The overarching objective of this project is to develop and validate a modeling framework that simulates the performance of an in situ colloidal activated carbon (CAC) barrier at a coastal site, to improve the prediction of CAC longevity at all coastal sites, and to determine realistic long-term remedial goals that may be attainable at coastal sites in general using this approach. The specific technical objectives include the following:

• high resolution delineation of the CAC distribution at one coastal site;
• modeling the maximum extent of CAC transport downgradient of the permeable adsorptive barrier (PAB), and several options for improving remedy performance when injecting CAC into a tidal-influenced flow system;
• estimating the average field-scale per- and polyfluoroalkyl substances (PFAS)-CAC adsorption isotherms for short- and long-chain perfluoroalkyl acids and common precursors at a coastal site;
• evaluating the representativeness of site-specific laboratory batch tests for predicting field-scale CAC longevity;
• characterizing PFAS desorption behavior and timeframes in the region between the PAB and the shoreline;
• using the demonstrated model and input parameters to support the design and analysis of a CAC pilot test at another coastal site; and
• development of a manual for the design, installation, and operation of CAC barriers at coastal sites.

CAC for In Situ PFAS Remediation at Coastal Sites: Field Assessment and Modeling of Long-Term Efficacy

Colloidal (1-2 µm) activated carbon has been demonstrated to disperse into aquifer settings under low pressure injection. After injection, this material coats the aquifer matrix with a thin layer of immobilized, highly sorbing CAC, which leads to increased adsorptive capacity of the aquifer while minimally altering hydraulic properties. Field and laboratory studies have shown this approach to be an effective option for sequestering many PFAS including perfluorooctanoic acid and perfluorooctanesulfonic acid, thus slowing plume migration and reducing mass flux in an aquifer. The approach is innovative as it employs a treatment technology in situ that has been most successfully applied to date for ex situ treatment of PFAS (i.e., granular activated carbon in pump and treat systems). However, it is currently difficult to set expectations for this technology and/or optimize its application at coastal PFAS sites where flux reduction to vulnerable water bodies (e.g., bays, harbors) is a desired outcome. The influence of coastal site tidal flow and variable geochemistry on both CAC distribution and adsorption capacity for different PFAS is uncertain. The goal of this ESTCP project is to improve the application of CAC technology at coastal sites by characterizing and modeling CAC distribution and CAC-PFAS sorption in a coastal environment.

The expected benefits at the site level include: (1) more efficient CAC distributions at coastal sites; (2) greater remedy longevity; (3) reduced overall costs for CAC remedy implementation at coastal sites; (4) more realistic expectations of attainable remedial goals; and (5) increased regulatory acceptance. Due to the simplicity and low cost associated with this passive technology, CAC may be implemented at dozens to hundreds of sites in the future, including many coastal sites. The results of this study are expected to optimize CAC application approaches at coastal sites, leading to both improved technology performance and reduced overall costs. Results from this work will provide an improved capability to cost-effectively characterize, remediate, and manage soil and waters impacted by chemicals of concern. (Anticipated Project Completion - 2028)