Isotopic Characteristics of East Indian Geothermal Water

Abstract

Long term sustenance of a geothermal power plant depends largely on the availability of geothermal water, which in turn is related to its origin. In this study, an attempt has been made to determine the origin of geothermal waters occurring in the eastern part of India. For this purpose the springs located in the Indian states of West Bengal, Jharkhand and Odisha were considered. New isotopic data (δ18O, δD) for the thermal area of Odisha, including thermal water, groundwater, river water, cold springs, and precipitation events were obtained for this study. Oxygen and hydrogen isotopic ratios demonstrate that the thermal water of Odisha is of local meteoric origin. Some of the thermal water samples show substantial evaporation. The absence of a significant oxygen shift and relatively low TDS values in the thermal water indicate a limited rock-water interaction. Six of the total eight thermal spring areas of Odisha are located along the northern and the southern boundary of the Mahanadi graben. Their position along the graben boundary helps the surface water to percolate to greater depths at a faster rate without providing sufficient time for rock-water interaction. Isotopic composition of the two precipitation events measured from the Odisha region fits well the Local Meteoric Water Line (LMWL) defined from the GNIP station at Sagar, indicating that the line may also be used for the Odisha region. However, some of the thermal water follow the Global Meteoric Water Line (GMWL) demonstrating that more precipitation samples from the Odisha region are needed to describe in detail the Odisha local precipitation line. The thermal springs of the state of West Bengal and Jharkhand are highly saline, however, their isotopic signatures clearly indicate that the thermal water is of meteoric origin, and higher salinity is attributed to prolonged rock-water interaction. The isotopic signature of the non-thermal waters in these regions show a negative shift from the GMWL indicating that the non-thermal water has a significant fraction of recycled moisture.