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Mr. Buehler February 9, 2024 Page 5 of 7 dichloroethane (DCA) by 826013, gasoline range organics (GRO) by 8015, and methanol and ethanol by 8015M. INSTALLATION OF SOIL VAPOR MONITORING WELLS The soil vapor investigation will focus on petroleum vapor intrusion to indoor air criteria for future development based on the LTCP. Three on-Site soil vapor monitoring wells (SVP-1 through SVP-3) will be installed at the approximate locations shown on Figure 2. Borings will be advanced using a hand-auger to a target depth of 5 feet bgs.Soil samples will be collected and logged by a field scientist in general accordance with the Unified Soil Classification System (USCS) and field- screened for volatile organic compounds (VOCs) using a hand-held photo-ionization detector (PID). The soil vapor monitoring points will be constructed using 1/4-inch diameter TeflonTM tubing, fitted with a vapor probe implant installed at the target depth with a three-way valve at the surface. The annulus surrounding the probe will be filled with #2/12 Monterey sand to six inches above and below the probe, followed by a layer of dry granulated bentonite up to 1-foot thick, and hydrated bentonite chips to near ground surface. The soil vapor monitoring points will be protected by a traffic-rated vault box installed in concrete. Following installation, the soil vapor monitoring wells will be allowed to equilibrate for a period of at least 48 hours prior to sampling. Citadel will sample the newly installed soil vapor probes in accordance with the Department of Toxic Substances Control (DTSC) / RWQCB Advisory-Active Soil Gas Investigations document dated July 2015 (Advisory). In accordance with the Advisory, a shut-in test and leak test will be conducted prior to sampling. A helium shroud will be used to conduct a leak test at each sample location and approximately 20 percent (%) helium will be monitored and maintained during the sample collection process. Helium monitoring will be conducted using a field meter capable of reporting helium detections between 0.1% and 100%with a precision of at least +10% at 0.1%. After the shut-in test is completed, sample locations will be purged of approximately three sample train volumes to ensure that stagnant or ambient air is removed from the sampling system and to ensure samples collected are representative of subsurface conditions. The soil vapor will be purged through the manifold into a purge canister/hand-held vacuum pump by opening the manifold valve to the purge canister/hand-held vacuum pump and then opening the canister valve. Following purging, these valves will be closed and the manifold valve to the sample canister will be opened. The canister valve on the sample canister will then be opened, and the sample will be collected in a 1-liter Summa canister. A flow rate of 100 to 200 milliliters per minute (mL/min) and vacuum less than 100 inches of water will be maintained during sample collection. Following collection of each sample, the canister valve will be closed, and the sample canister will be prepared for shipment to the laboratory. The sample containers will be labeled with sample-point identification, date, and time of collection. Clean laboratory-provided sampling manifolds, flow controllers, and canisters will be used at each sample location. One duplicate soil vapor sample will be collected for quality assurance/quality control (QA/QC) purposes. Samples will be submitted under chain-of-custody (COC) protocol to a laboratory for analysis of VOCs using USEPA Method TO-15 and helium and fixed gases (oxygen, carbon dioxide, and methane) using modified American Society for Testing and Materials (ASTM) Method D-1946. Soil vapor samples will not be collected if more than inch of precipitation has occurred in the five