Formatted Title
Large-Scale In Situ Remediation of PFAS in Groundwater Using Colliodal Activated Carbon
Background/Objectives
Aqueous film-forming foam, used to extinguish fires, is a major source of PFAS in groundwater worldwide. PFAS has been shown to be effectively removed from groundwater using several different types of adsorptive media, including activated carbon. Colloidal activated carbon (CAC) is a remediation product composed of very fine particles of activated carbon suspended in polymer that can be injected into the subsurface to facilitate in situ remediation of PFAS. After injection, the CAC binds to the aquifer matrix and serves as an in situ filter to remove PFAS from groundwater as it comes in contact with the activated carbon particles. This project was performed at a former Fire Training Area (FTA) where groundwater is contaminated with PFAS from historic aqueous film forming foam (AFFF) usage. Elevated PFAS concentrations were identified (as high as 190 parts per billion) in a shallow, perched aquifer. The subsurface consists of a permeable industrial fill and sand, underlain by a continuous clay layer.
Approach/Activities
The approach for achieving the remedial objective was to install an in situ permeable treatment barrier perpendicular to groundwater flow along the perimeter of the FTA, upgradient of the surface water body. A modelling software, PlumeForce™, was used as a tool to support the remediation design and data interpretations. In this software, the PFAS mass between multiple compartments (i.e., FOC in soil, groundwater, and CAC) is modelled for multiple species in parallel, including biotic and abiotic transformations of amenable species. Competitive adsorption on CAC is accommodated, with individual soil adsorption coefficients (Kd) recalculated for each finite difference. The software also allows hidden competition to be factored in based on informed engineering estimates of likely composition of the impact of the hidden / non-quantified PFAS. The model predictions are then compared to field observations to determine the agreement between the modelled and observed data. Based on pilot study results and the modeling process, a full-scale CAC remedy was designed and implemented. The full-scale approach consisted of injecting over 292,000 pounds of CAC into over 650 injection points to create a 1,625 foot-long barrier perpendicular to groundwater flow. CAC loading in certain areas was modified based on the presence of co-contaminants. To date, this is the largest application of PlumeStop to treat PFAS-contaminated groundwater.
Results/Lessons Learned
Data collected during the field activities indicated that CAC could be distributed at least 3 to 4 feet away from each injection point and PFAS concentrations in groundwater (specifically PFOS and PFOA, which are regulated in surface water) were reduced by more than 99.5 percent. During the field implementation, several challenges were encountered due to the heterogeneous nature of the subsurface and shallow groundwater. Several lessons were learned, and best practices were developed which will be shared at the conference. Furthermore, 2.5 years of performance monitoring data will be compared to the modeled results determine agreement and usefulness of the model as a predictive tool for PlumeStop performance at this site.