Theme: 09. Vapor Intrusion

Topic Area: 9b. Vapor Intrusion Mitigation and Effectiveness

Formatted Title:

Subfloor Tunnels and Floating Slabs in a 7-acre Building: The Case of the Accidental VI Mitigation System(s)

Background/Objectives:

A 7-acre building at a Navy facility was historically used for various manufacturing functions since WWII and stored tetrachloroethene (PCE) and trichloroethene (TCE) in aboveground storage tanks (ASTs) in the northern part of the building. These ASTs leaked, creating a TCE DNAPL body (outside the building), as well as dissolved TCE contamination (inside and outside the building). The building has numerous tunnels and utility corridors, some extending to the water table at approximately 7 feet bgs. Volatilization of cVOCs from the shallow groundwater has created a cVOC vapor plume underneath the building with initial VOC levels > 100,000 µg/m³. The North SVE system has been in operation north of the building since 2016, along with an enhanced anaerobic bioremediation (EAB) system for dissolved cVOCs. Levels of VOCs in indoor air were elevated, resulting in building evacuation and some occupants being relocated. One of the many utility corridors runs north-south along the entire length of the building. cVOCs were detected at low levels outside the southern edge of the building, and it was initially hypothesized that the utility corridor was the transport pathway for cVOCs from the north of the building to the south. However, it was later determined (2021) that there is a discrete source of cVOCs in the south of the building, with dissolved TCE > 25 mg/L. During a SVE pilot test for VI in the northern part of the building in 2015, no vacuum responses were observed within a few feet of the extraction well. It was then discovered that most of the 7-acre (!) building was on piles, and there was an air gap of a few inches between the bottom of the floor and underlying soil. Subsequently, a small cavern was discovered under a closet room, which was connected to the air gap. Rather than install actual SVE wells, the North VI Mitigation System (VIMS) was designed (and installed in 2016) by simply placing a screened PVC pipe in the cavern and adding a seal around the pipe. A few years later, when cVOCs were found the southern part of the building, a similar design was employed, initially using a large vault (which was connected to the air gap) to extract soil gas, leading to installation of the South VIMS in 2023. The second SFVS is co-located with a conventional South SVE system (with wells inside the building unlike the North SVE system). Both systems are operating as of October 2023.

Approach/Activities:

Taking advantage of the air gap (which was likely the main pathway for VI) to design the North VIMS and South VIMS resulted in significant cost savings by eliminating the need for wells. Collectively, the two VIMSs along with the South SVE system have been successful in maintaining acceptable cVOC levels in indoor air. The South VIMS is also designed to act as a pneumatic barrier to ensure that the SVE wells for the South SVE system (which are screened in the vadose zone just above the water table) do not short circuit.

Results/Lessons Learned:

The sealing of the cracks/joints coupled with operation of the SVE system has been successful in maintaining acceptable levels of cVOCs in indoor air since 2016. The levels of cVOCs in sub-slab soil gas have also decreased significantly, with a number of locations showing decreases of several orders of magnitude. The South VIMS has increased the effectiveness of the South SVE system, based on vacuum responses in proximal soil gas probes.

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Nicholas Shih