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Ms. Lori Duncan <br /> March 31, 2008 <br /> Page 6 of 7 <br /> Each sample point will have dedicated equipment. Any sampling and drilling equipment that is <br /> not dedicated will be washed thoroughly with laboratory-grade detergent and water between uses <br /> to minimize the potential for cross contamination. After sample collection, the borings will be <br /> backfilled with bentonite pellets and the surface pavement restored. <br /> Mobile Laboratory Analysis <br /> Soil vapor samples from each location will be submitted, under chain of custody protocol, to a <br /> State-certified mobile laboratory located onsite. The vapor samples will be analyzed for TPHg, <br /> BTEX, gasoline oxygenates and additives and VOCs by GC/MS (EPA Method 8260). <br /> Vapor Intrusion and Risk Evaluation <br /> Ground Zero will evaluate the potential health risk to occupants of the site due to potential exposure <br /> to contaminant vapors. RWQCB utilized the Johnson and Ettinger vapor intrusion model to prepare <br /> the document entitled Screening for Environmental Concerns at Sites with Contaminated Soil and <br /> Groundwater — Interim Final (CA Regional Water Quality Control Board, San Francisco Bay <br /> Region,November 2007). If soil vapor concentrations exceed the Environmental Screening Levels <br /> (ESLs) published by RWQCB, potential indoor air concentrations of contaminants will be <br /> computed using the Johnson and Ettinger model. The evaluation will then quantify the expected <br /> excess lifetime cancer risk due to exposure to those contaminants as well as non-cancer toxic <br /> effects. The purpose of the evaluation will be to determine if"significant risk"exists for potentially <br /> exposed individuals. <br /> Potential Conduit Evaluation <br /> In recent correspondence, the State Water Resources Control Board Underground Storage Tank <br /> Cleanup Fund expressed concern that free product may have migrated to groundwater as a result <br /> of the construction of well MW3. We will evaluate this issue and, if it appears that there is a risk <br /> of such migration, we will propose an alternative construction for a replacement well. <br /> Protocols <br /> All field work will be conducted in accordance with the Quality Assurance/Quality Control and <br /> Field Procedures Plan and the Site Specific Health and Safety Plan on file with EHD. All <br /> investigation-derived waste will be characterized and managed in accordance with state and local <br /> regulations. <br /> G:\GROUNDZE\GABBARD\workp1ws\wp_0308.doc <br />