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Additional Site Assessment Report and Human Health Risk Screening Evaluation www.arcadis.com 5 The soil samples were stored on ice in coolers. Samples collected in December 2019 and October 2020 were sent to BC Laboratories, Inc., located in Bakersfield, California, a California state-certified laboratory, under proper chain of custody procedures. Samples collected in February 2022 were sent to Pace Analytical National Center for Testing and Innovation, located in Mt. Juliet, Tennessee, a California State-certified laboratory, under proper chain of custody procedures. The soil samples were analyzed for the following analytes: • Total petroleum hydrocarbons as gasoline (TPH-g) in the C5-C12 range via United States Environmental Protection Agency (US EPA) Method 8015B • Total petroleum hydrocarbons as diesel (TPH-d) in the C12-C22 range via USEPA Method 8015B • Benzene, toluene, ethylbenzene, and xylenes (BTEX) and naphthalene via USEPA Method 8260B. 3.2.3 Soil Vapor Assessment Dual-nested soil vapor probes were installed in three of the five borings (B-13, B-14, and B-16). Soil vapor probes were not installed at the two remaining borings (B-12 and B-15) due to the presence of water. The soil vapor probes were screened at depths of 4.5 to 5.0 feet bgs and 7.5 to 8.0 feet bgs. The soil vapor probes were constructed using a stainless-steel weighted probe tip and screen, Teflon™ tubing, and a compression cap. Each 6-inch-long stainless-steel screen was centered within a 1-foot sand filter pack. One foot of dry, granular bentonite was installed above each sand filter pack followed by 1 foot of hydrated bentonite. The remainder of the borehole was filled with neat cement grout as an annular seal. The probes were completed with a 6-inch well box. Soil vapor probe construction diagrams are provided in Appendix C. A limited soil vapor sampling event was conducted on January 13, 2020. A soil vapor sample was collected from the dual-nested soil vapor probes at B-13. A comprehensive soil vapor sample event was conducted on April 18, 2022, as outlined in the Work Plan (Arcadis 2019a). The comprehensive soil vapor sample event targeted the four existing soil vapor probes (B-8 through B- 11) and the three newly installed dual-nested soil vapor probes on residential properties (B-13, B-14, and B-16). This represents a total of seven targeted soil vapor probe locations. Soil vapor sampling was conducted at only one of the seven targeted soil vapor probe locations during 2022. Arcadis was unable to locate soil vapor probes B-8 through B-11 and B-13 during the 2022 event; however, B-13 was previously sampled during 2020, as noted above. Arcadis attempted to collect soil vapor samples from B-14; however, field personnel encountered water in the shallow probe and downhole pressure in the deep probe that exceeded DTSC guidance for sample collection. Arcadis was only able to collect a soil vapor sample from the shallow soil vapor probe at B-16, including a duplicate sample. A soil vapor sample could not be collected from the deep soil vapor probe at B-16 due to water observed in the probe tubing during the syringe test. During the 2020 and 2022 sampling events, several quality control (QC) measures, including shut-in tests and helium tracer leak tests, were performed at each probe before sampling to verify the integrity of the sampling system. All samples were collected in accordance with Advisory – Active Soil Gas Investigations (Department of Toxic Substances Control [DTSC] 2015). A shut-in leak detection test was conducted on each sample train prior to sampling. While the port valve and sample valves were left in the closed position, a syringe was used to remove approximately 25 milliliters (mL) from the sample train through the purge valve to induce a vacuum of greater than 5 inches of mercury within the sample train. The purge valve was then closed and the vacuum within the sample train was monitored for 2