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

Topic Area: 7m. PFAS Site Characterization

Formatted Title:

PFAS Migration to Groundwater: Measuring In Situ Partitioning

Background/Objectives:

Understanding how per- and polyfluoroalkyl substances (PFAS) partition between soil and aqueous phases is critical for assessing migration potential and risk to groundwater. Because PFAS are generally released at the surface, it is important to estimate the transport of PFAS from the initial release, through the unsaturated zone, and into groundwater. Unlike more conventional contaminants such as chlorinated solvents, several additional retention processes are likely to influence PFAS transport in the unsaturated zone, such as storage at air-water interfaces. Many of these factors cannot be fully explained under all environmentally relevant conditions, indicating a need to determine PFAS migration potential on a site-specific basis. This work aims to develop and evaluate a simple leaching method for estimating impact to groundwater from PFAS-contaminated soil and to evaluate the use of lysimeters in measuring PFAS migration through the vadose zone. The studies were conducted at two Superfund sites with known PFAS releases, one in Pennsylvania, a facility that formerly applied fluorocarbon stain repellent to fabrics resulting in PFAS and TCE contamination, and the other in Alaska, a U.S. Air Force installation that has known releases of aqueous film-forming foam (AFFF).

Approach/Activities:

Continuous soil cores in the vadose zone down to the top of the water table were collected from the Valmont Superfund site in Pennsylvania. Groundwater was sampled from wells located next to the soil cores. The cores were analyzed for PFAS compounds listed in EPA Method 537 and other associated geochemistry, including grain size, mineralogy, organic/inorganic carbon content, and pH. The synthetic precipitation leaching procedure (SPLP) method was used with modifications made to avoid interferences: only PFAS-free equipment and materials were used, and filtration was replaced by centrifugation. An additional extraction using basic methanol in place of the extraction fluid was performed in parallel to produce both a “leachable PFAS” value and a “total PFAS” value. Further changes to enhance PFAS recovery and sensitivity are under investigation. Additional SPLP modifications under investigation include extractions with five different extraction fluids.

As part of a larger study, the U.S. Air Force installed suction lysimeters at nine AFFF release locations at Eielson AFB and collected porewater during four sampling events between May and September 2023. Lysimeters were placed at a variety of depths to help determine PFAS migration to groundwater. During the lysimeter installation, continuous soil cores in the vadose and saturated zone were collected. EPA samples were collected in the soil horizon directly above the lysimeter screens. Co-contaminants at these locations include petroleum hydrocarbons, pentachlorophenol, chlorinated solvents, polychlorinated biphenyls, and naturally occurring metals. Samples were extracted using SPLP and results will be compared to porewater concentrations, historic SPLP data, and groundwater samples from adjacent wells.

Results/Lessons Learned:

Our results show that:

No single default value of K_d or K_OC for the studied PFAS is appropriate for use in determining PFAS soil/water partitioning,
SPLP is effective at extracting PFAS with minimal modification,
Site-specific K_d values can be calculated easily using SPLP data to improve groundwater vulnerability and risk assessments,
SPLP extraction fluid composition has little effect on extraction efficiency at 20:1 ratio, and
Lysimeter installation is limited by physical site conditions such as soil type and depth to groundwater.

This is an abstract of a proposed poster presentation and does not necessarily reflect EPA policy.

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Jed Costanza