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RECOVE4 <br /> Or, 1 5 2005 <br /> ENSR International <br /> h - � I�HEl1��F� 10411 Old Placerville Road <br /> PERMIT/SERVICES Suite 210 <br /> Sacramento,CA 95827-2508 <br /> August 11, 2005 (916)362-7100 <br /> FAX (916)362-8100 <br /> www.ensr.com <br /> Mr. James L. Barton <br /> Central Valley Water Board <br /> 11020 Sun Center Drive, Suite 200 <br /> Rancho Cordova, CA 95670-6114 <br /> RE: Soil Vapor Extraction System Sampling Data— May 2005 <br /> Former Unocal Facility No. 5098 <br /> 5400 and 5606 Pacific Avenue, Stockton, California <br /> Dear Mr. Barton: <br /> On behalf of the Union Oil Company of California (Unocal), ENSR Corporation (ENSR) has <br /> prepared this letter in regards to the May 2005 soil vapor extraction (SVE) system sampling and <br /> laboratory analysis at the above referenced site. Former site facilities were located at both the <br /> former Montgomery Ward, LLC (Wards) site located at 5400 Pacific Avenue and at the former <br /> Unocal site located at 5606 Pacific Avenue, Stockton, California. The two properties are <br /> heretofore referred to as the Site. <br /> The Site has ongoing remediation consisting of an air sparge (AS) component and an SVE <br /> component. Every month, ENSR collects vapor samples from the influent, midfluent (intermediate), <br /> and effluent vapor streams. The purpose of the sampling is two-fold: <br /> 1. to verify compliance with the air discharge permits associated with the SVE system; and <br /> 2. to calculate the mass removal of petroleum hydrocarbons. <br /> In ENSR's Updated Site Conceptual Model, Quarterly Monitoring and Remediation Status <br /> Report, Second Quarter 2005 (Report) submitted on July 27, 2005, ENSR stated that the <br /> April 2005 SVE system sampling influent concentration for total petroleum hydrocarbons as <br /> gasoline (TPHg) was unexpectedly low at 1.3 micrograms per liter (Ng/L). ENSR collected the <br /> next series of SVE system samples on May 19, 2005. The influent concentration for TPHg from <br /> that event was 0.73 pg/L; also much lower than previous influent sample results. ENSR later <br /> determined that the May 2005 SVE samples were inadvertently collected while open to ambient <br /> air thereby diluting the samples. ENSR field personnel collected a new series of samples as <br /> quickly as possible following the May 2005 laboratory sample results. The June 2005 SVE <br /> system samples were collected on June 17, 2005 and included a 33 ug/L influent concentration <br /> for TPHg which is consistent with other recent sampling events. <br /> May 2005 SVE Sample Data.doc <br /> ® Celebrating 35 Years of Excellence in Environmental Services <br /> 6 <br />