Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7h. PFAS Fate and Transport

Formatted Title:

Lysimeters to Evaluate PFAS Leaching at AFFF-Impacted Sites

Background/Objectives:

Leaching of poly- and perfluoroalkyl substances (PFAS) from soil to underlying groundwater in aqueous film-forming foam (AFFF) impacted source areas remains a poorly understood environmental challenge. While porous cup suction lysimeters are being used to characterize these source areas, questions remain with regard to variability of PFAS concentrations in the collected porewater (due to variations in moisture content due to infiltration or other variables), how PFAS concentrations vary over time as the source becomes slowly depleted, and the extent to which bench-scale leaching/desorption tests inform on PFAS porewater concentrations. Using data collected from several sites, these questions are addressed herein.

Approach/Activities:

For this evaluation, several AFFF-impacted source areas were evaluated, including one site where in situ flushing was performed within a highly instrumented test cell. Multiple rounds of PFAS porewater data collected via porous cup suction lysimeters were examined. Soils were also analyzed to determine PFAS concentrations, grain size distribution, and moisture content. In addition, bench-scale testing that included both batch kinetic desorption studies and determination of desorption isotherms was performed.

Results/Lessons Learned:

Results to date have yielded several important findings. First, for many sites, the local equilibrium assumption has been shown to be valid. This was verified by comparing field porewater concentrations to those predicted based by bench-scale equilibrium measurements using site soils. This finding suggests that use of batch testing is likely to provide useful information for many AFFF-impacted sites. Second, results have shown that PFAS accumulation at air-water interfaces, as well as the release of these accumulated PFAS during wetting cycles that cause the collapse of air-water interfaces, can substantially impact PFAS porewater concentrations and impact transient PFAS mass fluxes to underlying groundwater. Interestingly, at some sites, PFAS accumulation at the air-water interface appears much less than expected; parallel studies suggest that this might be due to competitive sorption from other surface-active species. Third, not all the PFAS mass is readily desorbed from the soil, and PFAS porewater concentrations can decrease much more rapidly than their corresponding soil concentrations. Finally, kinetically controlled PFAS desorption at the grain scale correlated with long-term desorption behavior observed in situ.

Overall, these findings not only highlight the value of lysimetry for evaluating PFAS leaching in AFFF-impacted soils, but also show how bench-scale testing can be useful for evaluating the observed field-scale behavior. Further testing and evaluation are needed to determine the site and soil conditions over which our current findings are applicable.

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Nicholas Gonda