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1 � f <br /> ENSR AECOM <br /> 1 ENSR <br /> 10461 Old Placerville Road, Suite 170, Sacramento,California,95827-2508 <br /> 1 <br /> T916.362.7100 F916.362,8100 w .ensr.aecom.com [R 7 C�� U <br /> \UU/❑ED, <br /> Lt <br /> January 17, 2008 ILS <br /> 1 JAN 2 3 2008 <br /> Mr. James L. Barton <br /> Central Valley Water Board ENVIRONMENT HEALTH <br /> 1 11020 Sun Center Drive, Suite 200 PERMIT/SERVICES <br /> Rancho Cordova, CA 95670 <br /> 1 Subject: Addendum to Work Plan for Air Sparging/Soil Vapor Extraction Well Installation <br /> Cleanup and Abatement Order No. 5-00-707 <br /> 5400 and 5606 Pacific Avenue, Stockton, California <br /> 1 Chevron Facility 306620 (former Unocal 5098) <br /> ENSR Project No: 01231-154-A1C <br /> 1 Dear Mr. Barton: <br /> ENSR Corporation (ENSR), on behalf of Chevron Environmental Management Company (EMC), is <br /> 1 pleased to present this addendum to the August 2, 2007 Work Plan for-Air Sparging/Soil Vapor Extraction <br /> Well Installation (Work Plan) for both the former Montgomery Ward, LLC (Wards) service station located at <br /> 5400 Pacific Avenue and the former Unocal service station, located at 5606 Pacific Avenue, Stockton, <br /> 1 California (Figure 1 —Site Location Map), referenced collectively as the Site. The locations of the former <br /> and current site features, as well as the proposed air sparge (AS) and soil vapor extraction (SVE) well <br /> locations, are identified on Figure 2. Well construction diagrams for the proposed wells are included as <br /> 1 Figure 3. This addendum addresses comments provided by the Central Valley Water Board (CVWB) in a <br /> Document Review letter dated December 21, 2007 (Attachment A). <br /> 1 Comment No. 1 <br /> It is my understanding from previous conversations with ENSR that there is a question whether the <br /> 1 treatment system could adequately affect the groundwater plume to the north under Robinhood Drive. <br /> Please submit a letter addendum to the Workplan, which provides a detailed explanation of the existing <br /> SVE/AS system testing procedures, compares the testing results to the expected performance <br /> 1 specifications for a similar designed system, and shows that the entire groundwater plume will be <br /> addressed by the proposed upgrades. In other words, the technical justification for replacing all, or a <br /> portion of, the existing system needs to be presented in a manner that the State Water Resources Control <br /> 1 Board, Underground Storage Tanks Cleanup Fund and Regional Board staffs can concur to ENSR's and <br /> CEMC's determination that the upgrades are necessary and appropriate. <br /> 1 Response No. 1 <br /> Under EMC's management of remediation sites, the standards of operation include a remediation plan <br /> review by a technical team consisting of lead remediation personnel from Chevron and its key consultants. <br /> 1 This remediation review process draws on Chevron's experience having remediated over 1,000 retail and <br /> terminal sites in the United States. The Site was reviewed by this Remediation System Review Team <br /> (RSRT) in October 2006, and the RSRT recommended a complete redesign of the air sparge(AS) system <br /> 1 to provide a cost-effective remediation system with the ability to deliver effective AS/SVE application <br /> across the vertical and lateral extent of groundwater impacts at the Site. To effectively remediate the <br /> 1 01231-154 ENSR AECOM <br />