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Ms. Lori Duncan March 31,2008 Page 5 of 7 Soil Vapor Survey,Vapor Intrusion Evaluation and Risk Evaluation In order to delineate the extent of hydrocarbon vapors in the soil and to investigate the potential for vapor migration and intrusion to indoor air, Ground Zero proposes to conduct a soil vapor survey. Five soil vapor samples will be collected at the approximate locations shown on Figure 3. The sampling locations are intended to provide data on the average concentrations of contaminant vapors beneath the building along with a biased sampling location at the former UST tank pit. Soil vapor samples will be collected in general accordance with the Department of Toxic Substances Control (DTSC) and RWQCB guidelines set forth in their report Advisory—Active Soil Gas Investigations (DTSC and RWQCB, 2003). An onsite mobile laboratory will analyze the vapor samples for constituents of concern. Drilline and Soil Vapor Sampling Drilling and soil vapor sampling will be conducted under the supervision of an experienced geologist. A C-57 licensed drilling contractor will advance five (5) soil borings to depths of approximately 5 feet bgs using push probe equipment. Soil vapor samples will be collected from each boring at approximately 5 feet bgs. Up to three (3) additional samples may be collected based upon field observations. At each sampling location, a direct push vapor sampling probe will be advanced to the desired sampling interval. The vapor collection system uses a steel vapor tip that is shielded during the driving process. Once the rod reached the desired depth of 5 feet bgs, the rod will be pulled back to expose a screen. To prevent ambient air intrusion a bentonite seal will be placed around the rod/surface interface. The seal will be hydrated and the subsurface conditions allowed to equilibrate for no less than 20 minutes after probe advancement. A leak test and a purge volume versus contaminant concentration test will be conducted per DTSC guidelines prior to the collection of the first sample. Samples will be collected from SGl at one, three and seven purge volumes and analyzed for TPHg, BTEX, gasoline oxygenates and additives and VOCs by GC/MS (EPA Method 8260). Diflouroethane (DFA) or similar leak compound will be sprayed around the rod/surface interface and tubing connections prior to collecting each vapor sample. The soil vapor samples will be collected at approximately 100 milliliters per minute (ml/min) into dedicated syringes. The samples will be uniquely identified and transferred under chain of custody protocol to the State- certified mobile laboratory on site for analysis. The purge volume containing the highest concentration of the constituent(s) of concern will be used as the purge volume for the remaining borings. If no constituents of concern are detected, a default of three purge volumes will be used. Sampling of the remaining sample point locations will continue using the same sampling protocol at the selected purge volume. A duplicate sample will be run from the sample location with the highest detected concentration of the constituent(s) of concern. G.\GROUNDZE\GABBARD\workpi ns\wp_0308.doc