Theme: 01. Remediation Technology Innovations

Topic Area: 01q. Electrical Resistance Heating: Best Practices and Lessons Learned

Formatted Title:

Fast-Track Implementation of the First In Situ Thermal Treatment System at an Active Air National Guard Base

Background/Objectives:

Historical operations within former Building 1304 at the Montgomery (Alabama) Air National Guard Base impacted soil and groundwater with chlorinated volatile organic compounds, primarily trichloroethene (TCE). Although Building 1304 and the suspected contamination source, an oil/water separator, were removed, the presence of TCE concentrations of up to 3,200 milligrams per kilogram (mg/kg) hindered site redevelopment to accommodate the planned F-35 beddown mission. In October 2021, a non-time critical removal action (NTCRA) was initiated to address TCE, vinyl chloride, and 1,2,3-trichloropropane concentrations in soil that exceeded their respective Alabama Department of Environmental Management screening levels and prepare the site for construction of the F-35 facilities. While excavation and in situ chemical oxidation via soil mixing were identified as the preferred remedial technologies in the 2020 Feasibility Study, these alternatives presented significant technical, safety, schedule, and cost challenges, especially considering the transportation and offsite disposal of hazardous soil. Accordingly, an alternate remediation strategy was necessary to meet the stringent treatment goals, including reducing the 95% upper confidence level (UCL) TCE concentration to 0.940 mg/kg by July 2023.

Approach/Activities:

To meet the aggressive schedule and achieve the soil remediation goals, in situ thermal treatment (ISTT) was selected for the NTCRA because this approach offered greater treatment certainty. However, the spatial distribution of site contaminants was not fully defined and presented significant challenges to a fast-track ISTT system design and implementation. The project schedule required that ISTT design and planning proceed concurrently with the data gap investigation (DGI) to finalize the target treatment zone (TTZ). Between October 2021 and March 2022, real-time observations from the DGI, including membrane interface probe data and historical site investigation results from 2014 and 2020, were integrated into a 3D spatial model of contaminant distribution to finalize the 37,400 square foot and 26,400 cubic yard TTZ necessary to compete the ISTT design. The final TTZ was 67% larger in volume and flatter in morphology compared to the initial NTCRA design. In addition, the maximum TCE concentration was 260% greater than previously known. Collectively, these characteristics significantly impacted the ISTT design, construction sequencing, power requirements, and treatment duration.

Results/Lessons Learned:

An electrical resistance heating (ERH) system was constructed between June and November 2022. Field efforts were sequenced across the site to allow subsurface installation to proceed concurrently with electrode completion, surface cap construction, extraction piping installation, and power conductor terminations. System construction included 7,000 linear feet of drilling, 26,000 feet of electrical conductors, and 3,600 feet of high temperature conveyance piping to support operation of the 208 electrodes installed within the TTZ. Concurrently, Base stakeholders worked with the Alabama Power Company and the Base electrical contractor to install the infrastructure to deliver 9 million kilowatt-hours of electricity to the ERH system, while avoiding interference with the existing Base electrical system. The Base Environmental Management Office (EMO) negotiated with the Montgomery Water Works and Sanitary Sewer Board to secure a discharge permit for the treated condensate from the ERH system operation. The ERH system operated for 190 days between November 2022 and June 2023. Based on more than 60 soil confirmation samples, the treatment goals were achieved; the maximum TCE concentration was reduced from 3,200 to 0.620 mg/kg and a 95% UCL of 0.038 mg/kg was achieved for the treatment zone, well below the goal of 0.940 mg/kg. This result allowed the F-35 beddown facility construction to proceed on schedule. In the presentation, the administrative, characterization, design, construction, and operation challenges that were mitigated to complete the time-critical removal action under an aggressive and non-flexible timeline will be discussed further.

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Sarah Meyers