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I 1.0 INTRODUCTION SECOR International Incorporated (SECOR), on behalf of Atlantic Richfield Company (ARCO), presents this Risk-Based Corrective Action .(RBCA) report for the ARCO Service Station No. 6080 (the Site), ' located at 85 Louise Avenue, Lathrop, California 95330. This RBCA was prepared in response to fhe request by the California Regional Water Quality Control Board (RWQCB) Central Valley Region and the San Joaquin County Environmental Health Department (SJCIEHD) to provide an assessment'of the soil contamination remaining at the Site for vapor emissions, direct exposure, and gross contamination (SJCIEHD, 2006) (Appendix A). 1.1 PURPOSE AND SCOPE OF WORK The. objectives of this RBCA were to determine potential compliance to the RWQCB Central':Region's water quality objectives (RWQCB, 2003) and to evaluate current and potential risks to public health and ! safety (RWQCB, 2004a) as a result of exposure to residual petroleum hydrocarbons in soil and groundwater at the Site. In addition, as requested by the RWQCB and the SJCIEHD (SJCIEHD, 2006), this RBCA will provide an assessment of the soil contamination remaining at the Site for vapor emissions, direct exposure, and gross contamination. The methodology used for this RBCA analysis was based on risk assessment guidance documents from the American Standards and Testing Materials (ASTM) (ASTM, 2002), United States Environmental Protection Agency (USEPA) (USEPA, 1989, 2002, and 2004a), the California Environmental Protection. Agency(Cal-EPA) (Cal-EPA,1992, 2005a; RWQCB, 2005). i 1.2 REPORT ORGANIZATION The remainder of this report is divided into seven sections organized as follows: • Site Background and Characterization (Section 2.0), providing information on the site description, geology and hydrogeology, and previous investigations; • RBCA Methodology(Section 3.0), presenting water quality objectives and RBCA methodology; • Site Conceptual Model (SCM) (Section 4.0), including current and future land uses, potentially exposed populations, water surveys, and complete/potentially complete exposure pathways; • Determination of chemicals of potential concern (COPCs), exposure point concentrations(EPCs), and screening levels (SLs) (Section 5.0), providing methodology for selecting COPCs, estimating EPCs, and identifying SLs; F • Tier 1 RBCA Results(Section 6.0), presenting the Tier 1 RBCA results; • Tier 2 RBCA Results(Section 7.0), presenting the Tier 2 RBCA results; • Conclusions(Section 8.0), discussing the results; and • References(Section 9.0). i i - i 1 i i F