Legacy and Emerging Organophosphate Esters (OPEs) in a Rural–Urban Transition Watershed: Spatiotemporal Distribution, Sources, and Toxicity Screening

Agricultural watersheds are undergoing rapid rural-urban transitions, yet the relative contributions of diffuse agricultural runoff versus rural domestic and point sources to organophosphate esters (OPEs) pollution remain poorly understood. This study investigated the occurrence, spatiotemporal distribution, and potential risks of 17 legacy and emerging OPEs in the Dalongdong River, China. Combined non-target and target analyses revealed mean OPE concentrations of 111.94 ng/L in water and 8.76 ng/g in sediments. Spatially, total OPE concentrations increased progressively from upstream to downstream, with pronounced hotspots downstream of townships and near wastewater treatment facilities, indicating that rural domestic effluents and urban runoff, alongside agricultural activities, are critical contributors to OPE pollution in this watershed. Seasonally, concentrations of six legacy OPEs were significantly higher during the wet season. Furthermore, high-throughput phenotypic screening using <i>Caenorhabditis elegans</i>, combined with toxicological priority index analysis, showed that emerging OPEs generally pose higher integrated health and ecological risks, although certain legacy compounds, such as triphenyl phosphate, still display substantial toxic potential. These findings clarify the potential biological hazards of these compounds and provide baseline data on the fate of OPEs in riverine systems influenced by mixed agricultural and rural-urban anthropogenic activities.