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Additional Site Assessment Report and Human Health Risk Screening Evaluation www.arcadis.com 1 1 Introduction On behalf of Chevron Environmental Management Company (CEMC), Arcadis U.S., Inc. (Arcadis) prepared this Additional Site Assessment Report and Human Health Risk Screening Evaluation (Report) for the former Tidewater Associated Oil Company (TAOC)/Old Valley Pipeline (OVP) right of way (ROW), near the Surland Homes residential development, located in Tracy, California (Site; Figure 1). On January 24, 2019, Leidos Engineering LLC (Leidos) submitted the Subsurface Investigation Report and Health Risk Evaluation (Leidos 2019). The Central Valley Regional Water Quality Control Board (CVRWQCB) determined that additional soil vapor samples should be collected closer to the residences due to the soil vapor concentration observed at location B-10, and that the estimated risk calculations should be refined using the additional data, as documented in emails uploaded to GeoTracker. On May 16, 2019, Arcadis submitted an Additional Subsurface Investigation Work Plan (Work Plan; Arcadis 2019a) to conduct soil and soil vapor assessment activities at the Site. The Work Plan included two proposed soil vapor probe locations and one proposed alternate soil vapor probe location, which spanned three of the residential properties. In a letter dated June 3, 2019 (CVRWQCB 2019), the CVRWQCB conditionally approved the Work Plan, and requested installation and sampling of a third soil vapor probe in the western portion of the Site to provide data for a more spatially representative health risk screening evaluation. On September 26, 2019, Arcadis provided an updated site figure showing three proposed soil vapor probe locations and two proposed alternate soil vapor probe locations, which spanned five of the residential properties. The scope of work outlined in the Work Plan (Arcadis 2019a), Work Plan approval letter (CVRWQCB 2019), and subsequent email correspondence required Arcadis to obtain access agreements with five residential properties. Arcadis was only able to secure access agreements for two of the residential properties, at 2532 Dorset Lane and 2502 Dorset Lane. Arcadis made multiple attempts to execute access agreements with the other three residential properties with no success. On January 31, 2022, Arcadis was able to secure an access agreement with Union Pacific Railroad (UPRR) for the comprehensive soil vapor sampling event. Arcadis provided a status update for the Site in an email dated October 5, 2020. The CVRWQCB requested additional status updates in emails dated January 11, May 10, and June 28, 2021; Arcadis provided responses in emails dated June 29, 2021 and January 18 and May 10, 2022. The email correspondence is provided in Appendix A. In a letter dated June 30, 2021 (CVRWQCB 2021), the CVRWQCB requested a more detailed status update of all activities completed to date, including laboratory analytical results. Arcadis provided a case status update on August 30, 2021. Due to ongoing access delays with UPRR, Arcadis submitted an extension request letter to the CVRWQCB on January 12, 2022 (Arcadis 2022). The extension request was approved by the CVRWQCB in a letter dated January 13, 2022 (CVRWQCB 2022). This Report presents the results of the soil and soil vapor assessment activities that were completed between 2019 through 2022 in general accordance with the Work Plan (Arcadis 2019a), Work Plan approval letter (CVRWQCB 2019), and follow-up email correspondence with the CVRWQCB. In addition, this Report provides an updated human health risk screening evaluation that was critically reviewed by Arcadis’ Principal Toxicologist. 2 Background This section discusses the site location and description, site history, and geology and hydrogeology at the Site.