Formatted Title
Tempe Cell Method for Quantifying Vadose Zone Leaching of PFAS Sources
Background/Objectives
The leaching of per- and polyfluoroalkyl substances (PFAS) at AFFF source areas is more complex than that of simple hydrophobic contaminants. Ongoing research has indicated that PFAS tend to partition at the air-water interface within vadose zone soil resulting in nonlinear, moisture dependent leaching behavior that requires a combination of numerical modeling and representative leaching methods. However, USEPA groundwater screening levels are very low and the conventional approach of using synthetic precipitate leaching procedure (SPLP) testing in a dilution-attenuation factor approach may be too conservative to provide realistic estimates of site-specific leaching for source mass loading at AFFF sources. The state of practice involves application of lysimeters to collect porewater samples that represent equilibrium between infiltrating water and PFAS-containing vadose zone soils. While this approach provides the best estimate of leaching concentrations, it is often difficult to collect representative samples under certain conditions, for example, in arid or semi-arid climates or highly permeable vadose zones where infiltration rates are rapid. The proposed Tempe Cell method of conducting leaching tests will provide an alternative to lysimeter porewater with the advantage of completing the tests under controlled conditions and providing data to parameterize vadose zone transport models.
Approach/Activities
The primary objective of the demonstration is to develop and validate a leaching method using Tempe cells by extending the retention curve analysis to allow sampling of porewater from intact cores under controlled conditions. Validation of the method will be completed using two primary lines of evidence: comparison with lysimeter porewater sampling through intact cores collected proximal to prior lysimeters and comparison on Transient Water Release and Imbibition (TWRI) sampling at various points on the soil moisture retention curve and numerical simulations. The ultimate goal is to develop a commercial method of leaching tests that can be reliably conducted by contract laboratories, enabling widespread adoption by practitioners and providing a realistic estimate of leaching potential for vadose zone soils containing PFAS. This demonstration will develop and validate a commercial laboratory method of leaching tests using the Tempe cell conducted on up to five different AFFF sources where prior lysimeter testing was completed to estimate PFAS leaching in the vadose zone. Up to 60 intact core samples will be collected for leaching tests.
Results/Lessons Learned
The presentation will discuss the results of the initial phase of testing conducted at Ellsworth AFB in Rapid City, South Dakota. The discussion will compare leaching results versus saturation to establish the variability of the behavior with composite leaching tests designed to simulate ambient or infiltration event conditions. Vadose zone transport modeling will be completed in parallel with the Tempe cell retention curve assessment to provide a theoretical basis to better understand the results of detailed leaching tests and how simpler composite tests can be used for representative leaching evaluations in the commercial method. Lessons learned from the initial testing will be applied to refine the approach for the four remaining sites.