Assessment of PFAS contamination in soils: non-target identification of precursors, fluorine mass balance and microcosm studies

The continuous release of perfluoroalkyl acids (PFAAs) from the transformation of per- and polyfluoroalkyl substances (PFAS) precursors presents a significant and often overlooked challenge in contaminated soils. In south-western Germany a large-scale agricultural topsoil contamination PFAS was discovered, which is known as the Rastatt case, and was traced back to the past application of paper sludge as soil amendment. In this study, 40 PFAS were monitored in eight topsoil samples from Rastatt according to the EPA 1633 method. Additionally, non-target screening was performed to identify PFAS precursors. FTMAPs, diPAPs, and diSAmPAP were identified and accounted for > 80% of the total PFAS burden, which ranged from ~ 280 to 9,700 ng PFAS g^-1. These levels were confirmed by both, non-target screening (semi)quantifications and chemical oxidation of precursors (TOP assay) in order to close the fluorine mass balance against extractable organic fluorine (EOF). Notably, in some organic carbon rich samples, repeated oxidation was needed to achieve a complete fluorine mass balance, highlighting the need to include EOF as quality assurance of TOP assays and (semi)quantifications derived from non-target screening approaches.

Batch microcosm incubations were additionally set up to assess short-chain PFAS production over time. The linear increase of short-chain PFAS concentrations in solution, in combination with TOP estimates, allows to derive respective production rate constants and, therefore, estimate contamination time scales. This methodology may potentially apply to other precursor-driven contaminant sources such as those present in aqueous film-forming foam (AFFF) sites. Contamination time scales in the assessed locations indicate that leaching of short-chain PFAS to groundwater resulting from ongoing precursor transformation will continue for decades. The variability in time scale estimates across the eight examined soils encouraged the examination of specific soil properties affecting PFAS production rates, particularly assessing the role of certain phosphatase enzymatic activities and microbial biomass carbon. FTMAPs, diPAPs, and diSAmPAP all contain a phosphate moiety which is hydrolyzed during biotransformation processes. A principal component analysis (PCA) indicated the positive role of both acid phosphomonoesterase activities and, in lesser extent, microbial biomass carbon on the production of short-chain PFAS in soils. Nonetheless, further research on isolated bacteria strains is needed to elucidate the role of phosphatases as well as other enzymatic activities in the decay of P-containing PFAS precursors.