The nature and extent of groundwater contamination by per- and polyfluoroalkyl substances (PFASs) at military sites due to the release of aqueous film forming foams (AFFFs) is difficult to characterize given the lack of information on the PFAS composition of AFFFs. Advanced analytical methods are needed to characterize AFFF formulations and AFFF-contaminated sites, to better understand the PFASs that are present, and identify those that are potentially precursors of persistent, dead-end products, including perfluorooctanoate (PFOA) and perfluorooctane sulfonate (PFOS) and their longer and shorter carbon-chain homologs. Identifying potential precursors requires understanding biotransformation pathways of polyfluorinated precursors, including the intermediates and dead-end products that will persist under in situ conditions.
The overall goals of this research were to identify individual PFASs and their oxidizable precursors, in AFFF formulations and in AFFF-contaminated groundwater, sediment, and soil, and to carry out biotransformation studies to determine the biodegradation pathways of precursors in AFFF-contaminated media.
Analytical methods were created to extract and quantify a wide array of anionic, zwitterionic, and cationic PFASs in groundwater. The TOP (total oxidizable precursor) assay was modified to quantify the precursors in environmental samples that can form PFCAs as dead-end products. The TOP assay is based on the oxidation of polyfluorinated precursors to PFCAs in the presence of thermally-activated persulfate. The developed analytical methods were then applied to field-collected samples. Biotransformation experiments (conducted under aerobic conditions) were performed using a number of polyfluorinated potential precursors found in fluorotelomer-based AFFFs. The impact of solvents in AFFF and trichloroethene on the transformation of precursors was also evaluated. Lastly, controlled laboratory experiments were conducted to determine the sorption properties of a mixture of anionic, zwitterionic, and cationic precursors in National Foam AFFF.
Before this project, little was known about the actual composition of AFFF formulations used in military sites. The compositions of the formulations used since the late 1980s at military sites were determined, including those manufactured by 3M, Ansul, Chemguard, National Foam, Buckeye Fire Equipment, and Angus. Over 50 separate classes of PFASs, comprising hundreds of individuals PFASs, were identified in AFFF formulations and in groundwater over the course of this project. The most abundant individual PFASs in AFFF-contaminated groundwater typically were PFSAs (e.g., PFOS) and PFCAs (e.g., PFOA), with concentrations that often exceeded the EPA health advisory values by several orders of magnitude. Fluorotelomer sulfonates (FTSAs) were also found in groundwater, at levels that were sometimes greater than PFOA and PFOS. Use of the TOP assay revealed that polyfluorinated precursors also can make up a significant fraction of total PFASs in AFFF-impacted groundwater, soil, and sediment.
PFCAs were found in the 3M AFFF formulations, but other sources of PFCAs were also found in the fluorotelomer formulations, such as that produced by Ansul. The fluorotelomer thioamido sulfonates (FtTAoS), found in Ansul AFFF, were biotransformed under aerobic conditions to FTSAs, which were slowly transformed further to PFCAs. Thus, the fluorotelomer-based formulations can act as long-term sources of PFCAs in groundwater and surface water. FTSAs are not primary components in AFFF formulations, so their presence at relatively high concentrations in groundwaters indicates they do not degrade under anaerobic conditions.
Sorption testing showed that the anionic 6:2 FTSA is less retained by soils and sediments than PFOS, even though they have the same number of total carbons, an observation that is consistent with the relatively high mobility of 6:2 FTSA in groundwater. Greater sorption was observed for the anionic 8:2 FTSA than for 6:2 FTSA, consistent with the greater chain length of 8:2 FTSA. Compared to the anionic FTSAs, zwitterionic fluorotelomer betaines and cationic 6:2 fluorotelomer sulfonamido amines are strongly sorbed to soils and sediments, and thus are likely to be associated with residual source zones. Measured sorption of PFASs proved difficult to predict, and did not correlate with likely soil parameters, including organic carbon content, and cation and anionic exchange capacity. More information is needed on the field conditions that promote or inhibit transport of zwitterionic and cationic PFASs in order to assess the potential for source zone soils and sediments to act as long-term PFAS sources.
The analytical tools developed for this project, including methods for quantifying individual PFASs as well as precursors, will improve the ability to characterize AFFF-contaminated sites. The potential risks can be understood by having identified the potential precursors at AFFF-contaminated sites, and by understanding their fate and mobility in the environment. Efforts can now focus on understanding the process that retain PFASs in source zones and the conditions that mobilize or immobilize them. Identifying precursors also can improve evaluations of the effectiveness of treatment technologies, such as the use of granulated activated carbon and other sorbents. The biotransformation pathway of the polyfluoroalkyl substances in Ansul AFFF provides a framework for understanding the fate of other precursors, and insight into the conditions (anaerobic) that lead to high concentrations of persistent FTSAs and the potential for intermediates to be ultimately transformed to persistent PFCAs.
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