Surface-groundwater interactions and recharge sources in the upper Yamuna River basin: Insights from stable isotopic signatures and hydrogeochemical processes.

The interaction between surface and groundwater is vital for sustaining the hydrological system and ecosystem balance. This study investigates river water-groundwater connectivity and recharge sources in the upper Yamuna River basin (UYRB) by integrating stable isotopes (δO and δH), hydrogeochemistry, and statistical analyses. Spanning the Himalayas and alluvial plains, the UYRB presents diverse topography, climatic conditions and human interferences, making it ideal for exploring spatial and seasonal effects on water resources. A total of 233 samples, including river water (n = 60), groundwater (n = 106), spring water (n = 18), rainwater (n = 35), and fresh snow (n = 14) were collected during 2022 to draw the results. Hydrochemical parameters exhibit a significant (p < 0.05) spatiotemporal influence, with glacial and snowmelt driving summer recharge in the upper catchment (UC), whereas ISM-driven precipitation dominates downstream recharge. River water isotopic composition is strongly influenced by altitude (summer: ILR = -0.36 ‰ per 100 m for δO, R = 0.4, p = 0.0004; winter: ILR = -0.17 ‰, R = 0.5, p < 0.0001), though this effect is less pronounced in rainfall, groundwater and spring water. Distinct ion chemistry characterises the UYRB with Ca.Mg-HCO water types prevail in the upper and mid-catchments, while mixed or saline types dominant in the lower catchment. Groundwater and river water isotopes closely align with the derived LMWL (δH = 7.63 × δO + 7.83; R = 0.99), indicating the signatures of regional precipitation in their recharge. Elevated d-excess values in UC rainfall (x‾ = 12.3 ‰) and snow (x‾ = 18.1 ‰) suggest a combined influence of ISM and WD on regional moisture distribution. The multivariate analyses (PCA and HCA) confirm strong river water and groundwater interactions, particularly in the lower catchment. This study highlights catchment-specific RW-GW interactions and recharge sources in the water-scarce UYRB, providing critical insights for regional climate-resilient water resource management.