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Geosynte& consultants approximately 5 feet bgs. The exact depth of probe installation may vary based on the geology encountered; soil vapor probes will not be placed within clay layers that could hinder sample collection. The annular space of the borehole will be filled with filter pack sand from the bottom of the borehole to 6 inches above the top of the screen, then with granular bentonite, hydrated bentonite, and cement grout to ground surface (Figure 9). Soil vapor probes will be installed with a flush-mount completion as indicated in Figure 9. For installation of the soil vapor probe in a dual-purpose boring, where a grab groundwater sample is planned to be collected(i.e., 81713-11, 81713-13, 81713-15, and 81713-16), the boring will be backfilled with cement grout to 6 inches below the bottom of the planned filter pack (approximately 6 feet bgs) followed by 6 inches of dry granular bentonite. The cement grout will be allowed to cure for two hours prior to constructing the soil vapor probe as described above. 4.2.5 Soil Vapor Sampling Following soil vapor probe installation, soil vapor samples will be collected, as outlined in Table 1. Soil vapor sampling techniques will include leak testing, shut-in testing, and purging in general accordance with DTSC guidance, including the Advisory (DTSC 2015) and the Supplemental Guidance (DTSC 2023), as described in the following sections. Soil vapor probes will be sampled a minimum of 48 hours following probe installation. 4.2.5.1 Sampling Equipment At each soil vapor probe location, the sample train will be set up with dedicated well fittings and new NylaflowTM tubing. The NylaflowTM tubing will be field screened with a PID for the presence of VOCs before use at each location. Field instruments (PID, multi-gas detector, and helium detector) will be calibrated prior to conducting sampling activities. 4.2.5.2 Shut In Testing After the aboveground sampling equipment is assembled at each location, a shut-in test will be conducted to test for leaks in the sample apparatus. A vacuum of approximately 100 inches of water will be applied to the sampling train, and the vacuum will be monitored for approximately one minute. If the vacuum dissipates during the shut-in test, the aboveground fittings will be tightened, and the test will be repeated until the vacuum is maintained for one minute, indicating a leak-free assembly. This protocol will be repeated after leak testing and before sampling to ensure that the integrity of the sampling equipment seals has been preserved. 4.2.5.3 Leak Testing After each soil gas probe location passes a shut-in test, a leak test will be conducted during vapor probe purging. Laboratory-grade helium will be used for the leak test to check for ambient air leakage through the annular seal between the soil vapor probe and the ground surface. A shroud will be temporarily installed at the ground surface around each well, and helium will be added to the shroud, with the helium concentration ranging approximately between 20% and 40%. The concentrations of helium in the shroud and the purge samples will be measured with a field helium detector. The purge samples will be considered acceptable if the helium concentration is Investigation Work Plan—UST 817B 9 December 2024