Some per- and polyfluoroalkyl substance (PFAS) exhibit high affinity for air-water interfaces and recently collected high-resolution field data indicate that changes in moisture content and air-water interfacial area caused by groundwater table (GWT) elevation changes may result in dynamic PFAS storage, release, and enrichment in the GWT fluctuation zone. This process can result in hidden, sorbed mass that can act as a long-term secondary source, drive complex temporal changes in groundwater PFAS concentrations, and influence PFAS distribution and mass flux. From a practical perspective, this phenomenon may make it challenging to reliably discern concentration trends, design robust and reliable remediation systems, and predict long-term plume behavior and assess risks to potential receptors. As such, this knowledge gap translates to increased uncertainty. The overarching objective of this project is to investigate the occurrence, effects, and magnitude of PFAS behavior in the GWT fluctuation zone.

During periods when the GWT elevation is high, PFAS are transported in groundwater and are subject to conventional processes, including advection, sorption, and matrix diffusion. As the GWT drops, the upper saturated zone becomes unsaturated as the zone drains and water content decreases. This results in significant air-water interfacial area. PFAS that have high air-water interfacial partition coefficient values are retained at these interfaces. It is conceivable that under some reasonable field conditions more PFAS mass may be present on air-water interfaces than in the porewater. When the GWT rises and this zone is re-saturated, air-water interfaces collapse, releasing PFAS back into mobile groundwater. Evidence of this process has been observed at several field sites.

This project will be completed in two phases. The first phase will consist of a desktop study of empirical field data from multiple representative sites to assess how widely this phenomenon is observed at aqueous film-forming foam-impacted sites, and to quantify observed enhanced PFAS concentration variability. This effort will also be designed to identify the site characteristics (hydrogeologic characteristics, PFAS concentrations, well construction, etc.) correlated with this behavior. After a go/no-go decision gate, high resolution field data will be collected at two sites to characterize the temporal PFAS distribution, storage, transformation, and mass flux in and near the GWT fluctuation zone, and correlate PFAS behavior to the most influential site characteristics. A simple model will be developed that predicts PFAS distribution and potential variability at the GWT fluctuation zone for use at other sites.

This project will further elucidate PFAS behavior in the GWT fluctuation zone and clarify how this process complicates interpretation of conventional groundwater PFAS concentration data, particularly as it is used for trend delineation, plume stability analysis, remedy design, and risk assessment. Although this work will not definitively resolve all influences, effects, and implications related to this phenomenon, the project will result in useful insights and allow practitioners to consider numerous important practical considerations, such as:

1. What the predicted occurrence and magnitude of this phenomenon is for a specific site (based on soil type, grain size, hydraulics, soil moisture content, PFAS concentrations, or other parameters).
2. Whether there is the potential for PFAS to be sorbed for years or decades and then suddenly released as a result of infrequent, temporary high GWT elevations.
3. How remedy designs and site management strategies need to account for this behavior. 

Successful completion of this demonstration will ultimately lead to more cost effective PFAS management, directly benefiting the warfighter and installation communities. (Anticipated Project Completion - 2028)