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r, <br />�a <br /> i <br /> Report of Soil Vapor Survey January 12, 1989 <br /> f UNOCAL Station No. 6981, Stockton, California AGS 38032-2V <br /> Vapor_Survey Equipment and Sampling• Procedure <br /> Equipment used to sample soil vapor included a hardened steel <br /> drive-tip, a downhole sampling assembly with vacuum vials, steel <br /> probe rods with an outside diameter of 1.75 inches, and a drivira <br /> unit. The probe rod and drive-tip were hydraulically pushed to a <br /> predetermined depth, at which point the drive-tip was opened. <br /> The sampling assembly was then lowered through the hollow center <br /> of the probe rod to collect the vapor sample. Contamination by <br /> atmospheric gas was thus eliminated., giving a reliable and <br /> reproducible reading of the concentration of the hydrocarbon <br /> vapor in the soil at the sample depth. Because each sample vial <br /> is the same size (i.e. , constant volume of gas) and is evacuated <br /> to the same degree, the number of variables associated with vapor <br /> ® sampling are reduced. Reducing variation in the vapor sampling <br /> procedure provides consistency in sampling at different depths <br /> and locations. <br /> Selection of Sample Locations and Depths <br /> Initial sample locations were based on the locations of the <br /> underground storage tanks, as well as results of previous <br /> investigations. Additional sampling locations were distributed <br /> 4 <br /> .4PPIled 1GOOSySAe77S <br />