ARCHIVED REPORTS_REPORT ON SOIL VAPOR SAMPLING AND HUMAN HEALTH RISK ASSESSMENT DEC 2010

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AECOM Report on Soil Vapor Sampling and Human Health Risk Assessment 1-1 1.0 Introduction 1.1 Background and Objectives At the request of Chevron Environmental Management Company(Chevron),AECOM has prepared this report for the former Montgomery Wards, LLC (Wards) auto service center site located at 5400 Pacific Avenue, and the former Unocal service station site (No. 5098) located at 5606 Pacific Avenue, Stockton, California (Figure 1). The Wards and Unocal sites are collectively referred to as the"Site". Environmental activities at the Site have been conducted pursuant to the requirements of Cleanup and Abatement Order(CAO) Number 5-00-707,which was issued by the Central Valley Regional Water Quality Control Board (RWQCB)to Wards, Unocal, Surry California Limited Partnership, and Stone Brothers and Associated General Partnership (Stone Brothers)on September 8, 2000. In a letter dated February 2, 2010,the RWQCB directed Chevron and Stone Brothers to"submit a work plan for soil vapor investigation for all potential areas of concern, such as the Macaroni Grill (former Wards)and the bank(former Unocal), and for a direct-push soil and groundwater investigation under Robin Hood Drive."AECOM submitted the Work Plan for Soil Vapor Investigation on April 2, 2010 (AECOM, 2010).The objective of the soil vapor sampling was to evaluate the potential for vapor inhalation risk to current and future occupants of on-site commercial buildings. Consistent with the work plan, on May 13, 2010,AECOM installed three soil vapor wells at the site—SV-1 in the vicinity of the location of the dispenser islands at the former Unocal station [and outside the footprint of the current bank building.]; SV-2 in the vicinity of dispenser islands formerly located at the Wards auto center, and SV-3 in the vicinity of LISTS formerly located at the Wards auto center. Initial sampling of these wells was conducted on May 20, 2010. A report documenting the well installation, soil vapor sampling activities, and risk evaluation was submitted to the RWQCB on July 15, 2010,which should be referred to for detailed information. The results of the May sampling event were considered to be indicative of failed quality control and additional sampling of SV-1 was conducted on September 9,2010,to confine the accuracy of the laboratory analytical results. In accordance with recommendations presented in the July 15, 2010, report a"seasonal" variation sampling event of all three wells was conducted on October 20, 2010. The assessment acbvites described in this report include a description of the sampling methods employed during all three sampling events,the laboratory analysis of the samples collected, a discussion of those results, evaluation of potential soil vapor inhalation risks using soil vapor concentration data, and conclusions and recommendations. 1.2 Site Location and Description The former Wards property is located at the southeast corner of the intersection of Pacific Avenue and Robin Hood Drive. The former Unocal service station property is located at the northeast corner of Parc Avenue and Robin Hood Drive.The Site is currently located in a commercial business and retail shopping district. The locations of current Site and former service station features are illustrated on Figure 2.The surface elevation at the Site is approximately 12 feet above mean sea level. December 15,2010 60146576-M10