Formatted Title
Implementing Effective In Situ Permeable Colloidal Activated Carbon (CAC) Barriers to Stop Hydrocarbon Plume Migration
Background/Objectives
Background/Objectives. Colloidal activated carbon (CAC) is increasingly being utilized for various application approaches, commonly to create long-lasting permeable reactive barriers (PRBs) for solvent or PFAS (Per- and polyfluoroalkyl substances) plumes, effectively mitigating plume migration. CAC PRBs for hydrocarbon sites are less common yet offer sustainable groundwater remediation technology with low maintenance costs, service longevity, and broad effectiveness for organic pollutants. The proven effectiveness of CAC PRBs in addressing hydrocarbon impacts offers practitioners a viable solution to address the nation's thousands of underground storage tanks (USTs) and hydrocarbon spill sites. Example applications include property line protection from a spill, protection of a water body, pre-emptive mitigation of plume migration, and intermittent barriers to replace injection grids on very large sites.
Informed by experience gained through the increasing use of CAC PRBs for hydrocarbon plumes over the last decade, this talk summarizes the state of the practice on design and implementation to help ensure barrier effectiveness. Furthermore, some performance results from implemented CAC barriers at selected sites will be highlighted to emphasize the best practices described.
Approach/Activities
Approach/Activities. Many variables can influence effectiveness, although several site parameters and practices were identified that impact performance more than others.
Underestimating adsorbable and biodegradable mass is of concern, particularly since typical hydrocarbon spill sites don’t have approval or don’t require the collection of total petroleum hydrocarbon analysis for gasoline or diesel (TPH-G, TPH-D). The development and use of a simple rule-of-thumb for TPH-G loading rates will be shared, and how they can overcome the lack of such dissolved concentration data.
The rate at which dissolved fuel constituents pass through a potential barrier location can vary vertically through an aquifer and is associated with soil permeability. General Darcy velocity calculations do not measure actual flux rates in preferential, high-mass flux corridors in heterogenous soils. The best practice of using flux meters, when appropriate, will be discussed and how their use overcomes the measurement of mass flux challenge.
The need to verify the distribution of CAC during injection is crucial, as sites that don’t verify underperform. Sites can vary from assumed porosity and conductivity parameters, and a low-cost, rapid approach is needed to overcome product gaps. A simple and practical field distribution verification approach will be shown by using the coloration of the amendment and how it improves performance. Two case studies outlining the successful emplacement of a CAC PRB using best application practices to address PHC’s will be shared.
Results/Lessons Learned
Results/Lessons Learned. Correctly estimating groundwater seepage velocity, soil grain analysis, and contaminant mass flux are crucial design parameters. Furthermore, using the dark color of the CAC injectate as a real-time field tracer to observe the efficacy of delivery to the target zones provides a simple and direct means of placement validation to ensure performance success. Adhering to best practices resulted in a PRB installed at Naval Base Point Loma to prevent the movement of residual diesel contamination from reaching San Diego Bay and at a PRB in MT to prevent a large BTEX plume from entering a reservoir above ND for three consecutive years.