Stable isotope and geochemical evidence on sources and mechanisms of groundwater recharge in the Nalanda-Rajgir Region of Eastern India

Relentless extraction of groundwater has led to falling water levels and widespread concerns over the long-term sustainability of extensive groundwater-based irrigated agriculture in India. Hence, effective water management strategies must be developed with a thorough understanding of groundwater recharge. Here, we assess the source, recharge mechanism, and hydrogeochemical evolution of groundwater in the Nalanda-Rajgir region of eastern India using stable isotopes, physico-chemical parameters, and multivariate statistics. Based on major ion chemistry, ~ 40% of the groundwater samples are categorized as Ca2+-Mg2+-HCO3− type, characteristic of active recharge zones. Therefore, groundwater in these regions has short residence time and hence limited water–rock interactions. The other major group (~ 45%) of groundwater samples is of Na+-HCO3− type, which indicates the dominance of ion-exchange processes. The distribution of δ18O in groundwater ranges from − 10 to − 6.9 ‰ and that of δ2H from − 57.9 to − 40.4 ‰. While the hot spring sample is characteristically depleted in heavier isotopes, the remaining samples have been categorized into two groups based on their isotopic composition. Group I samples have a slope similar to that of the Global Meteoric Water Line (GMWL) and signify rapid recharge of rainwater. In contrast, group II samples with evidence of extensive evaporation due to prolonged exposure to the atmosphere represent water retained from irrigation return flow in agricultural fields. Hierarchical cluster analysis further corroborates that the association of major ions and isotopes in different clusters primarily indicates a largely meteoric source of groundwater that has been significantly influenced by human intervention.