Spatio-temporal variations in river water chemistry and hydrochemical facies of the Kathmandu Valley River System, Nepal

Abstract

Understanding the hydrochemistry of river water is crucial for assessing hydrological processes, water-source contributions, and human impacts in rapidly urbanising basins. The Kathmandu Valley, a tectonic intermontane basin in central Nepal, have a dense river network, including major rivers such as the Bagmati, Bishnumati, Manahara, Dhobi, Hanumate, Godawari, Kodku, Nakhu, and Balkhu. Despite the importance of river systems for local water resources, spatial and temporal chemical investigations of these rivers remain limited. In the current study, 80 sampling sites were selected from nine rivers during the wet (August 2017) and dry (February 2018) seasons. Total 160 water samples were collected, 80 samples from each season. Ions were analysed by ion chromatography. Chemical analysis showed a seasonal shift from Ca²⁺ dominance in the wet season to Na⁺ in the dry season, with significant temporal variation in most ions (p < 0.01). Strong correlations among ions indicated human influence, particularly increased NH₄⁺-N and PO₄³⁻-P levels during the dry season, except in Godawari Khola. Hydrochemical facies analysis revealed that river water was predominantly Ca-HCO₃⁻ type during the wet season, shifting to Na-K-HCO₃⁻ type in the dry season for northern rivers. Conversely, southern rivers largely maintained their original water type. The river systems of the valley were grouped into four clusters by Hierarchical Cluster Analysis (HCA) based on similarity of chemical concentrations. Hanumante Khola exhibited the highest chemical concentrations, indicating a heavily polluted river within the valley during the dry season.