Occurrence of ultrashort-chain PFAS (US-PFAS) in groundwater: Attempts to elucidate some potential sources

In the immense class of per- and polyfluoroalkyl substances (PFAS), there is a subgroup called ultrashort-chain PFAS (US-PFAS) based on the number of perfluorinated carbons. Although some of them were synthesised from the 1950s on, their occurrence in the environment was only documented from the 2000s. The main US-PFAS sought and found was trifluoroacetic acid (TFA). In the present study, four US-PFAS and a suite of legacy and emerging PFAS were analysed in samples from one of the largest groundwater reserves in Europe, located in north-eastern France. Legacy or emerging PFAS were quantified in 85% of the samples. No significant relationships were found between the occurrence of legacy and emerging PFAS and that of US-PFAS. Among the US-PFAS, perfluoropropanoic acid (PFPrA) and perfluoropropane sulphonic acid (PFPrS) were rarely quantified (< 1.5%). The quantification frequency of trifluoromethane sulphonic acid (TFMeS) was 10% (27/258), but the highest concentrations were all measured in the same area, suggesting the presence of a point source. Conversely, TFA was quantified in 96% of the groundwater samples (247/258). Several parameters were used to elucidate the source of TFA, including land cover data and pesticide concentrations. Contrasting outcomes were obtained, suggesting that the clear identification of predominant TFA sources is a complex task. Approaches based on vast geographical areas require approximation of important parameters (e.g., soil types and hydrological conditions), thus increasing the uncertainty of conclusions. Smaller study areas, where these parameters are better controlled, may provide clearer evidence to support the relative contribution of different TFA sources.