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DRAFT Geosyntec consultants 5.1.3 Sampling Equipment At each soil vapor probe location, the sample train was set up with dedicated well fittings and new NylaflowTM tubing. The NylaflowTM tubing was field screened with a PID for the presence of VOCs before each location. Field instruments (PID, multi-gas detector, and helium detector) were calibrated prior to sampling. 5.1.4 Shut-In Tests After the aboveground sampling equipment was assembled at each location, a shut-in test was conducted to test for leaks in the sample apparatus. A vacuum of approximately 80 to 100 inches of water was applied to the sampling train and the vacuum was monitored for approximately one minute. If the vacuum dissipated during the shut-in test, the aboveground fittings were tightened, and the test was repeated until the vacuum was maintained for one minute, indicating a nominally leak-free assembly. This protocol was repeated after leak testing and before sampling, to ensure the integrity of the sampling equipment seals had been preserved. 5.1.5 Leak Testing After each soil vapor sampling location passed a shut-in test, a leak test was conducted during vapor well purging. Laboratory grade helium was used for the leak test to check for ambient air leakage through the annular seal between the vapor well and the ground surface. A shroud was temporarily installed at the ground surface around each well and helium was added to the shroud, with the minimum and maximum average helium concentration ranging between 23% and 43%, respectively. The concentrations of helium in the shroud and the purge samples were measured with a field helium detector. The purge samples were considered acceptable if the helium concentration was less than 5% of the minimum shroud concentration, consistent with recommendations in the Advisory. Helium was detected in the shroud during leak testing at concentrations ranging from 2,400 parts per million by volume (ppmv) to 17.5%by volume. Fittings were re-tightened after the first purge sample was collected; however, helium continued to be detected in subsequent purge samples. Analytical results indicated that helium concentrations were less than 5% of the helium concentration in the soil vapor sample. This indicates that the soil vapor sampling methodology conformed with DTSC guidance, which states that "an ambient air leak up to 5% is acceptable if quantitative tracer testing is performed" (DTSC, 2015). 5.1.6 Purge and Sampling Rate Prior to sampling, air was purged from each vapor well to remove stagnant or ambient air and collect samples that are representative of subsurface conditions. A minimum of three well volumes were purged prior to sampling, consistent with recommendations in the Advisory. Purge volumes were calculated based on the volume of the void space in the tubing plus an estimate of the void space in the sand filter pack of each vapor well. The wells were purged by drawing soil vapor from the formation using a lung box to fill a I-liter TedlarTM bag. The purged soil vapor was screened in the field using a PID, landfill gas meter, and a helium detector. The purge volume for each sample location was recorded. Soil vapor was purged from each location at an approximate flow rate of 200 milliliters per minute (mL/min). Draft Soil and Soil Vapor Investigation 11 August 30,2019