Isotopic signatures of mountain block recharge and groundwater flow paths inferred from EMMA analysis in a tropical mountainous river basin draining the Southern Western Ghats, India

Groundwater is a critical source of drinking water in arid and semi-arid regions of developing countries. Understanding groundwater recharge sources and mechanisms is crucial for sustainable resource management in a changing climate.  Mountain block recharge (MBR) plays an important role in sustaining groundwater in downslope and valleys; however, distinguishing MBR from local and shallow recharge processes remains complex. This study investigates the role of MBR in a tropical river basin- the Upper Bhavani River Basin, draining the Southern Western Ghats, India, using stable isotopes of oxygen (δ¹⁸O) and chloride (Cl^-) as conservative tracers. Groundwater samples collected from confined and unconfined aquifers were analyzed to understand spatial variability in isotopic and geochemical signatures across the basin. End-member mixing analysis (EMMA), applied to normalized δ¹⁸O and Cl^- data, indicates that groundwater in the basin results from conservative mixing among three conceptual recharge components: mountain-front recharge (MFR), mountain-block recharge (MBR), and front-slope recharge (FSR). Most samples plot within the defined mixing space, supporting the assumption of conservative tracer behaviour and the applicability of EMMA in this hard-rock setting. The results suggest that focused recharge at mountain fronts and shallow recharge along slopes play a dominant role in sustaining groundwater resources, while deep mountain block recharge exhibits a comparatively limited influence. Confined aquifers commonly display elevated total dissolved solids, reflecting prolonged subsurface residence times and enhanced water–rock interaction within the fractured crystalline aquifer, which indicates a contribution from deep mountain block flow. Overall, this study highlights the importance of shallow and focused recharge processes in mountainous hard-rock terrains and demonstrates the value of isotope-based end-member mixing analysis in understanding groundwater recharge mechanisms and flow paths. The findings provide valuable insights for groundwater resource management and protection strategies in water-stressed mountainous regions.