Sustainable River-Basin management under monsoon variability: Spatio-temporal hydrochemistry and water-quality suitability in the Kankai Basin of Nepal Himalaya
River-water quality is deteriorating globally due to intensified land use, expanding wastewater inputs, and climate-driven hydrological variability that reshapes sediment and solute transport. This study investigates these challenges in the Himalayan Kankai River Basin in Nepal, which was selected because it is a monsoon-driven Himalayan–Terai River system with strong elevation contrasts and major irrigation dependence. The objectives were to characterize seasonal surface-water hydrochemistry, identify controlling mechanisms, map spatio-temporal hotspot behavior, assess drinking and irrigation suitability, and develop a Sustainable River-Basin Management Strategy (SRBMS). Surface-water chemistry was evaluated from 76 samples (38 pre-monsoon and 38 post-monsoon) using field and laboratory analyses of physicochemical variables and major ions. These analyses were supported by Geographic Information System (GIS) mapping, multivariate statistics, and hydrochemical diagrams. Water was slightly alkaline (pH 6.3–8.9 pre-monsoon; 6.7–9.0 post-monsoon), and dissolved oxygen was low (1.9–4.6 and 2.3–4.5 mg/L). Turbidity peaks increased after the monsoon (144.74 to 319.66 NTU), and ammonium maxima rose from 23.59 to 39.76 mg/L, indicating stronger post-monsoon stress in hotspot areas. Ca–Mg–HCO3 facies dominated (72.3% pre-monsoon; 56.3% post-monsoon), while Gibbs plots showed rock-weathering control. Water Quality Index (WQI) was predominantly unsuitable (76.31% of pre- and 86.84% of post-monsoon samples) for drinking. Irrigation screening indicated low sodium hazard with excellent Sodium Adsorption Ratio (SAR) values, but magnesium hazard (MH) was frequently unsuitable. The proposed SRBMS and the integrated workflow provide actionable, season-aware evidence applicable to other monsoon-affected mountain plain basins facing similar conditions worldwide.
