Hydrogeochemical controlling mechanism and associated health risk assessment of trace elements in the Koshi River Basin, Central Himalaya

The Koshi River Basin, a transboundary river system with monsoon-driven hydrology, supports more than 15 million people by providing irrigation and potable water resources in Nepal and India. This study aims to evaluate the trace elements (TEs) spatiotemporal variation, identify their natural and anthropogenic sources, assess hydrological controls, and estimate human health risks. A total of 44 surface water samples were collected during dry and rainy seasons, and 17 dissolved TEs were measured in the laboratory. The Spatial Processes in Hydrology (SPHY) model was used to quantify contributions of glacier melt, snowmelt, base flow, and rainfall to the total runoff at each sampling site. The study revealed elevated concentrations of Sr, Fe, Li, Cs, Pb, U, As, Cr, and Ni in upstream regions, with most TEs declining during the rainy season. Geogenic weathering was the dominant TEs source in the KRB; however, the downstream enrichment was also linked to anthropogenic activities. Hydrological regimes played a key role: base flow dominated the dry season, enhanced TEs mobilization through rock-water interactions, whereas rainfall runoff dominated the rainy season, causing dilution. Notably, the average concentration of Al (303.73 µg/L) and Pb (10.20 µg/L) in the upstream exceeded the WHO drinking water guideline of Al (200.00 µg/L) and Pb (10.00 µg/L). Although non-carcinogenic risks were negligible, As posed moderate to high carcinogenic risk (up to 1.24 × 10^–3), surpassing the USEPA safety threshold (1.00 × 10^–3). This study provides new insights into the hydro-geochemical mechanisms driving TEs variability in the Himalayan River basin. Since a few potential cancer risks were found in the human habitation downstream area, in-depth water quality monitoring and management strategies focused on mitigating TEs exposure are needed.