The overall goal of this work is to determine the efficacy of particulate carbon amendment (PCA) in arresting down-gradient plume migration, and the operating parameters that maximize PCA performance. This work is motivated by the Department of Defense's (DoD's) liability of impacted groundwater sites, the initial success of PCAs in arresting plume migration in relatively homogeneous aquifers, and the lack of sufficient data indicating success is achievable in heterogeneous aquifers. The specific technical objectives are the following:
- Determine the effects of porous media heterogeneity on PCA transport and distribution, and the corresponding effects of PCA distribution on permeability alteration;
- Determine sorption kinetics and equilibrium capacities of selected chlorinated volatile organic compounds (CVOC) and per- and polyfluoroalkyl substances (PFAS) on PCAs;
- Identify the mechanisms controlling CVOC and PFAS attenuation by PCAs in groundwater, including biodegradation;
- Quantify how PCAs address the long-term release of chemicals from large, diffuse matrix diffusion sources.
While there are many PCAs, only activated carbon-based particulate amendments will be considered in this work, and PlumeStop® (Regenesis) or similar will be used because of its widespread application at DoD sites. The primary chemicals of study will be CVOCs and PFAS; much more is known about the former, so the mechanisms controlling PCA performance will be easier to interpret. However, there is a greater need to understand performance of the latter, due to the tremendous liability posed at DoD sites. Various configurations of heterogeneity in sand-sized media with silt/clay deposits will be evaluated in the laboratory, and these deposits will be obtained from a DoD field site in Jacksonville, FL. The PCA will be injected into individual columns, each containing material from a uniform stratigraphic unit taken from the field site, and both PCA distribution and permeability alteration will be determined as a function of relevant injection parameters. The same PCA injection will be performed on columns set up in parallel with materials from different stratigraphic units to explore the effects of heterogeneity. Sorption uptake kinetics and equilibrium will be evaluated for PCA using both CVOCs and PFAS. This will then be used to interpret breakthrough and elution profiles performed with column experiments, in the absence and presence of biodegradation. A single-well PCA injection will be demonstrated at the Jacksonville site, and results will be informed by the laboratory experiments. Various modeling approaches will be used to interpret the both laboratory and field data.
This work will fill key knowledge and data gaps regarding injection of PCA in heterogeneous groundwater aquifers and the ability of this technology to mitigate down-gradient plume migration. Specifically, it will address the following questions:
- How are PCAs distributed in heterogenous media, and what is their impact on groundwater flow paths;
- What is the capacity of PCAs for sorption, especially under competitive sorption and potential mass transfer limited conditions;
- How does competitive sorption affect plume mitigation;
- Under what desorption conditions can biomass grow and remain active for chemical degradation on PCAs;
- What unexpected outcomes might occur (e.g., groundwater bypass), and how can they be anticipated;
- What is the ultimate role of PCAs in site management (e.g., short term intervention or permanent remedy)?
(Anticipated project completion - 2024)