Wastewater inputs and hydrological variability shape catchment-scale transport of polychlorinated dibenzo-p-dioxin and dibenzofuran-like immunoreactivity.

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are persistent hazardous pollutants whose riverine transport is shaped by wastewater discharges and hydrological variability. This study quantified PCDD/F-like immunoreactivity, expressed as PCDD/F ELISA equivalents (PCDD/F ELISA-EQ), in untreated and treated wastewater from 17 municipal wastewater treatment plants (WWTPs)-seven small, five medium-sized, and five large-and at eight sites along the Pilica River over two annual cycles. The assay-derived PCDD/F ELISA-EQ metric used in this study is a screening measure distinct from congener-specific PCDD/F concentrations and World Health Organization Toxic Equivalents (WHO-TEQ). Wastewater treatment reduced PCDD/F ELISA-EQ concentrations and loads by 85.9-92.8% and 79.8-90.8%, respectively, across the 17 WWTPs. WWTP size did not significantly affect concentration-reduction efficiency (one-way analysis of variance: F = 0.410, p = 0.665), whereas large WWTPs generated the highest residual loads because of their greater hydraulic throughput. Riverine PCDD/F ELISA-EQ did not follow a consistent downstream gradient but occurred as spatially and seasonally variable pulses. The highest daily loads were recorded in the middle river section during winter high-flow conditions, indicating strong hydrological amplification of contaminant transport. The Sulejów Reservoir appeared to modify this pattern, potentially reflecting dilution and retention processes within the reservoir. Multivariate analyses linked PCDD/F ELISA-EQ mainly with organic matter, suspended solids and redox conditions. These findings show that concentration-based monitoring alone is insufficient to characterize PCDD/F-related pressure. Integrating screening-level PCDD/F ELISA-EQ data with wastewater discharge, river flow and load estimates provides a more robust framework for identifying and prioritizing critical periods, high-load facilities, and vulnerable river sections.