AFFF formulations have been used by DoD since the 1970s to suppress fires, and there are hundreds of sites with associated PFAS contamination. DoD used AFFF mixtures containing significant quantities of PFOS and related perfluoroalkyl sulfonates such as perfluorohexane sulfonate (PFHxS) until 2002, when production stopped. However the DoD continued (until only recently) to use PFOS-containing AFFF stocks. Although the DoD’s legacy use of AFFF included various fluorotelomer-based formulations, the vast majority of DoD’s environmental liability likely results from the use of PFOS-based AFFF. Additional research on PFAS is timely given USEPA’s recent drinking water health advisories for two common PFASs, perfluorooctanoic acid (PFOA) and PFOS as well as the numerous states that are beginning to promulgate drinking water standards.
Many of the PFASs found in AFFF formulations are highly soluble and migrate rapidly, while others are far less mobile. The more soluble PFASs are likely to become depleted through flushing from source zones over time. However, other PFAS compounds may be retained in the source zone, with varying degrees of potential for mass transfer into the aqueous phase, infiltration to groundwater (for vadose zone source areas), and/or groundwater migration, particularly after several years in the subsurface. PFOS and PFOA are relatively mobile, though their fates are complicated by the presence of potential precursors for these compounds in complex PFAS mixtures such as AFFF formulations.
The risks posed by PFASs retained in the original source zones are not fully understood, in part because several mechanisms may be involved. PFASs may be sorbed to organic or inorganic solids in the vadose zone or aquifer, and different mechanisms may be involved in such sorption. Soil properties and groundwater geochemistry within source zones will influence the nature of binding mechanisms. Also, the chemical nature of the PFASs that are likely to dominate the original source zones may lead to low risks to groundwater. For example, cationic and zwitterionic PFAS compounds found in AFFF formulations can be very tightly bound to ion exchange sites.
Additionally, precursor compounds may be present and biotransformed to PFAAs. However, the ultimate potential, rate, and pathways of in situ biotransformation remain unclear, particularly for PFOS and related compounds such as PFHxS. Furthermore, regulated PFASs (and their precursors) could be present in lower-conductivity regions, diffusing into the plume over time, or the remaining PFASs may be sequestered in inaccessible forms and pose little or no risk to downgradient groundwater. Finally, other materials often found in AFFF source zones (e.g., NAPLs, petroleum fuels, and chlorinated solvents) may impact the fate of PFASs. Closing the data gaps on biotransformation and potential transport of PFOS-containing AFFF mixtures is needed to manage these sites cost-effectively. Better characterization of PFAS in source zones can help managers determine how much, if any, active remediation is needed. However, the ability to make such risk-based characterizations is limited by the complexity of PFAS chemistry in general, and our understanding of PFAS interactions within the subsurface environment.
