Activated Carbon-Based Technology for In Situ Remediation
Guidance
Remedial Design
Injection of granular activated carbon (GAC)- or powdered activated carbon (PAC)-based products primarily targets contamination stored in low-permeability zones, which often serve as slow-releasing sources that sustain persistent groundwater plumes. Remedial design of the GAC- or PAC-injection system typically focuses on characterizing the contaminant mass and delineating its distribution, both horizontally and vertically, in the subsurface. The mass dictates the loading rate of AC based amendment, while the distribution determines the injection intervals. Various in situ characterization tools can be used to qualitatively characterize the level and vertical heterogeneity of contamination. Continuous soil sampling and multilevel groundwater sampling can be further employed to obtain more quantitative results. Recognizing contaminant distribution is impacted by geology, characterization of soil lithology is another important component during remedial design.
Injection of colloidal AC-based product also relies on defining contaminant mass and distribution and geology. However, the amendment loading mainly focuses on identifying permeable zones and quantifying contaminant mass flux across those zones.
Implementation
The approaches to emplacing AC-based amendments are largely like other amendment-based in situ technologies, such as zerovalent iron (ZVI). AC-based amendments are most frequently emplaced by slurry injection to form permeable reactive zones in the subsurface. For granular activated carbon (GAC)-based products, soil mixing or trenching have occasionally been applied to construct a permeable reactive barrier when hydrogeological conditions permit. For direct injection, GAC- and PAC- based products require high pressure (high flow and low volume) to fracture and open the soil formation. The injection is typically conducted using a Geoprobe® equipped with customized injection tip and powered by a high-pressure pump. The injection spacing is typically tight both horizontally (5–7.5 ft) and vertically (~2 ft) to ensure sufficient coverage. In contrast, colloidal AC-based products are injected to high-permeability zones under low pressure due to their smaller particle size. The injection can be done via either direct push tools or permanent wells. In fractured rock, a customized straddle packer is often used to seal off the areas above and below the target fractures for successful delivery.
Performance Monitoring
Typical performance monitoring concerns the verification of amendment distribution inside the treatment zone and measurements of contaminant concentration over time. Amendment distribution may be evaluated during injection using indirect methods, such as surface telemetry, or by monitoring changes in geochemical parameters. Distribution can also be evaluated post injection using direct soil sampling. The presence of AC in the monitoring well must be used cautiously for indicating the radius of influence of injection, especially for high pressure injection. This is because a monitoring well, when in proximity to injection points during high pressure injection, may serve as a preferential pathway for amendment delivery. Wells impacted by AC should not be used for performance evaluation. AC should be removed before sampling when feasible or new wells that are free of AC should be installed. The most unbiased way to evaluate treatment performance is to compare mass flux coming out of the treatment zone before and after injection, which requires placement of a monitoring well immediately downgradient of the treatment zone.
The common analytical suite for performance monitoring of AC-based technologies includes concentrations of parent contaminants, daughter products, organic acids and dissolved gas. Simply measuring parent compounds may not tell whether concentration decline is due to adsorption or degradation. Anions, such as chloride, may be used to indicate dechlorination if the concentrations of parent compounds are sufficiently higher than the background chloride concentration.
Molecular diagnostic tools can also be used to provide direct or indirect evidence of degradation, especially for the AC-based products that have biologically active components.
Additional Information
In Situ Treatment Performance Monitoring: Issues and Best Practices
EPA 542-F-18-002, 2018
The purpose of this issue paper is to describe how in situ treatment technologies may impact sampling and analysis results used to monitor treatment performance and provide best practices to identify and mitigate issues that may affect sampling or analysis. This paper discusses eight potential sampling or analytical issues associated with groundwater monitoring at sites where in situ treatment technologies are applied. These issues are grouped under three topic areas: Issues related to monitoring wells (Section 2); Representativeness of monitoring wells (Section 3); Post-sampling artifacts (Section 4).
Performance of Injected Powdered and Liquid Activated Carbon at a Petroleum Hydrocarbon Site
McGregor, R.
11th International Conference on Remediation of Chlorinated and Recalcitrant Compounds. Palm Springs, CA (April 2018)
This field-scale case study compared the distribution of colloidal activated carbon and powdered activated carbon following parallel injection into a sandy aquifer using the same injection equipment and pressure. The injection pressure was low, which is suitable for colloidal activated carbon but not optimal for powder activated carbon. Soil cores were collected after injection at incremental distances from the injection point up to 20 feet.
Remedial Technology Fact Sheet — Activated Carbon-Based Technology for In Situ Remediation
EPA 542-F-18-001, April 2018
This fact sheet provides information to practitioners and regulators for a better understanding of the science and current practice of AC-based remedial technologies for in situ applications. The uncertainties associated with the applications and performance of the technology are also discussed.
