Theme: 06. Petroleum and Heavy Hydrocarbon Site Strategies

Topic Area: 6b. In Situ Remediation of Petroleum Hydrocarbons

Formatted Title:

Addressing Residual Hydrocarbon Concentrations Using Micron-Scale Carbon Injections at Formerly Used Defense Sites

Background/Objectives:

Formerly used defense sites (FUDS) in and near the Outer Banks, North Carolina (NC) are being remediated for residual hydrocarbon contamination. The goal at these sites is no further action, which requires the groundwater hydrocarbon concentrations to be below the individual NC Groundwater Quality Standards (GWQS). Source removal by excavation was conducted prior to 2011. Following excavation, initial groundwater sampling results indicated a decline in hydrocarbon concentrations. Concentrations have plateaued with concentrations two to three times above the GWQS since 2014. All the sites are in ecologically sensitive locations, limiting remedial technologies that could be implemented.

Approach/Activities:

Several remediation approaches were evaluated, including air sparging, thermal treatment, in situ chemical oxidation (ISCO), and injection of a proprietary micron-scale carbon suspension with soluble anaerobic electron acceptors (MSC). Factors involved in the remedy selection were probability of success, effects on sensitive nearby environments, implementability, and cost. Once micron-scale carbon was selected, implementation started with online planning tools, followed by additional refinement and support from the company providing the MSC.

Two MSC injection events, in the summers of 2020 and 2021, were conducted using direct push technology to depths ranging from 3 to 16 feet below ground surface. Visual confirmation of the distribution of the MSC was completed by collecting soil cores before and after injections. Soil classification, screening with a photoionization detector (PID), and visual verification of the MSC to identify zones of lower and higher conductivity zones, allowing for real-time adjustments to the MSC injection volume and pressures. Small variations in grain size played a significant role in distribution of the MSC.

Results/Lessons Learned:

Groundwater CoCs, which include benzene, isopropylbenzene, naphthalene, and 1-methylnaphthalene, all showed reductions in groundwater concentrations after the MSC injections. Volatile petroleum hydrocarbon (VPH) and extractable petroleum hydrocarbons (EPH) concentrations initially increased but have since decreased below the GWQS in the last year. Following the injection events of the MSC one site was closed in 2022 and the second site is schedule for closure in 2023 and significant reduction of contaminants of concern (CoCs) have been observed at a third site. Based on these demonstrated successes, the MSC has been recommended remediating a site in South Carolina.

Remedial performance following the initial injection event may have been improved with a more detailed classification of the complex permeability zones at the three sites.

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Kevin Moon