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Workplan for Additional Assessment and Subslab Gas Sampling Tracy Office Plaza 324 E. i 1 th Street, Tracy,CA November 22,2013 Select groundwater and select soil samples will be analyzed for total petroleum hydrocarbons as motor oil (TPHmo), total petroleum hydrocarbons as diesel (TPHd), and total petroleum hydrocarbons as gasoline (TPHg)by EPA Method 8015Cm;and benzene,toluene,ethylbenzene,xylenes(BTEX),and methyl-tertiary butyl ether (MTBE) by EPA Method 8021B. To satisfy LTCP requirements, select samples will also be analyzed for oxgenates, lead scavengers(1,2-DCA and EDB),and naphthalene by EPA Method 8260 Task 3—Subslab Gas Sampling Pangea proposes to install two(2)subslab probes to facilitate collection of subslab gas samples.As the highest hydrocarbon concentrations detected in soil were Vencountered at a depth of approximately 1 ft bgs,Pangea proposes subslab gas sampling to determine if hydrocarbon vapors could migrate through underground conduits to below the onsite slab. Semi-permanent probes will be used to allow subsequent collection if merited.Pangea anticipates collecting initial soil gas samples during the cold season. If requested by SCEHD, additional soil gas samples will be collected during the warm season. The proposed sampling locations were selected to minimize disruption to existing businesses. As shown on Figure 2, the two subslab probe locations (SS-1 and SS-2)are proposed a the onsite building. Subslab gas samples will be collected within Summa canisters and submitted to a state-certified laboratory for analysis. Subslab gas samples will be analyzed for total petroleum hydrocarbons as gasoline(TPHg),benzene,toluene, ethylbenzene, xylene(s) (BTEX), methyl-tertiary butyl ether (MTBE), and naphthalene by modified Total Organics Method 15(TO-15);and for percent oxygen and helium(leak check compound)by Method ASTM D-1946.The oxygen analyses will help evaluate the potential for future degradation and attenuation of detected hydrocarbons,and will help assess soil column characteristics(>=4%oxygen in soil gas is referenced in the S WRCB's Underground Storage Tank Low-Threat Site Closure Policy).Additionally,for naphthalene analysis by TO-15, the analytical laboratory will utilize procedures for recovery, carryover,canister cleanliness,age, and matrix spikes and matrix spike duplicates as outlined in the April 2012 Cal/EPA Advisory Appendix E. The subslab gas probe installation procedure involves using a rotohammer to drill a 1 '/z-inch diameter,4-inch deep hole in the approximately 6-inch thick concrete slab of the building, drilling a '/z-inch diameter hole through the remaining concrete,installing a rubber stopper with stainless steel tubing(capped on one end with a Swagelok fitting)and placing a bentonite seal from the top of the stopper to within an inch of the surface.A second rubber stopper will be placed over the subslab probe to protect it and the probes will be allowed to equilibrate for at least 48 hours,prior to sampling. An analytical laboratory will provide sampling assemblies and certified Summa canisters for sampling. The Summa canisters will come under a complete vacuum of approximately 30 inches of mercury.Prior to sample collection a shut in test will be conducted on the sampling assembly with a vacuum pump to confirm no leak 4