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Geosyntec consultants 4.4.3 Leak Testing After each soil vapor probe location passes a shut-in test,a leak test will be conducted during vapor well 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 less than 5% of the minimum shroud concentration, consistent with recommendations in the Advisory (DTSC, 2015). 4.4.4 Purge and Sampling Rate Prior to sampling,air will be purged from each soil vapor probe to remove stagnant or ambient air and collect samples that are representative of subsurface conditions. A minimum of three casing volumes will be purged prior to sampling,consistent with recommendations in the Advisory.Purge volumes will be 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 soil vapor probe. The wells will be purged by drawing soil vapor from the formation using a lung box to fill a 1-liter TedlarT M bag.The purged soil vapor will be screened in the field using a PID and a helium detector. The purge volume for each sample location will be recorded. Soil vapor will be purged from each location at an approximate flow rate of 200 milliliters per minute. 4.4.5 Soil Vapor Sampling and Analysis After completion of shut-in testing, leak testing, and purging, soil vapor samples will be collected through dedicated Nylaflow tubing attached to each probe with airtight fittings into 1-liter SummaTM canisters. Soil vapor samples will be collected until the vacuum on the SummaTM canister decreases from 30 inches of mercury to approximately 5 inches of mercury. One duplicate sample will be collected using a "tee"-fitting that splits the soil vapor sample between to two canisters. SummaTM canisters will be shipped to an analytical laboratory for analysis of 1,2-DCP, 1,2,3-TCP, DBCP and chloroform. 4.5 Groundwater Sampling Immediately after completion of the soil vapor sampling, samples will be collected from groundwater monitoring wells MW-513 and MW-713 for analysis of 1,2-DCP, 1,2,3-TCP, DBCP and chloroform. Groundwater sampling will be performed after conclusion of soil gas sampling to avoid potential issues associated with perturbing the soil gas in the vadose zone near to the soil gas probes, which may occur during the purging and/or sampling of the monitoring wells. 4.5.1 Depth to Groundwater Measurements Depth to groundwater will be measured to the nearest 0.01 foot in MW-513 and MW-713 with a graduated electronic water level sounder from the surveyed datum located at the top of the well casing. The electronic sounder will be decontaminated before and after use at each monitoring well. Work Plan for Vapor Intrusion Investigation 8 July 30,2021