Perfluoroalkyl acid precursor discharge from engineered water systems: composition and treatment impacts

Conventional monitoring for per- and polyfluoroalkyl substances (PFAS) in engineered water systems can underestimate total PFAS discharge by using targeted methods that capture mostly terminal perfluoroalkyl acids (PFAAs) but omit many important PFAS such as oxidizable precursors. We combined targeted LC-MS/MS with the total oxidizable precursor (TOP) assay to quantify PFAS composition and precursor contributions in municipal wastewater and urban stormwater in six communities. Target PFAS concentrations ranged from 25-108 ng L^-1 in wastewater treatment plant (WWTP) influent, from 20-231 ng L^-1 in WWTP effluent, and from 16-85 ng L^-1 in stormwater. However, total oxidizable precursors comprised up to 92% of total PFAS in WWTP influent, 70% in effluent, and 59% in stormwater. In WWTP effluent, the proportion of untargeted precursor PFAS was significantly higher at facilities with secondary treatment hydraulic retention times (HRTs) <1 h (mean = 59.7%) than at facilities with longer HRTs (>1 h; mean = 39.3%; Welch's t-test, p = 0.008), indicating that limited biological treatment duration may constrain precursor biotransformation. Collectively, the six WWTPs discharged approximately 26 kg per year of total PFAS, of which 47% (12.3 kg per year) consisted of untargeted PFAA precursors. Total stormwater PFAS loads were episodic (approximately 1.2-31.5 g per event) and similarly deliver substantial untargeted PFAA precursor mass (approximately 0.7-8.7 g per event) to receiving waters. These results demonstrate that precursor PFAS represent a major and underrecognized component of PFAS flux from engineered water systems and should be incorporated into regulatory monitoring and source control strategies.