The overall goal of this project is to improve the understanding and develop effective simulation tools for the transport of per- and polyfluoroalkyl substances (PFAS) in unsaturated systems, including their transport through the vadose zone and capillary fringe to the water table. This project will use a combination of one- and two-dimensional (1D and 2D) laboratory experiments complemented by numerical simulations and comparisons to lysimeter-based field data in a comprehensive investigation of non-equilibrium behavior. The specific objectives are as follows: 

• Determine the appropriateness of isotherm-based models for predicting the accumulation of PFAS at air-water interfaces under conditions representative of aqueous film-forming foam (AFFF)-impacted source areas.
• Assess the combined effects of heterogeneity in both permeability and water saturation on the transport of PFAS through unsaturated soils.
• Investigate impacts of the capillary fringe on overall PFAS flux to underlying groundwater.
• Evaluate the kinetics of PFAS retention and release during hysteretic drainage and imbibition due to transient infiltration and water table fluctuations.
• Validate and modify numerical models of PFAS transport in heterogeneous systems and compare simulations to laboratory and field lysimeter data from AFFF source areas. 

The 1D column experiments will be conducted under both static (Task1) and flowing (Task2) conditions to investigate PFAS partitioning to air-water interfaces under non-equilibrium conditions. Clean sands will be used to isolate the effects of interfacial partitioning, and influent solutions will progress in complexity from individual PFAS, to synthetic mixtures of PFAS and hydrocarbon surfactants, to AFFF-impacted porewater. Building on the results of these column experiments, 2D experiments will be conducted in small-scale (Task3) and intermediate-scale (Task4) flow cells to investigate the effects of heterogeneity and the collapse of air-water interfaces during transient unsaturated flow. These experiments will use clean sands as well as AFFF-impacted soils, along with individual PFAS and AFFF-impacted porewater to evaluate PFAS flux. The intermediate-scale experiments will incorporate a capillary fringe and trapped air (quasi-saturated conditions) to investigate the effect of the capillary fringe on long-term PFAS release. High-resolution visualization techniques will be used to measure unsaturated flow. The results of all experiments, as well as data from field lysimeters collected in ongoing SERDP and ESTCP projects, will be used to validate and modify numerical models of PFAS fate and transport (Task5) to quantify and predict non-equilibrium effects. 

The results of this research will help guide site management and remediation efforts by clarifying the possible relationships between soil and porewater concentrations observed at field sites. This includes guidance concerning the conditions under which co-occurring chemicals (other PFAS and hydrocarbon surfactants) should be accounted for, and identifying the role of heterogeneity in the vertical and lateral transport of PFAS from source areas through the unsaturated zone to groundwater. Of particular interest are the conditions under which simpler models (homogeneous, single species, equilibrium) can adequately describe PFAS fate and transport in the vadose zone. In addition, the results of this research will produce an extensive dataset that can serve as the benchmark for the validation of multi-dimensional PFAS models currently available or under development. (Anticipated Project Completion - 2028)