Hydrological connectivity between riparian groundwater and streamwater system in Kishanganga Basin: insights from solutes and stable water isotopes

Riparian groundwater plays a critical role in sustaining river baseflow, supporting ecosystems and ensuring water availability in the hydrologically dynamic regions of the Himalaya. However, hydrological studies in many transboundary catchments like Kishanganga Basin (KGB) remain unexplored due to the security and logistic reasons. In the present study, a comprehensive assessment of the hydrochemical characteristics, solute sources, water quality and stable isotopes of riparian groundwater and streamwater was conducted to understand the major geochemical processes controlling solute sources, hydrochemical evolution and hydrological connectivity. The results suggested that the groundwater chemistry is primarily controlled by water-rock interactions with Ca-Mg-HCO₃ type water facies revealing carbonate weathering as a dominant process. Geochemical modeling suggested that groundwater is undersaturated with respect to calcite, dolomite, gypsum and fluorite that facilitates mineral dissolution, while saturation with Fe(III) oxides suggests localized precipitation of ferrous minerals. The regression line of streamwater (δ²H = 6.8 × δ¹⁸O + 18, R² = 0.97) and groundwater (δ²H = 7.2 × δ¹⁸O + 22, R² = 0.97) revealed minor variation in slope, attributed to subtle differences in fractionation processes influenced by local environmental factors that promote evaporation of streamwater. The isotopic similarity between streamwater and riparian groundwater underscores a high degree of hydraulic connectivity. Contaminant assessment highlights arsenic (As) and fluoride (F⁻) as main geogenic pollutants in a few wells and springs, with potential health risks for local populations, particularly infants and children. The Water Quality Index (WQI) and irrigation suitability indices suggest that groundwater is fit for drinking and agricultural use, except at few locations with elevated As and F⁻ concentration. Self-organizing maps (SOM) and Pearson correlation analyses further illustrate the spatial variability of hydrochemical parameters and contamination hotspots. The findings emphasize the need for sustainable groundwater management strategies to mitigate contamination risks and ensure long-term water security in the KGB.