Theme: 09. Vapor Intrusion

Topic Area: 9b. Vapor Intrusion Mitigation and Effectiveness

Formatted Title:

Reduction of Vapor Intrusion Risk from an Off-Site Source Using Remedial Measures and a Horizontal Pressure Break

Background/Objectives:

Background/Objectives. The Site, located in Hollywood, California, USA was a theatre and commercial film studio. An initial indoor air survey was completed as part of pre‐acquisition environmental due diligence activities on November 12, 2020, and found tetrachloroethene (PCE) concentrations in indoor air samples collected within the Green Screen Room, where cleaning supplies and costumes were stored at the time. The room was emptied of all materials and was resampled on December 8, 2020. The second sampling event saw a reduction of PCE concentrations from 30 μg/m³ to 2.5 μg/m³. A third sampling event was proposed to see if the reducing trend of PCE in indoor air continued. The Green Screen Room was sampled for the third time on January 14, 2021, and the PCE concentration was 21 μg/m³, which did not support a continued reduction of PCE in indoor air and indicated the volatile organic compound (VOC) source was not from aboveground use. To support this conclusion, 13 soil vapor probes and seven sub‐slab vapor pins were installed and sampled to assess whether the presence of VOCs in indoor air was the result of a vapor intrusion condition at the Site in February 2022. Benzene, chloroform, cis‐1,2‐dichloroethene (cis 1,2‐DCE), PCE, and trichloroethene (TCE) were present in soil vapor at concentrations exceeding commercial screening levels. Given its prevalence across the Site and presence in indoor air, PCE was considered the primary constituent of concern in soil vapor and sub‐slab vapor. The highest concentrations of VOCs were present along the eastern property line, indicative of an offsite source. It is believed that former dry-cleaning operations associated with the neighboring property to the east of Site is the primary contributor to PCE soil vapor impacts at the Site. However, to facilitate the property transaction following these findings, it was recommended that VOC concentrations observed beneath the Site be reduced through both remediation and mitigation efforts.

Approach/Activities:

Site improvements constructed in the second quarter of 2022 to address the elevated VOC concentrations consisted of permanent engineering controls (ECs) in the form of a passive sub‐slab depressurization (SSD) system along with a temporary soil vapor extraction system (SVE) utilizing three vapor extraction wells connected via an aboveground pipe network to an off‐gas treatment system of two 1,000‐pound granular activated carbon (GAC) absorbers connected in series. The systems were designed to mitigate soil gases trapped below the existing building slab to prevent potential vapor intrusion through the slab and into the building. Two horizontal wells were installed along the property line at depths of 5 and 10 feet below ground surface (bgs) to act as a “pressure break” between the Site and the off-site source area, to prevent pulling off-site soil vapor impacts onto the property. The SVE system and pressure break operated between July 11, 2022, and December 19, 2022, before being shut down.

Results/Lessons Learned:

The majority of mass was effectively removed from the subsurface within the first month of SVE system operation between July 17, 2022 and August 8, 2022. After August 8, 2022, the rate of mass removal was marginal until the system was shut down in December 2022. The total mass removed by the SVE system was approximately 62 pounds. In December 2022, during the rebound testing, it can be observed that some soil vapor probes rebounded (or became slightly recharged with PCE). Although much of the PCE was removed from the subsurface, some rebounding in concentrations existed following the shutdown. This effect was most visible at soil vapor probes located near the east property boundary. For example SV‐4 had PCE at 100,000 µg/m³ (Feb. 2022), 160 µg/m³ (Nov. 2022), system shutdown for several weeks then resampled at 900 µg/m³. Following this evaluation of soil vapor and SVE system operational data, the SVE system has effectively reduced soil vapor concentrations beneath the Site. Therefore, based on asymptotic VOC mass removal rate, the SVE system was shut down in December 2022 and removed from the site in January 2023. Continued SVE operations were terminated to prevent pulling any potential VOCs from the off‐site VOC source area located to the east of the Site. It is our understanding that the neighboring facility has recently installed their own SVE system that will continue to remove contaminants from the subsurface.

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Joshua Long