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AECOM <br /> AECOM Environment RECORD <br /> 10461 Old Placerville Road,Suite 170 <br /> Sacramento,California, 95827-2508 SEP 0 3 2069 <br /> T916.362.7100 F916.362.8100 www.aecom.com <br /> ENVIRONMENT HEALTH <br /> PERMIT/SERVICES <br /> August 31, 2009 <br /> Mr.James L. Barton <br /> Central Valley Water Board <br /> 11020 Sun Center Drive, Suite 200 <br /> Rancho Cordova, California 95670-6114 <br /> RE: Proposed Remediation System Rebound Testing Schedule <br /> Cleanup and Abatement Order No. 5-00-707 <br /> 5400 and 5606 Pacific Avenue, Stockton, California <br /> Chevron#306620 (former Unocal#5098) <br /> AECOM Project#01231-154-M109 <br /> Dear Mr. Barton: <br /> AECOM Environment(AECOM), on behalf of Chevron Environmental Management Company(CEMC), is <br /> presenting a proposed rebound testing schedule for the air sparge with soil vapor extraction remediation <br /> system (AS/SVE)for the former Montgomery Ward, LLC (Wards) auto service center located at 5400 <br /> Pacific Avenue and former Unocal Service Station #5098 located at 5606 Pacific Avenue, Stockton, <br /> California (Figure 1), referenced collectively as the Site. Environmental operations at the Site are <br /> conducted pursuant to requirements of Cleanup and Abatement Order(CAO) #5-00-707 issued to Wards, <br /> Unocal, Surry California Limited Partnership, and Stone Brothers and Associated General Partnership <br /> (Stone Bros.) on September 8, 2000 by the Central Valley Water Board (CVWB). The locations of former <br /> and current site features and monitoring wells are presented on Figure 2. <br /> Proposed Remediation System Rebound Testing <br /> An upgraded AS/SVE remediation system commenced operation on July 31, 2008. Since the startup of <br /> the upgraded system, SVE system influent petroleum hydrocarbon concentrations, as well as dissolved <br /> groundwater petroleum hydrocarbon concentrations, have decreased significantly. The SVE influent <br /> concentration has decreased from a high of 8,200 parts per million by volume (ppmv) in August 2008 to an <br /> average of less than 12 ppmv during the last six months. In four of the key monitoring wells, dissolved <br /> petroleum hydrocarbon concentrations in groundwater have decreased by two orders of magnitude since <br /> April 2008. AECOM presented data in the Quarterly Groundwater Monitoring and Remediation Status <br /> Report and Updated Site Conceptual Model for the Second Quarter of 2009 demonstrating the <br /> effectiveness of the AS/SVE remediation system in remediating the dissolved petroleum hydrocarbon <br /> concentrations in the groundwater. Based on the remedial progress and the observed concentration <br /> decreases, it is now appropriate to begin rebound testing. Detailed below, AECOM presents technical <br /> justification and proposed schedule for the system shut down and rebound testing for the CVW B's <br /> acceptance. The technical justification includes data from third quarter 2009 that will be presented in the <br /> upcoming report(Quarterly Groundwater Monitoring and Remediation Status Report and Updated Site <br /> Conceptual Model for the Third Quarter of 2009). <br /> SVE Influent Concentrations <br /> Since the July 2008 startup of the upgraded AS/SVE system, SVE influent concentrations and mass <br /> removal rates have shown a steady decrease, as shown in Figure 3. At the startup of the upgraded <br /> AS/SVE system, the total petroleum hydrocarbons as gasoline (TPHg) mass removal rate was over 145 <br /> pounds per day (ppd), and remained over 10 ppd for two months following startup. The SVE TPHg mass <br />