Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7h. PFAS Fate and Transport

Formatted Title:

Per- and Polyfluoroalkyl Substances Precursor Transformation: Aqueous Film-Forming Foam (AFFF) Release Area Case Study

Background/Objectives:

Quantifying per- and polyfluoroalkyl substance (PFAS) transformation from precursor compounds to terminal compounds with risk-based screening levels is critical to understanding potential impacts to groundwater, surface water, soil, and sediments. The total oxidizable precursor (TOP) assay is a useful method to determine potential downgradient transformation and to support forensic evaluation from samples that are all fully oxidized rather than samples with unknown degrees of oxidation at an aqueous film-forming foam (AFFF) release area in the great plains.

Approach/Activities:

Groundwater, surface water, and sediment samples were collected in December 2022 along a groundwater flow pathway and surface water pathway from an AFFF release area. Pathways were identified from historical trichloroethylene plumes and extensive lithologic and hydrological characterization that were performed as part of historical remedial investigations. PFAS samples were analyzed using EPA Method 1633 before and after TOP assay. Subsequently, samples were analyzed using suspect screen analysis, with the capability of semi-quantitatively identifying up to 500 PFAS compounds, to identify additional PFAS contributing to increases in terminal degradation products after oxidation and provide further insight into PFAS fate and transport.

Results/Lessons Learned:

Short-chain perfluoroalkyl carboxylic acids (PFCAs) had the largest increase in concentration following oxidation and resulted in signatures similar to oxidized AFFF from 3M. TOP assay indicated that perfluoroalkyl sulfonic acids (PFSAs) were not generated from oxidation in groundwater or surface water samples, although known/suspected precursors were detected with suspect screen analysis.

Suspect screen analysis identified several key PFAS precursors that were present in different media and are not quantified with EPA Method 1633. Groundwater and surface water generally had a higher amount of perfluoro sulfonamides (such as FHxSA, a precursor to PFHxS), while sediments had a higher amount of fluorotelomer sulfonamide alkylbetaines (such as 8:2 FTAB, a precursor to PFOA). The difference in relative precursor concentrations between media was used to make inferences about transport mechanisms. 6:2 FTAB and 8:2 FTAB appear to more strongly sorb to sediments and were detected in the highest concentrations at the farthest downstream sampling point at a location where sediment deposition is likely to occur.

Ratios between suspected precursors and terminal PFAS were compared to identify whether in situ precursor transformation may be occurring along a pathway. PFHxA to 6:2 FTS ratios did not decrease along either the groundwater or surface water pathway as would be expected if the transformation of 6:2 FTS to PFHxA was occurring. This suggests that in situ conditions at this site are not conducive to precursor transformation to terminal PFAS (at least of the quantitatively and semi-quantitatively determined precursors). However, the potential for transformation is present with oxidizing conditions and TOP assay results indicate that PFHxA has the possibility to exceed current EPA RSLs from oxidation alone.

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