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' l <br /> HUM <br /> 0 <br /> Report of soil Vapor Survey January 12, 1989 <br /> UNOCAL Station No. 6981, Stockton, California ' AGS 38032-2V <br /> around the site and surrounding vicinity, to delineate possible <br /> offsite migration, especially those areas in the inferred <br /> downgradient direction of ground-water flow from the underground <br /> storage tanks and service islands. The inference of gradient was <br /> based on local topography. The soil probe locations are shown on <br /> Plate P-2. The maximum sample depth was based primarily on the . <br /> depth to ground saturation because little or no vapor transport <br /> occurs below this depth. The saturated zone varies in thickness <br /> depending on soil type but is generally assumed to extend upward . <br /> from the water table a few feet. The sampling depth at this site <br /> was based on the maximum depth the sampler could. be pushed. <br /> often, the subsurface geologic conditions prevent panetration to <br /> the desired depth. The samples were retrieved from an average <br /> depth of 17 feet, with some minor variation due to the <br /> aforementioned variations in the ability to penetrate subsurface <br /> sediments. <br /> After the samples were collected, the probe holes were backfilled <br /> with bentonite pellets to within a,few inches of the ground <br /> surface and then topped off with asphalt to the surface. A Wild <br /> ® NA-24 automatic level was used to survey probe-hole locations <br /> relative to other facilities at the site. <br /> b <br /> 5 <br /> .4pwlied L�,eta,Syst�lal� — - <br />