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November 2, 2004 RECEIVE® NOV 0 5 1004 ENVIRONMENT HEAL1i1 Mr. James Barton PERMR/$ERVICES California Regional Water Quality Control Board Central Valley Region 11020 Sun Center Drive, Suite 200 Rancho Cordova, CA 95670 RE: Work Plan to Evaluate Potential SVE/AS System Enhancements and Additional Well Installation within the Deeper Aquifer Units Former Unocal Station No. 5098 5400 and 5606 Pacific Avenue, Stockton, California Dear Mr. Barton: On behalf of Union Oil Company (Unocal) of California, ENSR Corporation (ENSR) has prepared this work plan for the Former Unocal Station No. 5098 at 5606 Pacific Avenue, and Former Montgomery Wards (Wards) Auto Service located at 5400 Pacific Avenue, Stockton California. The location of the site is shown in Figure 1 and site features are illustrated on Figure 2. Wards and Unocal are named jointly responsible for actions necessary to abate the effects of contamination in the vicinity of the sites. In August 2000 Wards and Unocal executed an agreement for Coordinated Corrective Action for Underground Storage Tank (UST) Fund Claim No. 1760 and UST Fund Claim No. 6864. On November 2, 2001, Unocal was named as the Appointed Representative for the PACIFIC/ROBINHOOD COMMINGLED PLUME. In this work plan, reference to the "site" includes the former Wards and Unocal sites. The California Regional Water Quality Control Board, Central Valley Region (RWQCB) requested this work plan in a letter dated, October 4, 2004. A copy of the RWQCB letter is included in Attachment A. Proposed Outline and Schedule to Evaluate System Enhancement Options The following seven activities are currently being conducted or planned to evaluate potential enhancements to the existing AS/SVE system, to help control plume movement, and to achieve regulatory closure: 1. Conduct a Remedial System Evaluation (RSE) to objectively review program goals (containment, receptor protection, and cleanup) in accordance with the Cleanup and Abatement Order (CAO) 5-00-707, and critically evaluate primary cost categories (labor, regulatory compliance/reporting, vendors/suppliers, and equipment/maintenance). The RSE will evaluate the placement of existing air sparge wells, vapor extraction wells and monitoring wells relative to the changing plume boundaries both vertically and horizontally. The results of the RSE will be used to either re-affirm the goals of the remedial system or to develop new goals for the existing system and propose enhancements and/or additional J:\Projects\06940\5098-293\Fina(Reports\Work Plans\5098 Work Plan-SVE AS System.doc