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AXj <br /> August 11, 1992 Working To Restore Nature <br /> ARCO Service Station 2168, Stockton, California <br /> Results of analyses of water samples indicated TPHg and BTEX were detected in ground- <br /> water samples from the three wells, with the exception of ethylbenzene which was not <br /> detected in well MW-3. TPHg concentrations ranged from 14,000 to 65,000 parts per billion <br /> (ppb), and benzene concentrations ranged from 2,100 to 13,000 ppb. TEH (quantified <br /> against a diesel fuel standard) and TOG were detected in the sample from well MW-2 at <br /> concentrations of 1,900 and 5,400 ppb, respectively Plate 24, Distribution of Dissolved <br /> Gasoline Hydrocarbons in Ground Water Map, shows the concentration of TPHg and <br /> benzene for each well. <br /> Purgeable halocarbons were below laboratory detection limits,except for 1,2 dichloroethane <br /> which was detected at a concentration of 19 ppb in monitoring well MW-2. Fecal and total <br /> coliform bacteria were not detected in the sample collected from well MW-3. <br /> Evaluation of Ground-Water__ radient <br /> i <br /> The three monitoring wells were surveyed relative to mean sea level as described in the <br /> attached Field Procedures. The ground-water gradient on May 11, 1992, was 0 003, with a <br /> flow direction to the east, as shown on the Potentiometric Map (Plate 25) The depth to <br /> water in the wells ranged from approximately 39 0 to 40 7 feet bsg, ground water elevation <br /> 1 data is shown on Table 6. <br /> WORK IN PROGRESS <br /> RESNA completed a vapor-extraction test performed on July 14, 1992. Results of this test <br /> will be discussed in the next quarterly status report <br /> CONCLUSIONS <br /> On the basis of this and our previous investigations we conclude the following. <br /> 1 <br /> Trace to low concentrations of TPHg are present in soil near the underground <br /> gasoline storage tanks; with concentrations decreasing with depth. The distribution <br /> of TPHg indicates that detectable gasoline hydrocarbons are limited to soil 25 to 26 5 <br /> feet bsg Further investigation in this area is not warranted. <br /> r TPHg exists in soil west of the dispenser islands at a depth of 31 feet bsg. Because <br /> the highest concentrations of TPHg were detected in borings B-9 and B-10, soil <br /> beneath the dispenser islands appear to contain gasoline hydrocarbons <br /> r � <br /> 05 <br /> 31009-4 5 <br /> r , <br />