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Vapor Intrusion Assessment Work Plan Project number:60653972 distributed across the study area(Figure 3).Results of sampling will inform the necessity of additional sampling to capture temporal/seasonal variability of the site. 4.5.1 Field Activities At each location, concrete coring will be conducted(if required),to allow access to the subsurface.Hand augers will be used for the first 5 feet,to clear the area of subsurface obstructions before probe installation, as needed. Boreholes will be drilled with direct push methods at 9 locations,to depths of 5 of and 15 feet below ground surface(bgs). At each location soil gas probes will be installed as dual nested wells as shown in Figure 3.The following steps will be followed to install wells. • Two screened intervals will be installed within one borehole with screens set at 5 and 15 feet. • Each sampling probe will be constructed using a 1/8 or 1/4-inch diameter tubing with a permeable probe tip at the end.After each probe is installed in the hole,the annulus around the probe tip will be filled with a sand pack centering the probe tip with at least 6 inches of sand above and below the level of the probe tip.After placing the sand pack,a dry granular bentonite transition seal will be added on top of each sand pack, followed by a hydrated bentonite annual seal(see attached Figure 3 showing typical sub-slab vapor probe design). The remainder of the hole will be filled with neat cement mixed with bentonite for a permanent probe. The concrete surface in the borehole will be cleaned 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 probe 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). Soil gas samples will be collected in accordance with the following procedures, and in accordance with the July 2015 DTSC/RWQCB Advisory for Active Soil Gas Investigations (DTSC, 2015): • 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 1 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 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 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 before sampling.The air flow rate during soil gas purging and sampling will be between 100 and 200 milliliters per minute, 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 will be plugged with cement grout, and a new probe will be installed at a new location within the sample area. Prepared for: California Regional Water Quality Control Board,Central Valley Region AECOM