Per- and Polyfluoroalkyl Substances (PFASs)
Aqueous Film Forming Foam (AFFF) formulations have been used by the Department of Defense (DoD) since the 1970s to extinguish fires. The DoD used AFFF mixtures containing significant quantities of perfluorooctane sulfonate (PFOS) and related perfluoroalkyl sulfonates such as perfluorohexane sulfonate (PFHxS) until 2002, when production ceased, although the DoD continued to use PFOS-containing AFFF stocks for some time after. Although the DoD’s legacy use of AFFF also included various fluorotelomer-based formulations, the majority of DoD’s environmental liability likely results from the use of PFOS-based AFFF formulations.
Per- and polyfluoroalkyl substances (PFASs) are anthropogenic chemicals and do not occur naturally in the environment. Due to their unique chemical structure, PFASs are very stable in the environment and are relatively resistant to biodegradation, photooxidation, direct photolysis, and hydrolysis. However, some PFASs, notably the polyfluoroalkyls, can be degraded in the environment, in some cases producing perfluoroalkyls, including perfluorooctanoic acid (PFOA). PFASs tend to bioaccumulate, and laboratory studies have shown that these compounds can have adverse health effects. The potential magnitude of the DoD’s PFAS liabilities will require a sustained effort to identify the best technologies to characterize, treat, and manage PFAS-contaminated sites.
Since FY11, SERDP and ESTCP have sponsored a number of projects aimed at developing a better understanding of occurrence, fate and transport, remedial treatment options, and ecotoxicity at AFFF-impacted sites, as well as next generation fluorine-free foams.
Occurrence, Fate and Transport
The relative solubility and recalcitrance of many of the PFASs, including PFOS, PFOA, and the other perfluoroalkyl acids (PFAAs) that are of the greatest environmental concern, gives these compounds the ability to migrate rapidly through groundwater, in some cases forming large plumes. Other less mobile PFASs that are also present in AFFF mixtures may be retained in the source zone, although to date the risks posed by these PFASs are not fully understood. Finally, some of the polyfluoroalkly PFASs found in AFFF mixtures may be transformed abiotically or biologically to PFAAs , including PFOS and PFOA, and the impacts of these precursor compounds should be included in risk assessments. Improved understanding in characterizing and modelling the nature of sources (and associated plumes) will lead to more targeted remediation strategies for sites posing the greatest risk. Several projects have been initiated to better understand the occurrence, fate and transport of PFASs; a description of the Statements of Need and related projects can be found in the overall program description graphic.
PFASs are highly resistant to biological and chemical degradation, particularly the PFAAs, due to the very strong carbon-fluorine bonds. Since options for treatment are limited due to the recalcitrance of PFAAs, especially PFOS, ex situ treatment with granular activated carbon (GAC) is common. As a result, management of AFFF sites is often very costly, and some of the PFAAs may break through GAC canisters more quickly than PFOA and PFOS, raising some concerns. Current research efforts aim to gain a better understanding of the degradation processes and develop new remediation technologies to treat PFASs. A summary of the in situ and ex situ remedial approaches that are currently being developed can be found in the overall program description graphic.
While PFASs have been found to negatively affect autotrophic and heterotrophic food webs, additional data are needed to improve the understanding of their ecotoxicity and set realistic criteria for protection of ecological receptors. While there is currently no MCL standard for PFASs, the USEPA has established a Health Advisory Level for PFOS and PFOA in drinking water at 70 parts per trillion (individually or as the sum of the two). This advisory level is often used as a cleanup standard, although it is based on human health protection and does not address ecotoxicity. Efforts to close knowledge gaps in ecotoxicity risk will aid in the development of appropriate site-specific risk assessments and help in decision-making related to mitigation of exposures and/or future environmental cleanup. A summary of those projects addressing ecotoxicity of these compounds can be found in the overall program description graphic.
Next Generation Fire Fighting Foams
The Weapons Systems and Platform program area under SERDP & ESTCP invests in studies aimed at developing fluorine-free surfactant formulations for use in AFFF fire-suppression operations. At this time, AFFFs used by the military must contain fluorinated surfactants (and other compounds) per MIL-F-24385F. Industry has identified potential fluorine-free alternative foams; however, none of these technologies meet the performance required for military applications. Current efforts aim to identify and test fluorine-free firefighting foam formulations as a replacement for existing AFFF that would meet performance requirements to ensure mission readiness and firefighter safety while also reducing adverse environmental impacts.
Drinking Water Advisory Levels