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' 11 August 1997 AGE-NC Project No 95-0173 ' Page 9 of 10 ' MW-7 are located down-gradient of the former UST area • Based on the apparent changes of ground water flow directions (northwest to northeast) and fluctuations in petroleum hydrocarbon concentrations, the dissolved petroleum-hydrocarbon plume appears to be in an "unstable" state • Four quarterly ground water events have been performed from wells MW-1 through MW-4 ' However, an overall trend of increasing/decreasing concentrations cannot be established from the analytical data • Field observations and laboratory data suggest that the impacted ground water beneath the ' site appears to have taken a relatively narrow, elongated oval shape (Figure 13-Appi oximate Extent of Dissolved Total Petroleum Hydrocarbons) Laterally, the dissolved plume extends ' beyond the property limits to the east • Concentrations of benzene detected in wells MW-1 through MW-4, MW-6 and MW-9 were above the State of California Maximum Contaminant Levels (MCLs) for drinking water of 1 0 ppb, toluene was above MCLs of 150 ppb in wells MW-1 through MW-4, ethylbenzene was above MCLs of 700 ppb in wells MW-2 through MW-4 and MW-6, xylenes were above • MCLs of 1,750 ppb in wells MW-2 through MW-4 and MW-6 • MTBE was not detected in ground water samples collected during this quarterly ground water sampling event However, due to matrix interference, the detection limits of several samples were higher than the State of California Action Limits of 35 ppb during this sampling round • Since the chromatogram of the hydrocarbons is not consistent with that of diesel fuel, the detected hydrocarbons are probably gasoline of origin I5.0. RECOMMENDATIONS IBased upon data collected during December 1996 and July 1997, AGE recommends the following • Upon regulatory approval, at least one in-situ air sparging (IAS) well should be installed at tl'1P CItP The I C pilot I OL 1V UUid Ve perlulliAIU to supplumr,1lt the J v L pllul JLUuy (performed on,14 July 1997) Field parameters and analytical results collected from both the I IAS pilot test and the SVE pilot study would assist in determining remedial feasibility at the site Upon completion of the IAS pilot test, a corrective action plan should be submitted to the PHS-EHD - Based on the likely gasoline origin of hydrocarbons, the requirement for TPH-d analysis should be waived and discontinued from future sampling events