. Certain ionic ratios coupled with very low tritium contents in the springs are indicative of prolonged subsurface circulation of the original water. XRD results of the spring site sediments and the bore hole rocks fairly corroborate the predicted saturation state of thermal water with respect to quartz, feldspar, calcite, fluorite etc., which is believed to control the major chemical compositions of the springs. A geothermal reservoir having probable temperature of about 100 ±5 °C and possibly occurring at a depth of 1 km is the source of thermal water for the springs. The present study is aimed at understanding the mixing trend between thermal springs and local groundwaters as well as the water–rock equilibria.