Theme: 09. Vapor Intrusion

Topic Area: 9b. Vapor Intrusion Mitigation and Effectiveness

Formatted Title:

Combined Long-Term Site Management Approaches: HRSC and SVE Technologies for Residual LNAPL Mass Removal and Vapor Intrusion Mitigation

Background/Objectives:

Defining the extent of residual light-nonaqueous phase liquid (LNAPL) at a site where approximately 20,000 gallons of petroleum fuel had been released from leaking underground storage tanks (LUSTs) at a commercial fuel station in order to further characterize the extent of the commingled gasoline-range total petroleum hydrocarbons (TPH) and chlorinated volatile organic compounds (VOC) plume for contaminant mass removal and to mitigate vapor intrusion (VI) into adjoining businesses was a priority at an Ecology Consent Decree site in the City of Yakima (the site), Washington. Soil vapor extraction (SVE) was selected for VI mitigation for the site.

SVE, a proven technology for VOC mass removal from the unsaturated and partially saturated zone soils, has recently been recognized as a potential VI mitigation technology. Conventional SVE and VI mitigation technologies such as sub-slab depressurization (SSD) operate under similar fundamentals of air exchanges and subsurface vacuum propagation but have different remedial objectives and therefore a different design basis. For SVE systems, typically the remedial objective is VOC mass removal, and the standard design basis is the pore volume (PV) air exchange (flushing) rate that is required to occur for effective treatment at a far distant point (i.e., radius of influence [ROI]) from the extraction well. Whereas, for SSD systems, typically the remedial objective is to maintain an outward pressure gradient from inside a building to the subsurface. The standard design basis for SSD systems is the minimum amount of required sub-slab vacuum influence (i.e., depressurization) that can be maintained at a far distant point (i.e., ROI) from the extraction point.

The objectives of this presentation are to: 1) show how the integration of high resolution site characterization (HRSC) into the site management strategy led to more efficient options for long-term management of the remaining contamination at the site, 2) identify conditions where SVE for vapor mitigation may be a preferred approach over conventional SSD systems based on cost and effectiveness, 3) present how to design a SVE system for VI mitigation, rather than mass removal, based on pilot testing results and subsequent pneumatic modeling performed for the site , and 4) provide the results of pilot testing and pneumatic modeling to support the design of the SVE system for VI mitigation.

Approach/Activities:

HRSC, involving the optical imaging profiler (OIP) and hydraulic profiling tool (HPT) to identify the storage and transport zones of the LNAPL and analysis of transmissivity of soil, respectively, provided the necessary data for evaluation of natural source zone depletion (NSZD) and monitored natural attenuation (MNA). Subsequently, a pilot test was performed at the site to assess the efficacy of a SSD or a barrier SVE approach for the site to mitigate petroleum hydrocarbons and chlorinated solvent-associated VOC (from an off-site source) vapor intrusion into the indoor air of two adjacent business buildings to the site and to determine the design parameters for the full-scale system. Based on the results of the pilot test and subsequent pneumatic modeling, a full-scale SSD system in the form of a barrier SVE system was designed and will be implemented at the site both for the VI mitigation and VOC mass/source removal purposes. The system was designed specifically to provide and maintain a minimum sub-slab vacuum influence of 0.1 inches of water (in. H₂O) underneath both buildings as well as to achieve a minimum air flushing rate of 3,000 PV exchanges per year in the subsurface throughout the target treatment area.

Results/Lessons Learned:

Long-term site management for LNAPL sites via NSZD and MNA is viable with a combination of HRSC and a SVE system for VI mitigation. Conventional SSD systems are not always an option due to site-specific constraints. SVE, when tailored for VI mitigation, can be an effective alternative to SSD. Pneumatic modeling supported by pilot test data are critical for designing SVE systems for VI mitigation purposes. Designing and operating an SVE system for VI mitigation (low flow rate and long operation time) differs from conventional SVE systems (high flow rate and shorter duration).

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Yen-Vy Van