Per- and polyfluoroalkyl substances (PFAS) are a group of chemicals that have become widespread in groundwater, largely through their use in aqueous film-forming foam (AFFF). Because PFAS are both highly recalcitrant and mobile, plumes continue to expand, potentially further increasing the overall treatment costs. To address PFAS plume control, ex situ treatment technologies involving groundwater extraction coupled with granular activated carbon and/or anion exchange resin (AER) are being employed. While there are currently no proven in situ technologies to destroy PFAS in groundwater aquifers, amendments designed to sorb PFAS have been evaluated in laboratory studies, and some have been applied in the field to create permeable adsorptive barriers (PABs) for the capture of PFAS. One of these applications is the funnel and gate (F&G) system installed and effectively removing PFAS in an impacted aquifer from a source area under ESTCP Project ER20-5252.

The objective of this project is to build on the previous project and perform long-term monitoring of the F&G system, while also employing a novel, off-the-grid pumping/groundwater recirculation approach, to significantly increase groundwater flow through the treatment gate and provide substantially more lateral capture of PFAS from the source area. This project is designed to improve the understanding of the efficacy of the F&G system for in situ PFAS remediation by studying the long-term removal of PFAS. This project leverages the previous site assessment, F&G system design, installation, modeling, and monitoring. Thus, evaluation of the long-term effectiveness of the existing F&G system is proposed to confirm that this approach can be a lower-cost, passive alternative to existing pump and treat approaches and can be readily implementable at many AFFF-impacted sites.

Permeable barriers have been applied in a range of configurations, using either sorptive or reactive media, for a variety of groundwater chemicals. One type of PAB design is the F&G, which consists of either (1) impermeable barriers or (2) high permeability trenches installed perpendicular to groundwater flow with impacted groundwater directed to a treatment gate or vessel that contains permeable adsorptive media. The barrier F&G approach is currently being evaluated for PFAS remediation via ESTCP Project ER20-5252. The main advantages of this F&G approach over injected PAB that typically employ particulate amendments are: (1) the sorbent media (AER) has a much a higher adsorption capacity for shorter chain perfluoroalkyl acids, and (2) the media in the gate/vessel can be removed and replaced when saturated with PFAS, thus removing PFAS from the aquifer, rather than sequestering it in place, and potentially allowing desorption and release over time.

This ESTCP demonstration project involves the continued performance monitoring of the F&G system (containing AER as the adsorptive medium) that was installed in August 2024. The overall system is approximately 123 ft wide and installed to 29.5 ft below ground surface, consisting of a treatment gate that is approximately 2 ft wide x 2 ft long x 30 ft high that is flanked by two 60.5 ft funnel walls, all installed to a depth of 29.5 feet below ground surface. A network of monitoring wells located within and outside of the treatment gate is used to monitor both hydraulic capture and treatment performance. An additional 3.5-years of performance monitoring will take place for this project (in addition to the 1.5 years of monitoring being conducted during ESTCP Project ER20-5252).

This project will provide follow-up monitoring and system optimization where an F&G has already been installed (> 1 million dollars of leveraged funding) and will provide critical insights into the long-term performance of this technology, including documenting AER adsorption capacity in situ, potential breakthrough for different PFAS, and possible impacts on AER permeability and groundwater capture. Studies documenting the adsorption capacity of aged AER also will help to improve estimates of barrier longevity and performance. These data will provide valuable information for future F&G designs, which may significantly impact cost and performance. Furthermore, the project will provide guidance to practitioners and support regulatory acceptance of this technology.

This technology is expected to provide a demonstrated in situ, passive, low-cost method to mitigate PFAS-impacted groundwater in shallow aquifers. There are many different PAB configurations possible for this technology, making it widely applicable at a variety of sites. The F&G approach can be implemented in either high concentration source areas to reduce mass flux, or in larger dilute plumes to protect downgradient receptors and/or mitigate expansion of the plume. Successful completion of this demonstration will ultimately lead to more cost effective PFAS management, directly benefiting the warfighter and installation communities. (Anticipated Project Completion - 2029)