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18 August 2023 AdvancedGeo Project No. 17-4073 An Employee-Owned Company Page 6of7 inventoried. Products that contain volatile chemicals or other chemicals that can potentially bias the results of the sampling will be listed on Building Survey and Indoor Air Source Screen Forms (Appendix C). The Building Survey and Indoor Air Source Screen Forms are presented in Attachment 6 of Supplemental Guidance: Screening and Evaluating Vapor Intrusion Final Draft (February 2023), prepared by the DTSC and California Water Resources Control Boards. Indoor contaminant sources and products that could potentially bias the sampling results will be located using an OVM equipped with a PID calibrated to a detection limit of parts per billion by volume (ppb MiniRae 3000). Any identified sources of indoor contamination will be removed from the building or sealed, if feasible, and the areas will be re-monitored with the PID. All field procedures will be overseen by an AGI representative under the supervision of a certified Industrial Hygienist and a California Professional Geologist. 2.6.2. Crawlspace and Ambient Air Sample Collection All crawlspace and ambient air samples will be collected in six-liter Summa canisters using passive integrated sampling procedures. Each canister's initial vacuum will be measured and recorded to ensure the initial vacuum is greater than 25 inches of mercury (in hg). The sampling inlet on the canisters will be connected to a mass flow controller containing a particulate filter and calibrated to 4.0 ml/min so that crawlspace and ambient air samples are passively collected over approximately 24 hours. The crawlspace air sample will be collected by attaching Teflon tubing to the inlet of the Summa cannister and routing the tubing into the crawlspace as near to the northeast corner of the residence as feasible. The outside ambient air samples will be collected to the east and west of the residence from approximately 4 to 6 feet above the ground surface. Once the air sampling canisters and tubing are placed and positioned properly, the Summa canister valves will be opened to begin air sample collection. Ambient air sample containers will be opened approximately 60 minutes before the crawlspace air sample can. Following approximately 24-hours of sample time, the crawlspace and ambient air sample containers will be retrieved, closed, and sealed. Ambient air sample containers will be retrieved and closed approximately 30 minutes prior to the retrieval of the crawlspace air sample can. The sample containers will then be labeled with the initial and final vacuum to ensure that the regulator was functioning properly. All air samples will be analyzed by a California ELAP-certified laboratory for VOCs in accordance with EPA Method TO-15. Laboratory reports for all analyses, testing methods, laboratory QA/QC reports, and sample chain-of-custody documentation will be presented in a report with the findings and recommendations. EDF files of the laboratory