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Chevron <br /> 40 Chevron Corporation <br /> 576 Standard Avenue, Richmond, CA 94802-0627 co <br /> June 8, 1988 <br /> O <br /> Addition to Tracy <br /> Bioventing Work Plan <br /> Mr. David L. Brent <br /> California Regional Water <br /> Quality Control Board <br /> Central Valley Region <br /> 3443 Rontier Road <br /> Sacramento, California 95827-3098 <br /> Dear Mr . Brent: <br /> Chevron is currently running a bioventing field test in the Alden <br /> Park area of Tracy, California. This field test is described in <br /> the work plan dated December 18, 1987, and is designed to test <br /> the feasibility of venting at Tracy and its effectiveness in <br /> removing oil from soil at the site. The purpose of this letter <br /> is to describe an additional test we intend to run but did not <br /> address in the work plan. <br /> One of the key parameters we need to define during the venting <br /> test is the area of influence or "reach" of the venting well. A <br /> tracer study is a safe and effective way to measure reach. It <br /> can be used to back up pressure profile reach measurements and <br /> possibly act as the primary measurement. We intend to run a <br /> tracer test using sulfur hexafluoride (SF6) as the tracer. <br /> This procedure can be performed safely. The attached Material <br /> Safety Data Sheet shows that SF6 is a colorless, odorless, and <br /> nontoxic gas. The SF6 is contained in a small compressed gas <br /> cylinder. The gas from the cylinder will be injected into one of <br /> the soil probes that we have installed at various distances from <br /> the well, and the flow will be controlled with a needle valve. <br /> After a few days of injection, the SF6 cylinder will be removed <br /> from the site. The compressed gas cylinder will not be left <br /> unattended when in use. It will be secured and capped inside of <br /> the locked test facility when not in use. <br /> Sulfur hexafluoride is an excellent tracer because it can be <br /> detected at extremely low levels ( 1 part per trillion) with a <br /> portable gas chromatograph equipped with an electron capture <br /> detector . After injecting the SF6 into the soil probe, gas sam- <br /> ples from the venting well will be analyzed with the gas <br />