Hydrochemistry and pollution status of the Bhotekoshi: a high gradient Himalayan river in Sindhupalchok District, Nepal.

This study evaluated the hydrochemistry, water quality, irrigation suitability, and pollution status of the Bhotekoshi River, Sindhupalchok District, Central Nepal. Water samples were collected from 31 sampling sites along the Bhotekoshi River and its tributaries during the pre-monsoon (April 2024) and post-monsoon (November 2023). The river exhibited slightly alkaline water with a mean pH of 7.9. The dominant order of major cations during both the pre- and post-monsoon was Ca2+ > Mg2+ > Na+ > K+. In the pre-monsoon, the dominant anionic order was HCO3- > Cl- > SO42- > NO3-, whereas it shifted to HCO3- > SO42- > Cl- > NO3- in the post-monsoon. The pH, Cl-, HCO3-, DO, total hardness (TH), NH3, Mg2+, biological oxygen demand (BOD), and chemical oxygen demand (COD) exhibit significant seasonal variations, while EC, Na+, K+, PO43-, NO3-, SO42-, and total dissolved solid (TDS) were relatively stable across the seasons. Multivariate and geochemical analysis demonstrate that river chemistry is primarily controlled by rock-water interaction, dominated by carbonate dissolution followed by silicate weathering. Based on the weighted arithmetic water quality index (WQI), river water quality is classified as good (WQI = 26.43) in the pre-monsoon and excellent (WQI = 23.88) in the post-monsoon. Irrigation indices confirm that the river water is highly suitable for agricultural use. Comprehensive pollution index (CPI) is 0.30 in the pre-monsoon and 0.24 in the post-monsoon, indicating sub-clean status of the river. Organic pollution index (OPI) reveals an excellent status of the river with the values of -0.33 during the pre-monsoon and -0.13 during the post-monsoon, suggesting minimal anthropogenic loadings. Eutrophication index (EI) of 13.08 suggests that the entire river is in an oligotrophic state, despite localized pre-monsoon mesotrophic shifts at urban sites. Hydropower development, road network expansion, sand mining, improper waste disposal, and rapid urbanization along the bank of the river are major anthropogenic stressors that impact the water quality in the Bhotekoshi Watershed. These findings provide a vital baseline for environmental monitoring and suitable river basin management in rapidly developing Himalayan basins.