Seasonal multiphase partitioning and driving mechanisms of eutrophicants and toxic contaminants in effluent-receiving rivers: Integrating field monitoring and hydrodynamic modeling.

Urban rivers in water-stressed regions increasingly depend on wastewater effluent as artificial baseflow. However, the influences of effluent input on seasonal multiphase partitioning of eutrophicants and toxic contaminants across dissolved, suspended particulate, and sediment phases remain poorly understood. Here, we conducted year-round field monitoring coupled with MIKE21 hydrodynamic modeling in two typical effluent-receiving urban rivers to reveal partitioning behaviors and driving mechanisms. Results showed that effluent discharge significantly elevated the proportions of nutrients and antibiotics in dissolved and suspended particulate phases within 0-1 km downstream of the outfall, but negligibly affected polycyclic aromatic hydrocarbons (PAHs) and heavy metals. Mechanistically, hydrodynamic dispersion dominated partitioning near the outfall (contribution: 34.2-69.6%), whereas sorption and degradation became predominant in further downstream sections (13.2-47.2%). Notably, we defined a novel indicator ecological buffering distance (Leq) to characterize phase equilibrium recovery, which reached 1-3 km in winter, much longer than in other seasons. Statistical analyses revealed that dissolved oxygen and dissolved organic matter molecular weight in effluent were associated with the dissolved-phase partitioning of eutrophicants and antibiotics, whereas electrical conductivity was associated with the partitioning of eutrophicants in the suspended particulate and sediment phases. Scenario simulations further demonstrated that dissolved-phase partitioning was more sensitive to effluent discharge fluctuations than other phases, especially in winter. This study provides mechanistic insights into the multiphase dynamics of co-occurring contaminants in effluent-receiving rivers and supports science-based management for urban aquatic ecosystems under global water scarcity.