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

Topic Area: 7j. PFAS Conceptual Site Model Approaches

Formatted Title:

Characterization of Relevant PFAS Fate and Transport Processes at Multiple AFFF Sites Using a Mass Balance Approach

Background/Objectives:

Understanding PFAS fate and transport in the subsurface is crucial for devising reliable and cost-effective management strategies. The objective of this study was to provide a comprehensive, field-scale, mass-based evaluation of the PFAS present at three AFFF release sites to illustrate the role of different retention processes on PFAS distribution. It involves converting soil and/or groundwater concentration data to mass demonstrate that a high percentage of PFAS mass at a site is associated with near-source areas, low-permeability zones, and/or precursor forms. Because PFAS mass in these compartments can serve as an indicator that PFAS retention mechanisms are influencing PFAS distribution within a site, this type of mass balance across a site can be used as a line of evidence (LOE) to support PFAS Monitored Retention (PMR) as a site management option. Mass discharge data can also be overlaid on mass balance diagrams to better show exchange between different compartments as another line of evidence for PMR. Because PFAS mass balances provide a quantitative assessment of PFAS mass in different compartments, they are also valuable for informing conceptual site models.

Approach/Activities:

The three example sites discussed here use PFAS characterization data that were generated from an earlier ESTCP project described in Adamson et al. (2022). At each site, data were obtained through a detailed soil and groundwater characterization program at suspected AFFF release sites and leveraging high resolution mass spectrometry to identify, quantify or semi-quantify a larger number of PFAS than would be possible using lower-resolution techniques.. The mass balance for Site 1 used a software-based interpolation approach, while the mass balances for Site 2 and Site 3 were developed using manual (spreadsheet-based) interpolation methods.

Results/Lessons Learned:

Site 1 contained a moderately high total PFAS mass (222 kg) with a source area mass discharge of 3.6 kg/yr. The remaining mass was primarily located in the saturated zone (95%) and in low-permeability soils (82%). Approximately half of the mass was still in the form of polyfluorinated precursors with lower migration potential. The results indicated that matrix diffusion, slow transformation (chemical retention), and solid-phase sorption were key retention processes that influenced migration at Site 1. Site 2 was characterized by higher total PFAS mass (450 kg) and mass discharge rates (36 kg/yr), and relatively low percentages of mass was in the form of precursors (13%) or present in lower permeability zones. However, 75% of the mass was still located with near-source areas, confirming that various processes were contributing to retention at this site. In particular, the high groundwater salinity (typically > 10 mS/cm) promoted PFAS salting out and increases retention at this site. Site 3 contained a relatively low total PFAS mass (86 kg) and the majority (80%) was encountered within the unsaturated zone, demonstrating the importance of air-water interfacial partitioning at this site. More than half of the mass was in precursor form (61%) and located with the near-source areas (61%), while 46% was associated with low-permeability soils at this site. The mass discharge from Site 3 (1.9 kg/yr) was the lowest of the three sites, and decreased at a bulk zero-order attenuation rate of 0.0013 kg/yr per ft. The Mass discharge also decreased with distance at Site 1 (0.002 kg/yr per ft) and Site 2 (0.011 kg/yr per ft). The finding that multiple processes were active at site-wide scales is consistent with expectations that these are naturally occurring reactions that should be relevant at most AFFF-impacted source zones.

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