Vapor Intrusion Assessment of Industrial and Commercial Facilities

A release of the chlorinated solvent trichloroethene (TCE) was identified at an industrial facility near downtown Seattle where Farallon is conducting a remedial investigation under an Agreed Order with the Washington State Department of Ecology. The release of TCE affected shallow groundwater and dispersed off site, extending below other commercial and industrial buildings in the area. Farallon developed a comprehensive vapor intrusion investigation component of the remedial investigation that included a decision tree for evaluating whether indoor air quality at eight buildings the groundwater plume extended beneath was being affected by TCE vapors volatilizing from the affected groundwater.

The evaluation work included sub-slab soil gas sampling at each building to evaluate whether TCE vapors were volatilizing from shallow groundwater and accumulating beneath the building slab at concentrations that could affect indoor air quality. Evaluations of the building construction, HVAC systems, and materials used at each facility also were included in the investigation. Farallon employed its sub-slab sampling system designed to mitigate potential cross-contamination of indoor air, which uses helium tracer gas to monitor leakage in the sampling methodology in real time, eliminating the potential need to resample.

Where sub-slab soil gas results indicated a potential risk to indoor air quality, an indoor air sampling program was implemented to evaluate whether TCE vapors were present at concentrations requiring mitigation while groundwater cleanup alternatives were being assessed during the remedial investigation and feasibility study. Ambient outdoor air samples were used to evaluate background concentrations in the sampling area. Chemical inventories were taken in the buildings and potential sources of TCE were removed in advance of the sampling.

Results of a vapor intrusion assessment at one facility indicated that TCE was present in the building at concentrations exceeding appropriate action levels set forth by Ecology. Farallon worked with the property owner and building tenants in designing and installing a sub-slab depressurization system that would effectively mitigate vapor intrusion risk and did not affect use of the building. Farallon performs periodic monitoring of the system vacuum to confirm that TCE vapors are not entering the facility. The system will remain in operation until TCE concentrations in shallow groundwater and indoor air have been confirmed to be below appropriate action levels.