Uncovering the Dominant Contribution of Untreated Domestic Wastewater to Antimicrobials in the Lower Reach of the Lhasa River on the Tibetan Plateau

A major tributary of the Yarlung Tsangpo River on the Tibetan Plateau, the Lhasa River, flows through Lhasa, one of the world’s highest-altitude cities. Along with the fast urbanization and booming tourism industry in Lhasa, pollution of its aquatic environment receives growing concerns. This study quantified the spatiotemporal distributions and potential ecological risk of common antimicrobials in the lower reach of the Lhasa River for the first time and found untreated domestic wastewater to be a key source of pollution. Thirty-four antimicrobials were detected, with concentrations spanning 4 orders of magnitude (0.01–629 ng/L). Spatial variations of antimicrobials were observed, with tetracyclines and macrolides dominated at the upstream and downstream sites of Lhasa city, respectively, due to varying source inputs and different attenuation of antimicrobials. Although only a few antimicrobials pose medium–high ecological risk individually, the simultaneous occurrence of multiple antimicrobials may result in significant ecological toxicity. While the treated wastewater effluent and agricultural runoff combined contributed to 14.2 and 23.5% of the total antimicrobials in the Lhasa River during wet and dry seasons, respectively, the untreated domestic wastewater discharge accounted for the rest. These findings indicate the urgent need for improving the collection system and treatment technology of domestic wastewater in Lhasa.