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Revised Soil Gas Sampling I w Workplan September 8, 2017 CA ARNG, FMS #24, Stocktch., CA Page 7 The temporary wells will be completed at the surface in well boxes, and tubing will be closed off using a luer lock valve, gas-tight valve, or a fitting for capping the tubing to minimize air intrusion from the surface. Well boxes will be traffic rated and completed 1/4-inch above the existing grade to prevent surface runoff in filtration. SOIL GAS SAMPLING AND ANALYSIS Sampling will be completed during the summer, so rainfall is not expected. Should a significant rain event occur, sampling will not take place less than 5 days following the rainfall and after the frontal system has passed the area. Subsurface conditions will most likely be disturbed during drilling and probe placement/installment. To allow for the subsurface to equilibrate back to representative conditions, the purge volume test, leak test, and soil gas sampling will not be conducted until approximately 1 week after installation. Soil vapor well installation method and equilibration will be recorded in the field log book or field form. All sampling activities, including shroud design, shut-in test, leak test, and purge volume test will be conducted per Advisory, Active Soil Gas Investigations (DTSC, et. al., 2015). Helium will be used as the tracer gas. Flow rates between 100 to 200 milliliters per minute (mL/min) and vacuums less than 100 inches of water will be maintained during purging and sampling to minimize stripping (partitioning of vapors from pore water to soil gas), to prevent ambient air from diluting the soil gas samples, and to reduce variability between contractors. Maintaining these flow rates and vacuums will increase the likelihood that representative samples will be collected. A vacuum gauge will be used between the soil gas sample tubing and the soil gas purging device to verify that 100 inches of water or less is maintained during sampling. During purging, subsurface soil vapor conditions will be field screened using a five-gas meter (oxygen, carbon monoxide, hydrogen sulfide, lower explosive limits [LEL], and VOCs). The analytical laboratory will provide sample collection equipment and sample analysis. Samples will be collected in 1-liter Summa® Canisters, each with a dedicated flow regulator. The field crew will record the pressure upon start and completion of the sampling. Typically, canisters are returned to the stationary laboratory with a slight vacuum. Pressure measurements will be collected using a calibrated pressure gauge, using the same gauge at the laboratory and in the field. Field crews will only rely on canister-dedicated pressure gauges if the gauges are calibrated and working properly. Samples will be analyzed by Environmental Protection Agency (EPA) modified analytical method TO-15 for TPHg and for VOCs. Sorbent tubes will be used to collect samples for analysis of diesel-range organics by EPA 10-17. The tubes will be capped with Swagelok®- type caps and combined Teflon® ferrules, rewrapped in aluminum foil, and placed in a chilled storage container immediately after sampling. A chain of custody form will be completed in the field and will include any relevant problems encountered during sample collection. The starting and ending pressures for passivated stainless-steel canisters should be recorded on the chain of custody form and the field notes. To