Phase partitioning and limited attenuation of neurotoxicants and estrogens sustain ecological risks in WWTP effluent-dominated rivers.

Wastewater treatment plant (WWTP) effluents release complex chemical mixtures into receiving rivers, but how contaminant partitioning, transport, and transformation shape spatially variable ecological risks remain poorly understood. Here, in a multi-site study of an effluent-dominated river concerning 135 emerging contaminants across 13 classes with a broad range of hydrophobicity (log KOW: -0.78 to 11), we found that WWTP effluent contributed over 90% of the contaminant load, yet abundance alone did not predict ecological risk. Although neurotoxicants (e.g., fipronil, chlorpyrifos) and estrogenic contaminants (e.g., 17-β-estradiol, estrone) were less abundant than fungicides and synthetic musks, they accounted for 61-99% of the predicted ecological risks. Phase partitioning further shaped downstream risk patterns. Highly hydrophobic contaminants, especially musks, progressively partitioned from water to sediments, redistributing contamination across compartments without reducing overall risk. In contrast, moderate- and low-hydrophobicity contaminants, including neurotoxicants and estrogenic contaminants, remained largely in the aqueous phase, where they exhibited limited in-stream attenuation (< 20%) and sustained risks along the river. Transformation introduced additional complexity, e.g., fipronil gradually converted to its more persistent and toxic desulfinyl transformation product, altering its hazard profile without decreasing overall risk. Collectively, these findings reveal that effluent dominance, phase partitioning, limited attenuation, and transformation combine to create spatially persistent ecological risk hotspots. This work provides a scientific basis for upgrading treatment technologies targeting persistent aqueous-phase toxicants and for revising effluent risk assessment frameworks to explicitly account for downstream phase transfer and transformation products.