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27 September 2024 AdvancedGeo p An Employet-Owned Company Project No. 23-7573 ' Page 3 of 6 of dry granulated bentonite will be placed atop the filter pack to 3 feet bsg. Following the dry bentonite, each sampling point will be grouted with Portland cement to near surface grade and water-tight well boxes will be installed over the points. 3.2. SOIL-GAS SAMPLE COLLECTION & ANALYSIS Following soil-vapor well installations, a minimum 48-hour stabilization period will be observed prior to soil-vapor sample collection from the vapor points. For sample collection, a one-liter Summa canister and purge pump calibrated to 150 milliliters per minute (ml/m) will be used to purge and collect samples from each point. Prior to sample collection, each soil-vapor well will be purged a total of three purge volumes. The purge volume will be determined by calculating the sum of the internal volume of the Teflon tubing and the volume of the boring trace. Once purging has been completed, laboratory-prepared one-liter Summa canisters will be connected to the above ground tubing. The Summa canister will be opened, and the sample start time and sample canister pressure will be recorded. Thereafter, each soil- vapor sampling point will be isolated from ambient air by enclosing the soil-vapor point and the canister assembly within a clear plastic shroud. Helium gas will be infused into the shroud during collection of soil-vapor samples to ensure an air-tight seal of the soil- vapor sampling assembly. Helium will be measured utilizing a Divesoft Analyzer SOLO 8300 helium meter to verify that a 20 to 25% concentration of helium is maintained within the shroud. Upon reaching at least -5 in hg or less, the sample canister valve will be closed, and final pressure and time recorded. The sampling port on the sampling canister will be capped with a brass end-cap. The soil-vapor samples will be labeled with sample ID, project name, date, time, and samplers' initials. The soil-vapor samples will be transported under chain-of-custody procedures to a California Environmental Lab Accreditation Program-certified (ELAP) laboratory for analysis. All soil vapor samples will be analyzed for VOCs by EPA method TO-15 and helium (as tracer gas) by Modified ASTM D-1946. 4.0. INDOOR AIR AND CRAWL SPACE SAMPLE COLLECTION Indoor air crawl space samples will be collected concurrently with soil-vapor samples. All indoor air/crawl space sampling field work will be performed utilizing procedures provided in the DTSC-prepared Final Guidance for The Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air (Vapor Intrusion Guidance) dated October 2011; and the DTSC Supplemental Guidance: Screening and Evaluating Vapor Intrusion Final Draft (February 2023). Procedures for sample collection and analysis are described below.