An integrated geochemical and hydrological approach for the conceptualization of surface-water/groundwater interactions in a transboundary river basin of the western Himalayas

Surface water–groundwater (SW–GW) interactions at the basin scale are critical for effective water resource management but remain poorly constrained in Himalayan river systems. This study integrates hydrogeochemical (major and trace elements), isotopic (²H, ³H, ¹⁸O), and hydrogeological data to investigate water origin, residence time, hydrochemical evolution, and SW–GW connectivity in the transboundary Upper Jhelum River Basin (UJRB), western Himalayas. Hydrogeochemical facies analysis reveals that recharge waters (RW) and shallow groundwater (SGW) are dominated by Ca²⁺–Mg²⁺–HCO₃⁻ facies, while deep groundwater (DGW) evolves towards Ca²⁺–Na⁺–HCO₃⁻ facies, reflecting prolonged water–rock interaction. Seasonal variability highlights the influence of aquifer residence time and localized anthropogenic inputs on water chemistry. SW–GW interactions are evident in the transition from Ca²⁺–Mg²⁺–HCO₃⁻ in tributary waters to mixed facies in groundwater, indicating active recharge and subsequent mineral dissolution. Mixing model results (δ¹⁸O and EC) show that groundwater is the dominant contributor to river baseflow, with contributions of 66 % ± 7 % in winter and 39 % ± 10 % in spring. River gaining conditions were identified along the alluvial and lacustrine plains, while localized losing stream conditions occurred near mountain front zones. Water–rock interactions, confirmed by Gibbs plots, govern basin hydrochemistry. Carbonate dissolution, gypsum dissolution, and silicate weathering are the primary processes, while Na-silicate weathering from the Panjal Traps shapes tributary chemistry. Ion exchange (Ca²⁺–Na⁺ and Mg²⁺–Na⁺ substitutions) further modifies groundwater composition along flow paths. Anthropogenic impacts, including wastewater infiltration and agricultural runoff, contribute to elevated Cl⁻, SO₄²⁻, and trace metal levels in specific zones. Evaporation effects are limited but elevate TDS locally. Glacier meltwater, characterized by Na⁺–Cl⁻–SO₄²⁻ facies, reflects atmospheric deposition and plays a minor hydrochemical role. These integrated findings underpin a conceptual flow model demonstrating how lithology, recharge dynamics, and anthropogenic pressures collectively shape SW–GW interactions. The results provide critical insights for managing transboundary Himalayan aquifers and sustaining river baseflows essential for regional water security.