Spatial heterogeneity and prioritization of chemical mixtures along the Yarlung Tsangpo River

High-altitude rivers act as unique "sinks" for the global long-range transport of emerging contaminants (ECs). This study employed a suspect screening strategy to investigate the occurrence, spatial patterns, and ecological risks of ECs in the Yarlung Tsangpo River (YTR), the highest major river on the Tibetan Plateau. In this study, 41 samples were collected along a 2000 km river stretch, and 74 compounds from the ToxCast database were identified, dominated by pesticides (n = 30) and industrial intermediates (n = 19), with over 50% of the compounds exhibiting a 100% detection frequency across the entire watershed. The PLS-DA model effectively transformed complex mass spectrometry data into intuitive spatial chemical maps, confirming a binary spatial partitioning of chemical fingerprints within the YTR. Analysis of co-occurrence patterns confirmed that the chemical community underwent a paradigm shift from being "stochastic process-driven" in the headwater-upstream (HU) regions to "deterministic process-driven" in the mid-downstream (MD) regions, characterized by the coexistence of increasing species richness and simplified network topology. Source apportionment using representative tracers revealed that this paradigm shift was likely because the HU chemical community was primarily influenced by long-range atmospheric transport and the high-altitude "cold-trapping" effect, manifesting as loose background signals, whereas intensive human activities in the mid-to-downstream regions, such as the mixed application of pesticides, strongly reshaped the aquatic chemical fingerprints. Risk prioritization using the Toxicological Priority Index (ToxPi) identified basin-wide high-risk chemicals, such as β-estradiol and perfluorooctanesulfonic acid (PFOS). Collectively, this study demonstrates that long-range transport, coupled with diverse local human activities (agriculture, domestic, traffic, and tourism), jointly shapes the spatial heterogeneity of chemicals in plateau rivers. These findings underscore the necessity of implementing section-specific, precision control strategies for the sustainable management of this iconic plateau river ecosystem.