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' 4 4 Groundwater Contamination <br /> Diesel was detected in groundwater samples at concentrations of 260 µg/f, (parts per <br /> billion), 390 ppb and 140 ppb in monitoring wells MW-1, MW-2 and MW-3, respectively <br /> Gasoline was detected only in the sample from MW-3, at 53 ppb <br />' No benzene, ethylbenzene or xylenes were detected in the water samples Toluene was <br /> detected in the samples from MW-1 and MW-3 at concentrations of 199 ppb and 2 4 ppb, <br /> respectively The analytical results of the water samples are listed in Table 4 The Iaboratory <br />' report and chain-of-custody are included in Appendix C <br />' TABLE 4 <br /> ANALYTICAL RESULTS of WATER SAMPLES <br /> µglL <br />' November 14, 1992 <br /> Sample TPH as TPH as Ethyl- <br /> ID Gasoline Diesel Benzene Toluene benzene Xylene <br /> MW-1 ND 260 ND 199 ND ND <br /> tMW-2 ND 390 ND ND ND ND <br /> MW-3 53 140 ND 24 ND ND <br /> µ = parts per Wiwi PP <br /> ND Not Detected <br />' 5.0 CONCLUSIONS <br /> 5.1 Soil Contamination <br /> During this phase of investigation, field (PID screening) and laboratory evidence indicates <br /> that soil contaminated with low concentrations of petroleum hydrocarbons remains at the <br /> site in a relatively narrow zone at depths between 45 to 55 feet (Figure 4) No <br /> contamination was detected above or below this zone <br />' The contaminated soil at depths between 45 and 55 feet appears to coincide with the <br /> seasonal high groundwater depth in 1986 (pre-drought conditions) Contaminants in the <br /> water were probably adsorbed to soil particles as the water level fluctuated, creating a smear <br /> zone When groundwater receded from 45 feet to the current 80 feet, over a relatively short <br /> period of time (5 years), some petroleum hydrocarbons remained m the smear zone <br />' e zone of residual soil contamination is generally less than 10 feet thick The zone <br /> T'h g Y <br /> appears to be thickest in the area lust down-gradient from the tank cavity (MW-1), and <br />' thins up-gradient and down-gradient toward MW-3 and MW-2 (Figure 4) It extends from <br /> B10 at the south to beyond MW-1 on the north, and approximately from MW-3 on the west <br /> 1 0 to beyond NiW-2 on the east This represents a large volume of contaminated soil, and <br /> 0_1n8-1 Mda« 1n. 10 <br /> e..d<C.OZzs <br /> I <br />