Mass-Based, Field-Scale Demonstration of PFAS Retention within AFFF-Associated Source Areas

Abstract

Transport of poly- and perfluoroalkyl substances (PFAS) at aqueous film-forming foam (AFFF)-impacted sites is limited by various processes that can retain PFAS mass within the source area. This study used concentration data obtained via a high-resolution sampling and analytical protocol to estimate the PFAS mass distribution in source and downgradient areas of a former firefighter training area. The total PFAS mass present at the site was approximately 222 kg, with 106 kg as perfluoroalkyl acids (PFAAs) and 116 kg as polyfluorinated precursors. Zwitterionic and cationic PFAS represented 83% of the total precursor mass and were found primarily in the source and up/side-gradient areas (75%), likely due to preferential hydrophobic partitioning, electrostatic interactions, and diffusion into lower-permeability soils. Based on the release history and the high percentage of total PFAS mass represented by precursors (primarily electrochemical fluorination-derived compounds), the estimated conversion rate of precursors to PFAAs was less than 2% annually. Eighty-two percent of the total PFAS mass was encountered in lower-permeability soils, which limited the potential for advection and transformation. This contributed to a 99% decrease in the mass discharge rate at the far-downgradient plume (0.048 kg/yr compared to the near-source area (3.6 kg/yr)). The results provide field-scale evidence of the importance of these PFAS retention processes at sites where AFFF has been released.

Monitored Natural Attenuation to Manage PFAS Impacts to Groundwater: Scientific Basis

“…• PFAAs and precursors are subject to several processes that can retain mass and limit mobility, leading to significant attenuation with distance. • PFAS retention can significantly reduce transport in both vadose zones (Anderson et al 2019) and groundwater plumes (Adamson et al 2020). • PFAS plumes are subject to matrix diffusion effects (Adamson et al 2020).…”

“…• PFAS retention can significantly reduce transport in both vadose zones (Anderson et al 2019) and groundwater plumes (Adamson et al 2020). • PFAS plumes are subject to matrix diffusion effects (Adamson et al 2020). • Compared to other COCs, the ratio of typical source concentrations to regulatory criteria may actually be lower for PFAS plumes compared to COCs that have conventionally impacted groundwater (Newell et al 2020).…”

“…These mechanisms include hydrophobic and electrostatic interactions, which are a function of pH, ionic strength, organic carbon content, the charge of soil particles, chain lengths, and charge of the functional group of the PFAS (Nguyen et al 2020). The relative importance of these processes has been documented at PFAS field sites (Weber et al 2017;Adamson et al 2020;Nickerson et al 2021). For example, a field-scale evaluation of a former fire training area where aqueous film-forming foam (AFFF) had been released showed that zwitterionic and cationic precursors were strongly retained within the source area relative to PFAAs due to preferential sorption characteristics (Adamson et al 2020;Nickerson et al 2021).…”

“…The relative importance of these processes has been documented at PFAS field sites (Weber et al 2017;Adamson et al 2020;Nickerson et al 2021). For example, a field-scale evaluation of a former fire training area where aqueous film-forming foam (AFFF) had been released showed that zwitterionic and cationic precursors were strongly retained within the source area relative to PFAAs due to preferential sorption characteristics (Adamson et al 2020;Nickerson et al 2021). Differences in PFAS distribution among solid and liquid phases as a function of carbon chain length were also noted.…”

Multidimensional simulation of PFAS transport and leaching in the vadose zone: impact of surfactant-induced flow and soil heterogeneities

“…The scientific understanding of the fate and transport of PFAS in the environment, especially in source zones under water unsaturated conditions, is still in its infancy. A growing body of field data demonstrates that PFAS concentrations in soils are orders of magnitude higher than those in groundwater beneath the source zones (up to seven orders of magnitude) and that the majority of PFAS remains near the top of the vadose zone (Anderson et al, 2016;Brusseau, et al, 2020b;Dauchy et al, 2019;McGuire et al, 2014;Xiao et al, 2015;Adamson et al, 2020). However, it remains poorly understood as to what primary mechanisms are controlling the long-term retention of PFAS in the vadose zone and the subsequent mass discharge to groundwater over time.…”