<p>This study presents a comprehensive hydrogeochemical assessment of the Burhi Gandak Basin, integrating major ion chemistry, statistical analyses, and hydrogeological dynamics. The basin, composed of interbedded sand, silt, clay, and gravel, experienced effective recharge and predominantly shallow groundwater levels, with marginal decline observed in 2019–20 relative to 2013–14, reflecting increasing water demands. Low-gradient groundwater flow enhanced prolonged soil–water interactions, reflected consistently higher EC, TH, and TDS in the calcareous alluvium, contributing to gradual facies maturation. Between 2013–14 and 2019–20, Cl, SO₄, Mg, Na, K, and NO₃ declined while HCO₃ and Ca increased, indicating a shift from silicate weathering to enhanced carbonate dissolution, supported by water–rock interaction analysis; Chloro alkaline indices (CAI) values further suggested increased reverse ion-exchange activities, resulting in a transition from Mg–HCO₃ and Mg–Na–HCO₃ to Ca–HCO₃ and Ca–Mg–HCO₃ facies, with reduced anthropogenic influence. Water quality indices (WQI) indicated marginal improvement for drinking across the basin but slight deterioration in the calcareous alluvium due to elevated TDS, F, and NO₃. Fluoride remained within limits but was influenced by pH, Na/Ca, Ca, TDS, F/Cl, and NO₃. Irrigation suitability showed slight basin-wide improvement, though localized high salinity, sodicity, and magnesium persisted. These findings provide an evidence base for sustainable groundwater management and policy development to ensure safe drinking water, effective irrigation planning, and long-term resource monitoring in the Burhi Gandak Basin.</p>