Aquifer characterization of Rajgir thermal spring field, Bihar, Eastern India, in the face of climate change and rapid urbanization

Abstract

Rajgir thermal spring field in Bihar, Eastern India, holds international significance due to its historical and multi-religio-cultural importance. Present study attempted to understand the aquifer system to arrest its diminishing discharge in the backdrop of climate change and urbanization. The study identified three aquifer systems, namely fractured quartzite, basement granite, and overlying alluvium, that are working in tandem toward the delicate balance in recharge and discharge in the thermal spring field controlled by the regional fault system. Geological study along with geophysical surveys through gradient resistivity profiling, vertical electrical sounding, and electrical resistivity tomography is carried out to identify local distribution of the fault system. Based on the study, a geological map of the area is prepared followed by preparation of water table contour map. Study of exploratory borehole tapping the fracture zone reveals that the fracture system is under thermo-artesian condition capable of 5 m³/h yield with 40 m drawdown. This translates into transmissivity of about 1 m²/day. Historical rainfall, spring discharge, and groundwater hydrograph of the area are correlated and show that an average time lag of about 20–30 days exists between peak rainfall and peak groundwater level and about 30 days between peak rainfall and peak discharge. It highlights the importance of local recharge for survival of the thermal spring field. Rainfall and potential evapotranspiration data analysis shows that recharge occurs only when annual rainfall exceeds 850 mm, making the variability in rainfall due to climate change a potential threat to adequate recharge. The area is also witnessing growing urbanization which is altering the local groundwater regime. Analysis of historical water level shows that groundwater table declined by about 2 m in the last decade. Comparison with earlier studies is used to identify the changes in groundwater flow pattern in the area. Spatial variation in groundwater temperature is mapped and statistically analyzed. Results show that temperature above 30.8 °C may be considered as samples having signature of thermal water component. It is also observed that thermal water is restricted in some fixed pathways. Discharge measurements are carried out to study diurnal variation and effect of pumping. A set of private borewells, where thermal water is reported, is also studied. Typically, these wells are limited to the weathered part of the rock formation and do not tap fractures. Based on hydrogeological scenario, a local hydrodynamic model is conceived. Further, a management plan is suggested for sustenance of the thermal spring field with potential solution involving establishment of a carefully regulated zone with prohibition of energized pumping. Based on the present study, Government of Bihar has accepted the recommendation and is in the process of implementation.