Formatted Title
Optimizing an In Situ Thermal Remedy via Pre-Remedial Investigation
Background/Objectives
The site consists of chlorinated solvent contamination in soil and groundwater resulting from release(es) from a former dry cleaner, in particular through an on-site septic system. The site is located near Camp Lejeune in Jacksonville, North Carolina, and consists of a mixed-use residential/commercial/industrial area. An operational retail business is located adjacent to the site, with only a small alley separating the edge of the site from the adjacent structure. Previous remedial activities to address the source area included removal of the septic system and soil vapor extraction; however, it was determined that significant mass remained following shutdown of the system in 2011, and a more robust remedy is warranted. Supplemental investigation in 2018 refined the understanding of the source area extent and supported development of a remedial design (RD) for in situ thermal remediation (ISTR). However, concerns about potential vapor intrusion risks associated with the adjacent commercial property and continued contaminant mass contributions to groundwater remained, and additional further investigation was warranted. Therefore, a pre-remedial investigation (PRI) was conducted in 2023 to refine the understanding of extent of contamination adjacent to/underneath the adjacent building and to optimize the planned ISTR.
Approach/Activities
The PRI was performed at the site in 2023 utilizing an adaptive management decision model and focused on several objectives: (1) closing existing data gaps including the lateral and vertical extent of source material along the shared boundary and beneath the adjacent off-site building, (2) establishment of baseline conditions including identifying any potential impacts on contaminant migration as a result of disturbance during septic removal, (3) verification of the previously estimated shallow treatment area; (4) verification of the transmissive zones within the proposed treatment areas, and (5) confirmation of geotechnical characteristics. To accomplish these, a series of four vertical borings, four angled borings, and three hydraulic profile tool borings were advanced throughout the site to a maximum depth of approximately 80 feet below ground surface. At each boring, a series of soil samples were collected from various depths (approximately every 6 feet) within the borings. The new data were combined with previous data to re-evaluate thermal treatment zones, including generation of a three-dimensional model of the contamination, as well as evaluate geotechnical conditions and transmissivity.
Results/Lessons Learned
Based on the results of the PRI, three primary optimizations of the planned ISTR were developed. First, the extent of the originally proposed five thermal treatment zones were refined to ensure full capture of the source area on site. Additionally, due to the determination that transmissivity at the site is likely higher than incorporated into the RD, the proposed depths of the heater wells were extended to provide additional heating, and supplemental steam injection wells were added to the proposed ISTR layout to prevent influx of cooler groundwater resulting in inefficient heating. Finally, a sixth treatment zone was defined and added to the ISTR plan to address contamination under the adjacent building, including installation of angled heaters in a fan array and a subslab depressurization system to capture generated vapors allowing the business to remain open during system installation and operation and continued access to the alleyway between the site and adjacent building. The system is currently being installed, and it is anticipated that early operational data will be briefly presented.