Soil Vapor/Indoor Air Investigation, Mitigation, and Monitoring

TRC's targeted approach was implemented for less than 25% of the original estimated cost and ensured the property could remain in use during investigation and implementation of the corrective measures.



Confidential Manufacturing Facility

Project Location


This active manufacturing facility supplies brake and axle components for the transportation sector. The presence of former vapor degreasing operations and solvent drum storage in several known locations in the manufacturing facility resulted in significant contaminant source areas that required investigation and characterization. Concentrations of chlorinated volatile organic compounds (CVOCs) in soil and groundwater underlying the manufacturing facility exceeded concentrations acceptable to be protective of the indoor air pathway. TRC worked closely with the client to ensure that the property could remain in use during the investigation and implementation of corrective measures.

TRC provided an effective, low‐cost solution to mitigate CVOCs in indoor air and satisfy conservative state cleanup standards in an over 300,000 square foot active manufacturing facility. Although preliminary estimates for construction of a facility‐wide sub‐slab depressurization system were costly and would result in significant disruption to plant operations, TRC developed a focused approach that targeted specific soil and groundwater source areas beneath the facility that was implemented for less than 25% of the original cost. The indoor air pathway was successfully mitigated, while other corrective measures were being performed.

Soil Vapor/Indoor Air Investigation
TRC conducted a preliminary assessment of CVOC concentrations in soil vapor beneath the main manufacturing building by installing a series of sub‐slab vapor sampling points and collecting sub‐slab soil vapor and indoor air samples to assess the vapor intrusion pathway. Data indicated the presence of CVOC concentrations above state regulated chronic screening levels (however, levels were well below OSHA permissible exposure limits). Based on the sub‐slab and indoor air sampling results, TRC and the client identified the need to address the volatilization to the indoor air pathway.

Soil Vapor Mitigation Design Workplan
TRC prepared a Remediation Work Plan (RWP) that proposed a sub‐slab ventilation (SSV) system to mitigate the presence of VOCs in indoor air at the site, and submitted it to the state regulatory agency for approval. The SSV system is an engineered control measure that was designed to achieve adequate advective air flow under the building sufficient to dilute VOCs diffusing from the soil or groundwater. The system essentially eliminates the potential for these vapors to migrate into the indoor air within the building by removing the soil gas at the potential source areas. The system was designed to target areas based on the location of known soil source areas, as well as elevated soil vapor, and indoor air data. A monitoring program was also included in the RWP that involved the collection of seasonally spaced quarterly indoor air samples, in order to evaluate the effectiveness of the SSV system. The agency agreed that the SSV system was appropriate and approved the installation of the proposed SSV system design and monitoring program.

Sub‐Slab Ventilation System Pilot Test
TRC performed a sub‐slab soil vapor extraction design test to determine the feasibility of using a full‐scale SSV system to mitigate the presence of VOCs in indoor air above the state‐regulated indoor air screening levels. The results of the pilot test were summarized and reviewed by the state agency.

Sub‐Slab Ventilation System Installation and Effectiveness Monitoring
TRC installed the indoor air mitigation system and submitted a report to the agency documenting the installation and demonstrating the effectiveness of the remedy. The customized SSV system included three horizontal sub‐slab screens vented outside through the roof of the facility using a high suction fan, six vacuum monitoring points to allow the measurement of vacuum during system operation to determine the radius of influence (ROI), and pipes that were installed and bedded within the permeable gravel base underlying the concrete floor. 

The indoor air and soil gas data collected after the SSV system was operational confirmed that the SSV system was working effectively to mitigate CVOCs in sub‐slab soil gas to concentrations below the state screening levels. In order to continue mitigation of the soil gas, the client continues to maintain and monitor the existing SSV system.

Throughout the completion of the investigation, remedial design, implementation, and performance monitoring activities, TRC has successfully addressed the potential for worker exposure by successfully mitigating CVOCs in the sub‐slab soil gas using a cost-effective, targeted approach over known source areas. Preliminary estimates for construction of a facility wide sub‐slab depressurization system were above $250,000 and would result in significant disruption to plant operations. TRC’s approach was implemented for less than $60,000, and the indoor air pathway has successfully been mitigated, while other corrective measures are being performed.

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