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2101 Webster Street <br /> 12th Floor <br /> Oakland, CA 94612 <br /> [51 O] 663-4100 • FAX (51 O) 663-4141 GEOMATRIX <br /> April 15, 2003 <br /> Project 7243.011.0 <br /> Mr. M. Scott Mansholt <br /> Chevron Environmental Management Company <br /> 6001 Bollinger Canyon Road, Room K2050 <br /> San Ramon, CA 94583-2324 <br /> Subject: April 6, 2003 Observation of Sheen and Corrective Measures Taken <br /> Tracy Westside Channel Outfall System <br /> Tracy, California <br /> Dear Mr. Mansholt: <br /> On behalf of Chevron Environmental Management Company(ChevronTexaco), Geomatrix <br /> Consultants, hie., (Geomatrix), has prepared this letter to describe the observation of a sheen <br /> on surface water on April 6, 2003 and the corrective measures taken by Geomatrix. The sheen <br /> appeared to be associated with a groundwater de-watering system that has been installed as <br /> part of a storm drain construction project along an approximately 1'/4 mile section of Byron <br /> Road,between Belconte Drive and Highway 205 (the Westside Channel Outfall System). <br /> Many of the de-watering wells for the construction project have been installed within <br /> petroleum hydrocarbon-affected soil. Treatment systems, consisting of settling tanks (to <br /> remove sediment and separate-phase petroleum) and granular activated carbon(GAC; to <br /> remove dissolved constituents in groundwater),have been constructed to treat extracted <br /> groundwater prior to discharge. One of the treatment systems discharges to a detention basin <br /> adjacent to the east side of Belconte Drive. At the point of discharge, the detention basin is <br /> clear and open in an area approximately 6 feet by 6 feet. Beyond this open area, the detention <br /> basin is densely overgrown with tules (cattails) for approximately 300 feet south. The <br /> detention basin is again clear and open south of the rules. The de-watering system that <br /> discharges to this detention basin was started on the evening of March 29, 2003. Samples of <br /> GAC influent and effluent were collected on March 31, April 1, April 2, and April 4, 2003; all <br /> samples were non-detect for petroleum constituents. <br /> During an inspection at approximately 7:00 a.m. on April 6, 2003, Geomatrix staff observed <br /> separate-phase petroleum in one of the settling tanks for the treatment system that discharges <br /> into the Belconte detention basin. At approximately 7:15 a.m., Geomatrix staff checked the <br /> Belconte detention basin and observed a sheen covering approximately 10 square feet in the <br /> open area near the discharge point. The sheen appeared to extend into the area that is densely <br /> overgrown with tules; however, it was very difficult to assess how far the sheen may have <br /> extended into this area. No sheen was observed in the open area south of the tules. <br /> Upon observation of the sheen, Geomatrix staff shut down all of the wells that pumped to the <br /> treatment system and placed sorbent pads on the sheen in the open area of the detention basin <br /> Geomatrix Consultants, Inc. <br /> Engineers, Geologists, and Environmental Scientists <br />