Public School Vapor Intrusion Mitigation
Elevated concentrations of trichloroethylene (TCE) were detected in indoor air at an active school facility in Oakland, California. The issue was identified during due diligence investigations for a property transaction, just one month before the start of the school’s summer break. The TCE levels were found to be two orders of magnitude higher than the regulatory action level. Terraphase rapidly implemented overnight emergency interim measures to protect occupants and ensure the continuity of school operations.
Scope of Work
Terraphase immediately constructed and installed a dozen air filtration units using granular activated carbon (GAC) to address areas impacted by TCE.
Additional interim controls included sealing wall and floor penetrations with expanding foam, sealing entire walls with plastic sheeting, and adjusting building heating, ventilation, and air conditioning (HVAC) systems to control vapor intrusion from below the building. Air sampling verified the effectiveness of the interim measures, and students only missed one day of school due to indoor air quality issues.
During the summer break, Terraphase installed a permanent, 200-cubic-feet-per-minute sub-slab depressurization system (SSDS) utilizing soil-vapor extraction (SVE) wells to control vapor intrusion from the subsurface. The system operated under an enforcement agreement with the Bay Area Air District (BAAD) until a permit could be issued following a mandatory 30-day public comment period.
Terraphase maintained and monitored the SSDS throughout the school year. Differential pressures and sub-slab volatile organic compound (VOC) concentrations were measured monthly and indoor air samples were collected quarterly for laboratory analysis. The school was permanently relocated at the end of the school year, and the SSDS was subsequently decommissioned.
Notable Accomplishments
- Deployed emergency air filtration overnight to restore indoor air safety
- Installed and operated a full-scale SSDS within weeks
- Maintained indoor air quality throughout the school year with minimal disruption
Former Metals Manufacturing Facility Chlorinated Solvent Vapor Intrusion Mitigation
Terraphase is managing a chlorinated volatile organic compound (VOC) assessment and remediation project at a former metal manufacturing facility. Following the completion of groundwater and soil vapor remediation, the project has entered the post-remediation monitoring phase. Despite previous cleanup efforts, sub-slab vapor concentrations of chlorinated VOCs remained above regulatory screening levels, prompting further evaluation for vapor intrusion risks.
Scope of Services
Terraphase is currently assessing potential vapor intrusion by collecting paired sub-slab vapor and indoor air samples and conducting human health risk assessments on a building-by-building basis.
In one facility, elevated indoor vinyl chloride concentrations—absent in sub-slab vapor—prompted a focused assessment to identify the source. Terraphase repurposed an existing soil-vapor extraction (SVE) blower with carbon abatement as an interim sub-slab depressurization system (SSDS) during site closure negotiations with the Los Angeles Regional Water Quality Control Board (RWQCB).
Notable Accomplishments
- Completed groundwater and soil vapor remediation and transitioned to post-remediation monitoring
- Performed detailed vapor intrusion risk assessments across multiple buildings
- Identified and traced unexpected indoor air vinyl chloride concentrations
- Used targeted field screening and lab analyses to confirm indoor air quality sources
- Installed an interim SSDS using existing infrastructure to support site closure negotiations with RWQCB
- Leveraged a permit exemption from the South Coast Air Quality Management District by repurposing the existing SVE equipment and installing shallow SSDS sumps, saving tens of thousands of dollars per year in permit fees and monitoring expenses
Former Dry Cleaner Site Investigation, Remediation, and Mitigation System Installation
Terraphase was retained to implement remedial activities at a former dry-cleaner site in Southern California impacted by volatile organic compounds (VOCs), including tetrachloroethene. Terraphase was tasked with achieving the site’s remedial objectives more efficiently and cost-effectively, while addressing data gaps and optimizing the original strategy.
Scope of Services
Terraphase performed additional site investigation to address data deficiencies and subsequently refined the originally proposed remedial approach. This included revising the soil-vapor extraction (SVE) system design, expanding the soil-gas monitoring infrastructure, and targeting vapor sampling across the former dry-cleaning site and adjacent units. Our team installed five multi-depth soil-gas monitoring points and two dual-nested SVE wells. A pilot test confirmed the feasibility—but limited mass removal potential—of SVE. Based on these results, Terraphase designed and implemented a sub-slab depressurization system (SSDS) as a long-term mitigation strategy. The SSDS consisted of a horizontal SVE line beneath the slab, connected to a rooftop automated blower system with telemetry and vapor treatment using granular activated carbon.
Notable Accomplishments
- Closed data gaps through targeted additional site investigation
- Optimized and implemented a more effective SVE system design
- Installed five soil-gas monitoring points and two dual-nested SVE wells
- Conducted SVE pilot testing to confirm performance and limitations
- Designed and installed a cost-effective SSDS beneath the former dry-cleaner site
- Reduced the client’s annual operating costs from ~$150,000 to under $2,000
- Delivered a regulatory-approved system currently in operation
