Design of Vapor Mitigation Associated with Redevelopment of Site Situated over a Solvent Plume in Shallow Groundwater

EKI assisted the owner with facilitating redevelopment of a property that overlies a plume of trichloroethylene (TCE) and tetrachloroethylene (PCE) in shallow groundwater at concentrations above regulatory agency thresholds that are migrating onto the Site from an upgradient source or sources. EKI evaluated human health risks associated with potential intrusion of volatile organic compound (VOC) vapors from the subsurface into the planned building via a 2-level subgrade parking structure that was designed to extend approximately 15 feet below the groundwater table containing VOCs. EKI worked closely with the Client and the San Mateo County Department of Environmental Health, Groundwater Protection Program (GPP) with regard to the development design and mitigation of potential human health risks, as well as to minimize further migration of VOCs in groundwater through redevelopment activities. EKI conducted the following:

• Investigated soil, groundwater, sub-slab soil vapor, and indoor air conditions in existing building to support design of new commercial building with vapor mitigation measures protective of future occupancy and acceptable to the San Mateo County GPP.
• Prepared a Risk Management Plan (RMP) that assured proper soil management during construction and minimized potential exposure to VOCs present in groundwater during and post-construction, for San Mateo County approval.
• Evaluated planned ventilation within the sub-grade parking garage to mitigate vapor intrusion into the overlying building spaces.
• Assisted with the design of waterproofing and vapor mitigation barriers around the portions of the parking garage that extend below and above the groundwater table.
• Designed a ventilated buffer wall outside a planned occupied space within the parking garage for vapor mitigation.
• Designed a subsurface passive venting system around the perimeter of the building for vapor mitigation.
• Facilitated off-site reuse of low-impacted soil from underground garage excavation to save substantial soil disposal costs.
• Preformed pre-occupancy indoor air sampling to confirm vapor mitigation measures are effective and protect indoor air quality.

Outcome

EKI worked closely with Client, architect, construction contractor, and San Mateo County GPP to integrate vapor intrusion mitigation into building design, maintain schedules, and obtain timely approval from San Mateo County GPP.