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IAECOM <br /> HMERW ED <br /> AECOM Environment <br /> 10461 Old Placerville Road, Suite 170 FEB 2 4 2009 <br /> Sacramento, California 95827-2508 <br /> T(916)362-7100 F(916)362-8100 ..aecam.cam ENVIRONMENT HEALTH <br /> PERMIT/SERVICES <br /> February 20, 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 /> Subject: Addendum to the Quarterly Groundwater Monitoring and Remediation Status Report and <br /> Updated Site Conceptual Model for the Fourth Quarter of 2008 <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-OM1 <br /> Dear Mr. Barton: <br /> AECOM Environment(AECOM), on behalf of Chevron Environmental Management Company (CEMC), is <br /> submitting this Addendum to the Fourth Quarter 2008 Groundwater Monitoring and Remediation Status <br /> Report and updated Site Conceptual Model. This addendum presents a remaining contaminant mass <br /> estimate at the time of the fourth quarter 2008 groundwater sampling event (October 17, 2008) for the <br /> former Montgomery Ward, LLC (Wards) auto service center located at 5400 Pacific Avenue and former <br /> Unocal Service Station #5098 located at 5606 Pacific Avenue, Stockton, California (Figure 1) referenced <br /> collectively as the Site. Groundwater monitoring and sampling have been conducted pursuant to the <br /> requirements of Cleanup and Abatement Order(CAO) No. 5-00-707 issued to Wards, Unocal, Surry <br /> California Limited Partnership, and Stone Brothers and Associated General Partnership(Stone Bros.) on <br /> September 8, 2000, by the Central Valley Water Board (CVWB). In a letter dated December 5, 2008, the <br /> CVWB, requested "An evaluation of the effectiveness of cleanup activities and an estimate of pollutants <br /> treated versus pollutants remaining(maybe submitted quarterly as an ancillary report)". The fourth quarter <br /> 2008 quarterly monitoring and remedial status report did not include the estimate of remaining mass which <br /> is presented in this addendum. Future quarterly monitoring and remediation status reports will contain an <br /> estimate of remaining mass in addition to an evaluation of the effectiveness of cleanup activities and an <br /> estimate of pollutants treated. <br /> Remainina Mass Estimate <br /> The remaining mass for total petroleum hydrocarbons as gasoline (TPHg) and benzene in the saturated <br /> zone(both groundwater and saturated soil) was estimated based on fourth quarter 2008 groundwater <br /> monitoring results. Estimates were calculated for the shallow hydrostratigraphic A/B and D units using the <br /> iso-concentration maps presented in the Quarterly Groundwater Monitoring and Remediation Status Report <br /> and Updated Site Conceptual Model for the Fourth Quarter of 2008 dated January 12, 2009. Estimates for <br /> Unit C were not calculated because neither TPHg nor benzene was detected in the fourth quarter 2008 in <br /> AECOM Environment <br />