Vapor Intrusion Evaluation and Mitigation

TRC was retained by the client to implement multiple remediation strategies to achieve site-specific risk-based closure levels for soil, indoor air, and groundwater.



Confidential Automotive Parts Manufacturer

Project Location


A former manufacturing facility had a long history in supplying hydraulic hose components for the aircraft and automotive sectors, with manufacturing pre-dating the 1940s. Because of the presence of degreasing and chrome plating operations throughout the facility, multiple known and unknown source areas required investigation and characterization. Studies performed by TRC on behalf of the client identified the presence of significant chlorinated solvent contamination in soil and groundwater inside and outside of the manufacturing facility. Concentrations of volatile organic compounds (VOCs) in soil and groundwater underlying the manufacturing facility were above concentrations accepted to be protective of the indoor air pathway. TRC worked with the client during the sale of the facility so that the property could remain in use by the purchaser, while corrective measures were performed.

The project's high degree of complexity required a variety of hydrogeological and engineering skills to mitigate the potential vapor intrusion pathway to levels below the remedial goals (RGs) established for the project. TRC performed final Corrective Measure Implementation (CMI) activities, which included in situ chemical oxidation (ISCO) and in situ thermal desorption (ISTD) to address shallow affected soil and groundwater, and in situ biostimulation/monitored natural attenuation (MNA) of the bedrock aquifer to achieve the negotiated RGs. Groundwater and indoor air monitoring are currently being performed to demonstrate that RGs have been achieved.

The presence of highly affected soil and groundwater under the main site building required the evaluation of the vapor intrusion pathway. Soil gas sampling performed by TRC indicated that soil gas samples exceeded screening levels. Additionally, indoor air samples were collected and submitted for analysis of VOCs. Data showed that, under existing conditions, chlorinated VOC concentrations in indoor air had the potential for long–term risk to facility workers.

Because the facility was in use during the implementation of corrective measures, as a short-term solution, TRC industrial ventilation experts developed a strategy to modify the HVAC operational conditions to reduce indoor air concentrations to levels below the RGs inside the manufacturing facility. During the remediation efforts, indoor air samples were collected on a quarterly basis to ensure that VOC concentrations did not pose an unacceptable risk to workers.

As recommended by TRC, and agreed to by the USEPA, the ISTD that was implemented to treat soil and groundwater beneath the building successfully removed the primary source of VOCs that were volatilizing to indoor air and assisted in achieving the indoor air concentrations protective of workers. In addition, the treatment of soil was completed to levels acceptable to the USEPA. Continued monitoring of groundwater shows that the primary VOC of concern, trichloroethene (TCE), have been reduced to concentrations below the site RGs. 

Because the RGs for indoor air established by the USEPA were very conservative, and the floor in the manufacturing facility was in poor condition, the potential for vapor intrusion was not completely eliminated by the operation of the ISTD. Therefore, TRC subcontracted an industrial flooring company to install an epoxy coating over the former plating area (approximately 10,000 sq. ft.) to act as a vapor barrier and assist in mitigating the potential for vapor intrusion. As a result, VOC concentrations in indoor air have met the RGs, and there is no need for long-term vapor mitigation strategies (e.g., sub-slab ventilation or sub-slab depressurization).

  • TRC’s remediation design experience and in-house expertise defined the nature and extent of affected media, quantified the site-specific risk, and subsequently negotiated RGs significantly above the USEPA’s standard default criteria. These negotiations were instrumental in determining appropriate technologies to achieve the required remediation goals in the expedited time frames required by the USEPA Final Decision.
  • TRC was not only able to assist the client with the design and execution of all of the strategies outlined in the CMI, but also tailor the design approach to utilize several remediation approaches to address multiple mediums wherever possible. This synergistic approach reduced the short-term capital costs for implementing additional treatment technologies and long-term monitoring costs for multiple media.
  • Additionally, TRC and the client partnered together in subcontractor management and execution with ISTD and ISCO vendors with an eye on managing and minimizing remediation costs, while maintaining the technical quality needed to meet the requirements of USEPA.

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