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6.5 Proposed Soil Gas Sample Collection Methodology The proposed soil gas sampling methodology common to the sub-slab soil gas and shallow soil gas samples is presented below 6.5.1 Shut-In Test Prior to purging and sampling the sub-slab and shallow soil gas probes a shut-in test will be performed on the above ground sample tubing. The test will be performed with an air-tight glass syringe constructed with a three-way valve. If the cause of any loss of vacuum cannot be corrected, the initial soil gas probe will be properly abandoned and a replacement soil gas probe will be installed at a distance of approximately five feet from the initial probe. 6.5.2 Leak Test Upon completing the shut-in test, a shroud consisting of a plastic tote or an overturned plastic five- gallon bucket will be used to cover the sample tubing that protrudes from the neat cement seal. A leak test will performed to evaluate the integrity of the borehole seal at the sub-slab probes. Introducing ambient air may result in an underestimation of actual target compound concentrations or, alternatively, may introduce external constituents into samples from ambient air. A tracer compound, 1,1-diflouroethane(1,1-DFA),will be used to evaluate sample integrity. The 1,1-DFA will be applied to a clean towel or rag near the borehole within the shroud in order to evaluate potential leaks of ambient air through the neat cement seal. Seal integrity will be evaluated by analyzing the soil gas sample for the tracer compound using the mobile laboratory. If the mobile laboratory results indicate a leak in the soil gas sampling system, the connections will be tightened, and the sample will be re-collected. If leaks continue to occur,the soil gas probe location will be abandoned and re-installed at a distance of approximately five feet from the initial probe. 6.5.3 Sample Flow Rate Soil gas samples will be collected with a gas-tight glass syringe at a flow rate of 150 milliliters per minute (ml/min)to reduce compound partitioning during soil gas sampling. Gas-tight syringes will be used to qualitatively determine if a high vacuum soil condition is present. If a high vacuum condition is present due to low permeability soil, the sampling technician can feel the suction while the plunger on the syringe is being withdrawn. Sampling and Analysis Workplan Tracy Corners Shopping Center 3225 North Tracy Boulevard Tracy,California 95376 Partner Project Number SM14-129814 Docket Number HAS-VCA 14/15-108 August 15,2016 Page 25