In Situ Treatment‐Train Remediation of Per‐ and Polyfluoroalkyl Substance–Impacted Groundwater
Authors: Pourabadehei Matt , Tarnocai David , Timlin Robert , Orquiza Renan , McRae Chris , Tam Andrew 
Published: February 2025

It has been more than a decade since per‐ and polyfluoroalkyl substances (PFAS) were listed as persistent organic pollutants (POPs) in the Stockholm Convention and, to date, PFAS are still considered emerging contaminants. Our understanding of the fate, transport, bioaccumulation, and toxicity of this group of chemicals is evolving and, subsequently, the PFAS regulations around the world are being updated on a frequent basis. To meet the current and progressively more stringent PFAS regulatory guidelines, enhanced and viable field‐deployable remediation technologies play a crucial role for mitigating risks associated with PFAS exposure, particularly for treating aqueous media. An innovative In Situ Treatment‐Train, in the form of a Permeable Reactive Barrier (“ISTT‐PRB”), was designed and a pilot‐scale ISTT‐PRB was constructed between August 2021 and March 2022. This pilot‐scale treatment system was designed to provide a proof‐of‐concept test of using a PRB to intercept and remediate PFAS‐impacted groundwater within a portion of a PFAS‐contaminated location at Canadian Forces Base Trenton in Ontario, Canada. Utilization of permeable retaining walls between reactive cells (where treatment‐train media are located) allows separate replacement of the tested treatment media (consisting of modified bentonite clay and granular activated carbon) in any single cell, or chamber, if and when the media become exhausted and reach PFAS breakthrough. Hence, PFAS can be removed from the site, even though the ISTT‐PRB is an in situ remediation system. The pilot‐scale ISTT‐PRB has successfully treated PFAS‐impacted groundwater (migrating from the source zone) during almost 2 years of operation, with up to a 99.9% PFAS removal efficiency in lag (downstream) reactive cells. Lower PFAS removal efficiency was observed after heavy precipitation events or when the ground and PRB were fully saturated (i.e., when the groundwater table was above the top of the treatment media). Saturated ground conditions may have resulted in untreated groundwater bypassing the lead (upstream) reactive cells, which adversely affected the quality of treated groundwater in the lag reactive cells, therefore adversely affecting the PFAS removal efficiencies. Despite the saturated ground conditions during certain periods of the year, the removal efficiencies in lag reactive cells remained above ~94% in the second year of operation. Construction of a full‐scale system in areas of higher hydraulic gradient and deeper water table may improve the performance of an ISTT‐PRB. After almost 2 years of operation, all three ISTT‐PRB chambers (with 2.5%. 5.0%, and 7.5% mixing ratios of treatment media with PFAS‐free fine sand) equally showed high PFAS removal efficiency. It is believed that the ISTT‐PRB pilot system can effectively intercept and treat PFAS‐impacted groundwater for years without original treatment media replacement. When the treatment media are approaching fully adsorbed conditions, the performance of each chamber with different sorbent media mixing ratios can be reassessed, and subsequently, an optimized mixing ratio can be determined.