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V made conduits such as buried utility lines, through cracks in the paving or foundations to indoor and outdoor air where they could be inhaled. • Direct contact with the shallow (53 meters or 10 feet bgs) impacted soil. Construction workers could be exposed to chemicals when excavating soils. Chemicals of Potential Concern COPCs are defined as chemicals that are potentially site-related and for which data are of sufficient quality and quantity for use in a quantitative risk assessment (USEPA, 1989). This section presents the selection of COPCs and estimation of EPCs for soil and groundwater at the Site. txf0S,Vre— p i(+ `rCMeG -irAI rr,� COPCs in this assessment include those chemicals for which a Tier 1 RWQCB ESL are available and were detected at the Site in ground and soil: benzene, toluene, ethylbenzene, xylenes, MTBE, TBA, and 1,2-DCA. — T 1i 4,J 14- 10.2 4 10.2 Tier 1 and Tier 2 RBCA Evaluation For this project, the risk evaluation will also include comparison of Site date�to the applicable RBCA Tier 1 and Tier 2 standards. This evaluation, described below, will proceed the s actually soil vapor intrusion modeling described in section 10.2.2. Prot 0C� - 10.2.1 Tier 1 RBCA Analysis jr � .;.yr-e p8,.vt- esnce,,v4-r,4L4 At the conclusion of the proposed soil sampling activity and analytical analyses described herein and consistent with the Tier 1 RBCA methodology, Site EoCs of chemicals detected in soil and groundwater will'8ompared to their appropriate Tier 1 ESLs (RWQCB, 2003)5. The RWQCB RBCA analysis for soil samples differentiates between exposure to shallow �,Vv soils (<3 meters deep) and exposure to deep soils (>3 meters deep). In addition, ESLs ✓`�`1 have also been developed for shallow soil gas. A discussion is presented below. Tier 1 Soil Vapor The reported concentrations will be compared to the California Regional Water Quality Control Board-San Francisco Bay Region (RWQB-SFR) Tier 1 ESLs for residential receptors for shallow soil gas levels (Table E-2). ESLs are documented in the "Screening For Environmental Concerns at Sites With Contaminated Soil and Groundwater California Regional Water Quality Control Board-San Francisco Bay Region", (Interim Final, November 2007 updated May 2008)1 . If it is determined that a soil vapor ESL for residential use is exceeded, an evaluation will performed to determine whether the detected COPCs could potentially migrate into indoor air and adversely affect human health. Tier 1 Soil The proposed soil sample location/results and historic Site soil analytical data will be compared to the CRWQB-SFBR Tier 1 ESLs for residential receptors for shallow soil levels (Table A). An evaluation will be made to determine if the resultant concentration(s)will pose V � \t, `Workplan to Conduct Subsurface Soil Vapor Intrusion Study \\V// CALM ATC Project 54.26874.0001 Page 10