Dissolved organic matter and water age modulate heavy metal(loid)s distribution in the Yarlung Tsangpo River, Tibetan Plateau.

Heavy metal distribution and ecological risks were of much concern, especially in high-altitude rivers which were sensitive to climate change and human activities. By choosing Yarlung Tsangpo River (YTR) as research object, the spatiotemporal distribution and ecological risks of 14 priority heavy metal(loid)s were comprehensively assessed, highlighting effects of dissolved organic matter (DOM) and water age on sediment-water metal partitioning (log Kd). Aqueous metal(loid)s concentrations were generally low (ND to 376.91 μg/L), whereas sedimentary metal(loid)s concentrations (0.11 ∼ 30817.76 mg/kg) frequently exceeded background values. Remarkably low DOM contents and optical indices (SUVA254, HIX, and BIX) were detected probably due to freeze-thaw alternation, limited microbial processing and the shear effect of ice on solid particles. They further controlled the log Kd of Tl, Co, Ni, Mn, and Se, with aromatic, humic-rich DOM enhancing sediment retention and microbially derived DOM promoting metal mobilization. Relationships of δ3H/δ2H-H2O with DOM/metals, confirmed direct contributions of "old water" (e.g., glacier melting with higher water age) to metal contents (Ni, Se) and indirect influence with SUVA254 as the medium. Ecological risk assessment identified Se in sediments and As in water/sediment as priority pollutants, while health risk evaluation revealed generally low risks except for localized As concerns, particularly for children. Future work should focus on metal sedimentation and migration with the participation of DOM (SUVA254), especially under the combined effect of climate change and hydropower development.