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Soil Gas Sample Collection After each soil gas sampling probe assembly.is complete, a minimum of 48 hours of stabilization time will be allowed before collecting each vapor sample. Once the gas sampling points have been allowed to stabilize,°the above ground sample collection equipment will be assembled. A vacuum/pressure gauge will be fitted to the three-way sample port assembly to measure the vacuum while purging is•conducted. Three purge volumes, using 60-millimeter (ml) plastic syringe, will be removed from'the vapor probe assembly. The purge volume will be calculated based on the internal volume of tubing and annular space around the "probe tip and-the tubing volume of the above ground assembly. Once purging is complete,.a 400 ml Summa canister, fitted with a 200 mllminute flow restrictor, will be connected to the vapor probe assembly and samples of soil gas will be collected in the Summa canisters.; Prior to sampling, a leak check compound derived from commercial computer "dust-off' spray (such as 1,1-Difluoroethane [1,1 DFA]), or similar will be applied to clean cloths and used to monitor leaks within the above ground vapor probe assembly. The "dust-off' spray will be sprayed into a .plastic bag containing several clean cotton cloths. - The cloths will be placed adjacent to, but not touching, each portion of the sample apparatus, including the area where the Teflon tubing exits the ground, Upon completion of the sampling, the vapor probes will be removed and the borings will be backfilled with native soil from the boring terminus to ground surface. Sample Analyses Soil vapor samples collected from the soil vapor_probes will be labeled and submitted utilizing chain-of-custody procedures to a State-certified laboratory. Each soil vapor sample will be analyzed using EPA Method TO-15. The soil vapor samples will also be analyzed for the leak check compound, ',using,a.method detection limit of less than 10"micrograms per liter (pg/1). Each sample will 3ibe screened at the laboratory for elevated concentrations. EPA Method 8260B may be employed for some samples to prevent an elevated dilution factor, which would affect the detection limit of some of the compounds: - For Quality Assurance/Quality Control purposes, a minimum of one duplicate samplewill be collected immediately after the original sample from a selected vapor probe location. One trip blank provided by the analytical laboratory will be analyzed for target compounds. . ATC will request detection limits for the analytes"of interest that are less than or equal to the residential scenario screening levels provided in,Table E of the Environmental Screening Levels (ESLs) for Indoor Air and Soil Gas (Vapor Intrusion Concerns)as.published by the RWQCB San Francisco Bay Region in'November 2007. Indoor Air Risk Assessment A tiered evaluation will be conducted using the:,laboratory analytical data. The soil vapor data will be compared to the-values listed in Table',E of the Tier 1 ESLs as..published by the San Francisco Bay Regional Water Quality Control Board (SF RWQCB) in.November 2007 and the California Human Health Screening Levels (CHHSLs).published by the California Environmental Protection Agency (CAL EPA) in January 2005. If any of the analytes of interest exceed the ESLs and/or CHHSLs, these analytes will be evaluated utilizing the Tier 2 Modeling program. ATC will use the appropriate Johnson and Ettinger Vapor Intrusion Model based on subsurface soil encountered i,and property usage to estimate the cumulative risk "of exposure to all applicable analytes of interest using site-specific data. Analytes of interest detected above SoiIGasWorkPlan.doc 4 ATC Associates Inc. 1 i