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Vapor Intrusion Assessment Work Plan Project number:60653972 with a damp towel to remove concrete dust prior to the hydrated bentonite being placed to create a seal. • Water used in the construction of the probes will be deionized and metal probe components will be thoroughly cleaned to remove manufacturer-applied cutting oils. • The probes will be allowed to sit for at least 48 hours to allow sufficient equilibrium time prior to sampling. • Each probe will be constructed with a gas tight fitting and a flush-mounted well box(as necessary). Samples will be collected in accordance with the July 2015 DTSC/RWQCB Advisory for Active Soil Gas Investigations (DTSC,2015). Once the soil gas probe(s)have been installed, soil gas samples will be collected from the air space immediately below the slab of each building, and, at depths of 5 and 15 feet, in accordance with the following procedures: • A shut-in test will be performed to check for leaks in the above ground sampling train.The shut-in test will involve connecting a vacuum pump with a built-in pressure gauge to the above ground sampling train using flexible tubing,applying vacuum pressure of greater than 100 inches of water, and checking vacuum pressure after one minute has elapsed to verify that the vacuum pressure has not decreased by more than 0.5 inches of water.If the sample train fails the shut-in test,the fittings and connections will be tightened or replaced, and the shut-in test will be repeated. • A leak test will be performed with a gas leak-test compound,helium,to check for leaks in the entire sampling system.The leak-test compound will be included in the analytes to be reported by the environmental laboratory.An American Society for Testing and Materials(ASTM)Modified Method D-1946 analysis will be conducted to analyze samples for helium.A shroud will be used to contain the entire sampling train and the helium compound for the duration of the leak test. • The tubing exiting the ground surface at the sub-slab soil gas probe will be connected to an electric vacuum pump, or other type of pump,which will remove approximately three sample-train volumes of soil gas from the subsurface prior to sampling.The air flow rate during soil gas purging and sampling will be between 100 and 200 ml/min, and the vacuum will be less than 100 inches of water. If the probe is found to hold a vacuum of at least 15 inches of mercury(200 inches of water)for at least 5 minutes, a wire will be used to attempt to unplug the probe. If this does not fix the problem, the probe will be removed,the hole plugged with cement grout,and a new probe will be installed at a new location within the building. • For each sub-slab soil gas sample location, samples will be collected in laboratory supplied,batch certified 1-liter SUMMA canisters.New tubing will be utilized at each sampling point to avoid cross- contamination. • Duplicate sub-slab gas samples will be collected at a rate of at least 1 per sample event or 10 percent, whichever is greater. • The SUMMA canister will be shipped to a qualified environmental laboratory for analysis of project- specific VOCs using U.S.Environmental Protection Agency(USEPA)Method TO-15. 4.3.2 Laboratory Analysis Samples will be sent under chain of custody to a certified laboratory for analysis. Samples will be analyzed for helium and VOCs by ASTM Modified Method D-1946 and USEPA TO-15 in accordance with Table 1. 4.4 Indoor and Outdoor Air Sampling Indoor and outdoor air sampling will be conducted to assess whether VOCs in the subsurface have intruded into the indoor space and whether they pose a significant risk to indoor air quality.Twelve IA Prepared for: California Regional Water Quality Control Board,Central Valley Region AECOM