Particle-Facilitated Transport of PFAS and their Precursors in Contrasting River Catchments

Rapid urbanization rates in combination with climate change may lead to increasing urban runoff volumes and pollutant loads. Many organic pollutants are transported sorbed to particles. Therefore, high discharge events present a major pathway for pollutant transport in rivers. In recent years, per- and polyfluoroalkyl substances (PFAS) have gained growing attention due to their persistent nature, ubiquitous occurrence, and toxicity. Many studies have focused on the transport of PFAS in rivers in the aqueous phase, often overlooking particle-facilitated transport, which is particularly relevant for PFAS precursors (i.e. polyfluoroalkyl substances that can degrade into perfluoroalkyl end-products) due to their generally strong sorption affinity to solids.

In this study, particle-facilitated PFAS transport, including precursor compounds of perfluorocarboxylic acids (PFCA), is investigated during high discharge events in contrasting river catchments in southwest Germany. Additionally, polycyclic aromatic hydrocarbons (PAH) are analyzed. 29 high discharge events were sampled at eight different rivers over 1.5 years. Concentrations of PFAS precursors (∑_PFCA,ox) on the suspended river sediments measured by a chemical oxidation assay (dTOP assay) were between 33.9 ± 0.4 and 100.9 ± 10.6 µg kg^-1, while PAH (∑_PAH16) concentrations ranged from 0.07 – 3.92 mg kg^-1. No apparent correlation was found between ∑_PFCA,ox and ∑_PAH16. While PAH have been shown to correlate with urban pressure strongly, PFAS precursors appear to exhibit an elevated ubiquitous signal in the environment, as they were detected in a remote river catchment at concentrations comparable to those in more urbanized areas. Further source apportionment included the sampling of stormwater overflows from residential and highway areas. PFCA precursor concentrations were more variable and generally higher than those observed in river samples, suggesting that, similar to PAH, one potential source is urban particles and street debris being washed into the river during heavy rainfall events.

PFAS precursor concentrations on suspended sediments in the rivers were more or less independent of the event, likely since rivers act as integrators of numerous small streams and inflows. This was particularly true for the Neckar River, the largest stream investigated, and also holds for PAH. By combining sediment yield data or online turbidity measurements with information on suspended sediment loading, it is possible to estimate the contaminant flux of PFAS precursors. The Neckar River alone transports approximately 1.7 kg year^-1 of PFAS precursors through the city of Tübingen, ultimately carrying them towards the North Sea, where they may degrade into stable PFCA over time.