Project Specifics

In the early eighties, a manufacturing facility (Site) was found to be the source of trichloroethene (TCE) impacts to local groundwater. Later in the decade, approximately 220 cubic yards of soil were removed from the Site’s central courtyard, an area where chlorinated solvents had previously been stored. The excavation was believed to have removed the majority of the source material, but a subsequent investigation showed elevated sub-slab soil gas concentrations of TCE centered around the courtyard with concentrations decreasing radially. The Site is currently a plastic recycling facility.

Geosyntec Scope of Services

Geosyntec started working at the Site in 2014 by installing and sampling ten sub-slab soil gas probes at t

he Site. TCE was detected at concentrations above the Michigan Department of Environmental Quality (MDEQ) nonresidential sub-slab screening level (SSSL). Geosyntec subsequently conducted four quarterly sub-slab and indoor air sampling events to assess temporal variability of sub-slab and indoor air TCE concentrations. After discussions with MDEQ, Geosyntec developed a comprehensive Conceptual Site Model (CSM) of vapor intrusion (VI) risks at the Site and evaluated potential options for mitigation.

High volume sampling (HVS) was conducted at fourteen locations throughout the facility to further evaluate the sub-slab distribution of TCE in soil gas and estimate the pneumatic properties of the sub-slab fill and floor slab. The HVS program included chemical testing, pneumatic testing, and mathematical analysis. Static vacuum measurements were collected, and transient response tests were performed to inform design decisions at locations where a VI mitigation system (VIMS) was contemplated. The static and transient vacuum data were analyzed using the Hantush-Jacob Leaky Aquifer Model. The HVS results, coupled with the earlier sub-slab and indoor air results, were used to define a target mitigation area, and the VIMS design incorporated the fact that building characteristics differ throughout the building. Geosyntec worked with a mitigation contractor to install the VIMS, and Geosyntec conducted performance monitoring of the VIMS, which indicates the VIMS is performing as designed.

Ongoing operation, maintenance, and monitoring services are being provided.

Notable Accomplishments

Geosyntec was able to successfully implement the HVS program which proved to be a quick and cost-effective way to assess sub-slab vapor impacts. It also provided site-specific data used to design a targeted VIMS. The mitigation system successfully mitigated intrusion of TCE into the building. Multiple lines of evidence were collected to characterize VI risks and to assess mitigation system performance.

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