PFAS Leaching at AFFF-Impacted Sites: Insight into Soil-to-Groundwater Ratios
Charles Schaefer | CDM Smith
The overall goal of this project is to demonstrate improved insight into per- and polyfluoroalkyl substances (PFAS) leaching through the unsaturated zone to serve as a basis for developing soil cleanup criteria and facilitating site management. Specific objectives are to:
- Assess the factors controlling PFAS leaching in the vadose zone;
- Determine how soil properties impact PFAS porewater migration and overall source longevity;
- Identify unsaturated zone characteristics that impact PFAS migration to groundwater;
- Attain insight into the role of air-water interfacial retention on PFAS vadose zone migration; and
- Develop a screening-level basis for PFAS soil concentrations protective of underlying groundwater.
The overall technical approach is to obtain detailed vadose zone soil data (i.e., PFAS levels and physical/chemical properties) coupled with in situ porewater leaching/partitioning information at a wide range of sites impacted by aqueous film-forming foam (AFFF). Specifically, this will involve a multi-phased approach that begins with a high-resolution soil core investigation at up to 8 AFFF source areas, collecting information on PFAS levels (both target and suspect analytes), soil grain size, and soil chemical properties (including pH, volatile organic compounds [VOCs], total organic carbon, and cation exchange capacity). Groundwater underlying the source area soils also will be analyzed for PFAS.
Next, intact soil cores collected at each site (from locations adjacent to where PFAS concentrations in the vadose zone soils were determined) will be shipped to the laboratory. Porewater will be extracted along the length of each soil core using micro-sized lysimeters. These “equilibrium” porewater samples will provide insight into the PFAS levels associated with the porewater and air-water interfaces under static (no porewater flow) conditions, and thus will serve as an important parameter needed to estimate PFAS flux and retention that would occur under the varying dynamic flow conditions that occur in situ.
Porewater samples also will be collected in situ at each site using up to three lysimeters. The lysimeters will be installed adjacent to the locations of the previously collected soil cores, and will be used to collect dynamic porewater samples. The lysimeters will be sampled for porewater using constant applied vacuum. A minimum of 2 water samples will be collected over a 1 to 2 day sampling period. If needed, to avoid difficulties due to limited porewater in the vadose zone, sample collection will be coordinated with rainfall events or rainy seasons. Collected lysimeters samples will be analyzed for PFAS with and without prior high-speed centrifugation to assess the role of natural colloids on PFAS migration in the vadose zone. In addition to the lysimeter installation and sampling, the moisture content in the vadose zone adjacent to the lysimeter also will be determined to facilitate the modeling needed to calculate PFAS flux to groundwater.
Lastly, using the data collected from the efforts described above, the local PFAS flux to groundwater for each site will be determined. Unsaturated zone transport modelling will be used to estimate the average PFAS flux due to percolating water, using percolation flow modeling and the PFAS levels measured in both the equilibrium and in situ porewater sampling. Although the data collected herein and the modeling will provide only localized (i.e., in the immediate vicinity of the lysimeters) PFAS, the measured soil: porewater relationship will provide an improved tool for overall site assessment related to potential soil impacts to groundwater.
This demonstration is expected to provide the Department of Defense (DoD) with practical information and field-based data that, coupled with ongoing mechanistic studies in SERDP, have potential to provide unique insights into developing/validating meaningful predictive models of PFAS soil-to-groundwater mass discharge, and ultimately guidance for determining soil concentrations protective of groundwater. Attainment of the unique dataset will help the DoD prioritize AFFF-impacted sites with respect to risks to groundwater and receptors.