The overall objective of this project is to develop a framework for evaluating site-specific soil-to-groundwater leaching and mass loading to facilitate site prioritization. This project will assess processes impacting fluxes of per- and polyfluoroalkyl substances (PFAS) and the applicability of current vadose zone models for quantitative prediction of PFAS migration. The objective is to apply the methods at two different source areas with differing depth-to-water, recharge rates, geology, and weather conditions to evaluate the broader applicability of the approach. Specific objectives include the following:

• Evaluation of tiered levels of model parameterization and model sophistication to determine if source behavior can be modeled with sufficient accuracy to predict groundwater concentrations.
• Demonstration of a protocol for deriving source-specific soil-to-groundwater leaching.
• A site-specific approach for identifying relevant sources and determining associated source strength (mass discharge over time) that can be applied to a range of conditions.
• A framework for source prioritization based on source strength.

This project will demonstrate the PFAS-LEACH decision-support modeling platform at the field-scale by using it to simulate site-specific attenuation and mass discharge at aqueous film-forming foam-impacted sources. The performance of the models to simulate leaching and mass discharge will be verified against porewater and groundwater concentrations observed in downgradient transects of vertebrae monitoring wells. The approach will reconcile datasets including recharge, soil moisture, porewater, underlying groundwater, and site data to identify the primary parameters governing PFAS mass loading from the vadose zone. Through the application of the field-scale modeling at two different sources under different weather, hydrogeologic, and depth-to-groundwater conditions, the project team will develop a framework for determining source-specific soil-to-groundwater pathway leaching standards and quantifying source strength through modeled long-term mass loading and mass discharge estimates. 

The research will take place at two sites. Three methods will be evaluated for estimating water recharge, as it is critical for modeling the vadose zone PFAS leaching behavior. Existing downgradient monitoring wells will be used to estimate mass discharge using measured PFAS concentrations, hydraulic conductivity, and PFAS/groundwater flux. The intent is to verify the capability of the models to match observed groundwater concentrations and determine the data density required to parameterize the different model tiers of PFAS-LEACH (from the most comprehensive three-dimensional model to the simplest one-dimensional model) reliably and accurately. Sensitivity analysis will be performed to determine which parameters govern flux from PFAS source areas.

This study will be the first to employ source-specific models to provide a framework by which to estimate mass discharge and a scientifically robust basis for site prioritization under field conditions. Source strength will be evaluated in terms of the magnitude and persistence of mass loading, providing a framework for ranking and prioritizing sources based on the relative mass loading contribution to groundwater plumes. The outcomes will equip users with a consistent framework for implementing source reduction or control at PFAS-impacted sites. Successful implementation of this demonstration holds profound implications for improving the DoD's management of PFAS, directly addressing mission readiness by safeguarding the warfighter and their communities. (Anticipated Project Completion - 2027)