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AM <br /> November 12, 1992 Working To Restore Nature <br /> ARCO Station No 6100, Tracy, Califorrua <br /> in boring B-14 were 760 ppm (20 feet bgs), 49 ppm (35 feet bgs), and below detection limits <br /> (65 and 80 feet bgs) In boring B-24 the concentrations of gasoline hydrocarbons (TPHg) <br /> detected were 14 ppm (16 5 feet bgs), 610 ppm (21.5 feet bgs), and 200 ppm (315 feet bgs) <br /> Residual gasoline hydrocarbons were not detected in soil samples 10, 25, and 35 feet bgs at <br /> boring B-16, which is 50 feet west of borings B-14 and B-24 and 70 feet northwest of the <br /> water-supply well At boring B-19, which is approximately 60 feet south of borings B-14 and <br /> B-24, and approximately 10 feet southeast of the water-supply well, only trace concentrations <br /> of toluene and total xylene isomers (0 0094 and 0 0068 ppm, respectively) were detected at <br /> r 10 feet bgs and trace concentrations of total xylene isomers (0 0053 ppm) were detected at <br /> 30 feet bgs. Gasoline hydrocarbons were not detected in a sample collected at 45 feet bgs <br /> from boring B-19 <br /> CONCLUSIONS <br /> i <br /> Based on results of this investigation, RESNA concludes that the lateral extent of gasoline <br /> hydrocarbons in soil beneath the site has been delineated north, northeast, and east of the <br /> former and new tank cavities and southwest, west, and northwest of the dispenser islands, <br /> however, the lateral extent has not been delineated offsite beyond boring B-25 Plate 19 <br /> shows the current known lateral extent of residual gasoline hydrocarbons in soil beneath the <br /> site <br /> Additional vapor-extraction wells VW-11, VW-12 and VW-14 have been constructed to <br /> remediate soil affected by gasoline hydrocarbons from surface grade to 30 feet below surface <br /> grade near portions of the dispenser islands and product pipelines and from approximately <br /> 10 feet to 30 feet below surface grade near borings B-7, B-11 and B-14. <br /> The depth to ground water beneath the site is not currently known Ground water in the <br /> water-supply well was measured to be 100 feet bgs in 1969, however, ground water was not <br /> encountered at 115 feet bgs during 1991 Artesian conditions exist nearby, and the water- <br /> supply well was constructed with perforated zones below 396 feet bgs, therefore, ground <br /> water appears to be deeper than 115 bgs and under confined conditions However, it <br /> appears that the upper portion of the gravel pack is within the depth interval that residual <br /> gasoline hydrocarbons were detected beneath the site The gravel pack for the water-supply <br /> well extends from the bottom of the well to 35 feet bgs However, only trace concentrations <br /> 1 of toluene and total xylene isomers are in soil near the water-supply well, and appear to be <br /> restricted to depths above the top of the gravel pack Because high concentrations of <br /> gasoline hydrocarbons are not present near the water-supply well and water is greater than <br /> 115 feet bgs, further evaluation of ground water quality beneath the site does not appear <br /> warranted <br /> 1� <br /> W23ccar <br /> 30041-3 4 <br /> 1 <br />