How Sand Mining is Shaping the Trishuli River in the Himalayas of South Asia

Abstract The Trishuli River in Nepal plays a vital role in hydropower generation, irrigation, fishing, and recreational activities. However, human activities such as sand and aggregate extraction, as well as agricultural practices, have been found to have a detrimental impact on the river. A comprehensive study was performed to assess the influence of these activities and natural processes on the river’s hydrochemistry and overall water quality. Water samples were collected from 42 sites along the river, and 17 physicochemical parameters were thoroughly analyzed. The findings revealed that activities such as sand mining and agriculture significantly contributed to the river’s high turbidity levels, PO 4 3− , and TSS. The study also indicated that the river was moderately polluted based on the average Nutrient Pollution Index value. Moreover, the Water Quality Index suggested poor water quality at upstream sites and very poor water quality at downstream and mining sites due to elevated levels of TSS, turbidity, PO 4 3− , and NO 3 − . The assessment also concluded that the water from most sampling sites was suitable for irrigation based on the US salinity diagram and various irrigation indices. Additionally, the water was characterized as Ca-HCO 3 dominant according to the Piper diagram, and the river’s hydrochemistry was mainly influenced by rock weathering, as indicated by the Gibbs diagram. In summary, the study concluded that while the downstream region of the Trishuli River is not suitable for drinking water due to negative impacts from sand mining and agriculture, it is ideal for irrigation purposes.