Synthesis analysis of hydrogeochemistry of nepal himalayan rivers: Perspective from major ions and trace elements

Rivers originating from the Himalayas play a crucial role as primary water sources for a significant proportion of the population in Asia downstream. This paper presents a comprehensive synthesis of hydrogeochemical data, incorporating major ions and trace elements from 153 papers spanning 2000 to 2023, with a detailed analysis of 20 papers specific to Nepal Himalayan rivers. To bridge the research gap due to sparse data, we aimed to enhance inclusivity by amalgamating both seasonal and one-time datasets, computing mean values for singular instances, and employing grand means for multiple seasonal data points. Our findings highlight varied research attention across different river basins, with the eastern Koshi Basin receiving extensive attention, while the middle Gandaki and Bagmati Basins garner moderate attention, and data scarcity observed in the western Karnali Basin. Predominant ionic composition, notably Ca2+ and HCO3–, reflect lithological influences, primarily driven by rock weathering. Although anthropogenic impacts remain minimal in most basins, certain lowland areas with higher population densities exhibit increasing significance. Trace element contamination, notably Cd, Cu, Sr, and Zn, poses ongoing threats, both from natural and anthropogenic sources. Water quality assessments conducted for agricultural and drinking suitability indicated good status, except in the densely polluted urban Bagmati stem. Health and ecological risks are low, as determined by pollution indices such as Pi, NPI, HI, and CI. The Bagmati River's middle segment, with high population density and moderate agricultural activity, shows pronounced anthropogenic footprints, while downstream areas exhibited greater impacts. Additionally, this article proposes the potential use of a Water Quality Index tool to comprehensively assess water quality across all parameters in Himalayan water bodies, providing a roadmap for future environmental conservation in the Himalayas. This pioneering review represents a significant effort to understand Himalayan rivers geochemistry on a large geographical scale and emphasizes the need for integrated observation and consideration of river water quality across the entire Himalayan region, from high mountains to lowlands.