Theme: 09. Vapor Intrusion

Topic Area: 9b. Vapor Intrusion Mitigation and Effectiveness

Formatted Title:

Using Active Subslab Depressurization to Mitigate Diffusion-Based Vapor Intrusion

Background/Objectives:

Vapor intrusion mitigation systems (VIMS) are used to disconnect the vapor intrusion (VI) pathway and to mitigate indoor concentrations due to subsurface vapor sources. Active subslab depressurization (ASD) is commonly used to disconnect the VI pathway by maintaining a negative pressure beneath the building slab, which prevents vapor from entering indoor air. However, some VI practitioners have speculated that maintaining a negative pressure beneath the slab may not be sufficient to prevent VI in cases where the primary mechanism for VI is diffusion across the slab. An ASD pilot test was conducted in a portion of a large industrial building where diffusion-driven VI was confirmed to determine whether this technology would still be an effective mitigation technique.

Approach/Activities:

Indoor air concentrations of TCE above risk-based screening levels were detected due to VI in a portion of a large industrial building. Investigation activities included HAPSITE investigation, building pressure cycling (BPC), and flux chamber diffusion testing. The results indicated that the primary mechanism for vapors to enter to the indoor space was diffusion across the building slab. BPC testing indicated that the slab in the area of interest was leaky. The flux chamber testing indicated that diffusion across the slab was occurring at a rate that, when extrapolated to the floor surface and volume of the area of interest, resulted in a predicted indoor air concentration equivalent to the actual TCE concentrations measured in indoor air. Based on the conclusion that VI was occurring primarily due to diffusion across the slab, a pilot study was initiated to determine if ASD could mitigate the diffusion-driven VI pathway. The subslab TCE concentrations would have to decrease to reduce the TCE concentrations diffusing through the slab.

Results/Lessons Learned:

Eight ASD nodes were installed based on the results of pressure field extension testing conducted prior to system installation. The ASD nodes were installed with differential pressure (dP) transmitters that were set with operator-adjustable high and low alarms. Subslab probes were installed throughout the area of interest and were used to measure dP and to collect subslab soil vapor samples. Five of the subslab probes were installed with dP transmitters that continuously collect subslab pressure readings which are used to inform automated adjustments of the variable speed motor via telemetry. Under vacuum from the ASD nodes, subslab vapor is drawn into a conveyance pipe where it is routed to a ground-mounted regenerative blower located outside the building and fitted with a variable-frequency drive. Performance monitoring conducted to-date indicates the VIMS is effectively mitigating the VI pathway and reducing diffusion across the slab. The VIMS has consistently met subslab depressurization target at all locations within the area of interest. Helium tracer data indicates a direct connection to the ASD nodes from the most-impacted subslab locations in the area of interest. Post-startup BPC testing indicated stable TCE indoor air concentrations and TCE concentrations in the flux chamber at the 1-month post-startup event were an order of magnitude lower than baseline. TCE concentrations in subslab soil gas generally declined by 2-3 orders or magnitude or became non-detect. Indoor air TCE concentrations also showed declining trend from baseline, with all reported concentrations below the risk-based screening level. The dP, helium tracer, subslab vapor, and indoor air data indicate the VIMS is mitigating the diffusion-driven VI pathway while contributing to subslab source mass removal.

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Jeffrey Minchak