Source Apportionment and Ecological Risk Assessment of Metal Elements in the Upper Reaches of the Yarlung Tsangpo River

Heavy metal (HM) pollution in the southern Tibetan Plateau has attracted global attention. Prior studies have noted HM enrichment and water issues in Tibetan rivers, but seasonal variation, sources, and controlling factors remain unclear. This study measured HM levels in high-frequency river water and suspended particulate matter (SPM) at the Lhaze on the Yarlung Tsangpo River (YTR), assessing pollution and ecological risks. The results showed that the overall surface water quality was excellent. The SPM overall showed a low potential ecological risk. Nevertheless, pollution risks were observed for As and B in river water samples during the dry season. Additionally, As and B were found to be in moderate-to-heavy pollution levels for SPM samples, and there was a moderate potential ecological risk for As during the dry season. The source identification results revealed geothermal spring input as the primary factor contributing to the ecological risks of As and B in the YTR water. While rock weathering dominates the origins of Al, Mn, and Fe in river water, with contributions ranging from 64% to 90% of their total amounts, water availability during weathering reactions in the dry and wet seasons serves as the primary control factor for their release, mobility in the YTR basin, and concentration in the river water. As an erosion product, SPM exhibited no significant seasonal changes in metal element concentrations and showed a moderate correlation with water discharge, indicating a stable HM ecological impact from the erosion process in the YTR basin.