AFFF formulations have been used by DoD since the 1970s to suppress liquid fuel fires and there are thousands of sites impacted by PFAS. 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 perfluorooctanesulfonic acid (PFOS)-based AFFF.
Due to their chemical structure, PFAS are very stable in the environment and are relatively resistant to biodegradation, photo-oxidation, direct photolysis, and hydrolysis. The perfluoroalkyl carboxylic acids and sulfonic acids have very low volatility due to their ionic nature and can leach from vadose zone sources into groundwater despite many operative soil retention processes. GAC is being used at several sites to specifically remove PFOS and perfluorooctanoic acid (PFOA) from contaminated water in ex situ systems; however, the cost-effectiveness over time of GAC systems remain questionable, specifically with regard to regeneration. Use of some types of technologies results in generation of ancillary waste streams (e.g., brines are generated during regeneration of ion exchange resins). Solutions are needed that include provisions for regeneration of spent media, and treatment and disposal of ancillary waste streams.
Further complicating the issue, many of the PFAS 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 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. Effective treatment options are needed that address the variety of matrices that may be impacted by PFAS.