The overall goal of this project is to assess and validate the use of in situ vadose zone soil flushing with water (using no added cosolvents or constituents) to remove per-and polyfluoroalkyl substance (PFAS) mass and to substantially reduce future PFAS mass discharge to groundwater. Lysimeters will be used for in situ measurements of PFAS mass discharge. Specific objectives are as follows:

1. Assess the mass of PFAS and adsorbable organic fluorine (AOF) removed from the vadose zone with flushing and the extent to which PFAS and AOF mass discharge to groundwater is reduced over time following flushing.
2. Compare the decrease in PFAS mass discharge to the fractional decrease in vadose zone soil mass to support the hypothesis that much of the PFAS mass in soils is not readily desorbed.
3. Determine the extent to which leaching tests can be employed to predict flushing performance.
4. Evaluate the cost-effectiveness of in situ soil flushing to conventional ex situ soil washing approaches, taking into consideration the need to capture and treat flushed water.

In situ soil flushing will be performed in the unsaturated zone at an aqueous film-forming foam (AFFF)-impacted site. The methodologies employed will be based largely on the project team’s experience under SERDP project ER18-1204. Initial testing will focus on determining soil desorption behavior and verifying that the unsaturated zone for the selected site is sufficiently permeable to facilitate water flushing. Soil sampling via coring and porewater sampling using porous cup suction lysimeters under ambient conditions will be used to determine baseline PFAS soil mass and porewater concentrations, respectively. An irrigation system will then be installed to flush the AFFF-impacted soils for a 5- to 9-month period of active flushing, with the duration dependent upon soil permeability and soil leaching kinetics. Active flushing will be suspended intermittently to assess rebound and develop the relationship between porewater mass flux and the fractional PFAS mass removal.

Decreases in PFAS mass discharge and AOF as a function of time and flushing volume will be evaluated using a network of lysimeters and moisture probes. A small groundwater extraction system will be used to capture the PFAS flushed through the vadose zone, as well as PFAS mass from groundwater underlying the treatment area. The extracted groundwater will be treated using a sorbent and the treated groundwater will be re-circulated for use as flushing water. An extended rebound period of 18 months will be performed following cessation of active flushing.

This study will provide a demonstrated in situ flushing approach to substantially reduce PFAS mass discharge to underlying groundwater, thereby serving as a potential alternative to more costly approaches that require excavation, off-site disposal, or high-energy treatment. This proposed approach capitalizes on the fact that, for many soils, equilibrium PFAS porewater concentrations and subsequently mass discharge to underlying groundwater will decrease much more quickly than soil concentrations during leaching. The collected data are also anticipated to demonstrate that a substantial fraction of the sorbed PFAS does not readily desorb from the AFFF-impacted soils. The appropriateness of bench-scale desorption tests for predicting in situ leaching behavior during flushing will also be evaluated. Field-scale data of these leaching behaviors will strengthen science-based arguments for reasonable and site-specific soil clean-up criteria for PFAS, ultimately leading to improved site management of PFAS-impacted sites to directly strengthen the Department of Defense’s operational readiness and ensure warfighter preparedness. (Anticipated Project Completion - 2028)