Lessons learnt from a field trial of colloidal activated carbon injection to reduce PFAS migration from a contaminated site

Due to their extreme persistence, often combined with high aqueous mobility, per- and polyfluoroalkyl substances (PFAS) are challenging to remediate and remove from the environment. Stabilisation by activated carbon is a method that can potentially stop or at least reduce the migration of PFAS and limit further spreading to the environment from contaminated hot spots. In the StopPFAS project, the use of sorbents to limit PFAS migration in groundwater has been investigated, including in a field trial of colloidal activated carbon (CAC) injection at a PFAS contaminated site in Arboga, Sweden. After CAC injection at the field site, there was an initial reduction in PFAS concentrations by approximately 72% for a sum of 11 PFAS. This indicates significant reduction, but that there were still flow paths where the PFAS transport was not intercepted by CAC. Later however, the PFAS concentrations rebounded to levels equal or higher than before CAC injection. After the rebound concentrations again declined to lower than before CAC injection. The reduction in PFAS concentrations by sorption is related to the amount of CAC that the PFAS come in contact with. A sorptive barrier application is likely to be less effective if the CAC emplacement is uncertain or does not reach all flow paths, if the contaminant transport is dominated by a few preferential flow channels or if the barrier is bypassed due to changes in flow directions. Furthermore, our laboratory experiments indicate that competition with natural organic matter (NOM) does not have a large net effect on the PFAS sorption to CAC on the site. The most reasonable explanation to the post-CAC-injection changes in downstream PFAS concentrations was deemed to be changes in the groundwater flow direction causing bypass or partial bypass of the injected CAC. It was concluded that the distribution of CAC in the subsurface and the extent to which CAC intercepts the PFAS-transporting groundwater are critical to the reduction in PFAS concentrations leaving the contaminated site. The soil characteristics including texture, heterogeneity and existence of preferential flow channels are important factors for the resulting CAC distribution. In a treatment with CAC to limit PFAS migration the goal is to catch as much as possible of the contaminant transport with the injected CAC. Our results show that complex hydrogeological conditions pose a challenge to this goal. For optimal design and placement of a sorptive CAC barrier, it is essential to have a detailed characterization and understanding of soil conditions, groundwater flow directions, including seasonal variations, and flux-zone locations on the local scale of flow through and around the CAC barrier.