Disentangling urban wastewater sources using isotopic tracers and CEC-based diagnostic indicators

Urban rivers receiving multiple wastewater inputs require reliable diagnostic tools to disentangle overlapping nitrogen sources. Here, we investigate wastewater source attribution in a densely urbanized basin using isotopic tracers together with diagnostic indicators based on contaminants of emerging concern (CECs), rather than CEC occurrence alone. Monthly water sampling over 2 years at five stations in an urban river system included nitrate isotopes, conventional hydrochemistry, and a broad suite of CECs. Nitrate concentrations reached up to 18.3 mg L^-1 as NO₃^--N, and δ¹⁵N-NO₃^- values ranged from +6.1 ‰ to +24.2 ‰, consistent with variations in total nitrogen loads along the river continuum. Although isotope signals indicated increasing wastewater influence downstream, isotopic information alone was insufficient to distinguish between human and animal derived wastewater sources. The integration of nitrate isotopes with CEC-derived diagnostic indices, including the Human Wastewater Index (HWI) and the Hospital Index (HI), within a Bayesian mixing framework based on characterized source signatures revealed that human wastewater contributions exceeded 60 % at all monitoring stations. Elevated HI values further indicated hospital wastewater inputs that were not detectable using isotopic variables alone. An ecological risk assessment indicated that, in 70 % of the samples, the combined risk from detected CECs was high, with over half of the individual compounds present at concentrations exceeding their respective acute toxicity thresholds. These results demonstrate that CEC-based diagnostic indicators provide complementary information to isotopic tracers and substantially improve wastewater source attribution in complex urban river systems, with direct implications for monitoring and management of urban water quality.