Desorptive behavior of perfluoroalkyl acids and perfluoroalkyl acid precursors from aqueous film-forming foam impacted soils and aquifer materials

Abstract

Aqueous film-forming foams (AFFFs) have been used to fight hydrocarbon fuel fires and for firefighter training at a number of U.S. military sites. As a result, soils, aquifer materials, and groundwater near these firefighter training sites are contaminated with a broad suite of poly- and perfluorinated substances (PFASs), including many perfluoroalkyl acids (PFAAs). Though plumes resulting from such activities are well documented, the extent to which surface soils can serve as long term sources of both PFAAs and PFASs remains unclear. To this end, 14 day infinite sink batch desorption experiments were conducted to measure the desorption behavior of PFAAs and PFAA precursors using four field-contaminated surface soils and two subsurface aquifer materials collected at two AFFF-impacted sites. The desorption of precursors to eight different perfluorocarboxylates (PFCAs) from these materials was measured through use of the recently-developed total oxidizable precursor (TOP) assay. PFAA and PFAA precursor desorption data were modeled using either a two-site or a one-site kinetic model. PFCAs and perfluorosulfonates (PFSAs) showed similar desorption rates for equivalent perfluorinated carbon chain lengths, and both showed decreasing desorption rates with increasing perfluorinated chain length. PFAA precursors desorbed more slowly than PFAAs from the contaminated materials; all PFAAs could be modeled with a single fast desorbing compartment while many PFAA precursors required an additional, slower compartment containing up to 96% of precursor mass with desorption rate constants 3 to 15 times smaller than in the fast desorbing compartment. PFAAs had larger fast desorption rate constants than PFAA precursors with average values of 2.27 and 1.65 (day[superscript -1]), respectively. Significant differences in desorption rate were also observed between the six sediment samples. Despite slower desorption rates, PFAA precursors may still constitute a significant portion of potentially mobile PFASs at sites impacted by AFFF use.