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• 4.0 CONCLUSIONS • The groundwater sample collected from monitoring well MW- 1 contained 08 ppb benzene, 88 ppb MTBE, 11 ppb tert-butonal (TBA), and 150 ppb TPH-G a The groundwater samples collected from monitoring well MW-2 contained 2 2 ppb benzene, 22 ppb ethyl benzene, 2 9 ppb total xylenes, 280 ppb MTBE, 1 2 ppb tert-amyl methyl ether (TAME), 94 ppb TBA, and 340 ppb TPH-G The groundwater samples collected from monitoring well MW-4 contained 2 6 ppb MTBE • The groundwater samples collected fro m monitoring well MW-5 contained 14 ppb MTBE, 31 ppb TBA, and 74 ppb TPH-G • The groundwater samples collected from monitoring well MW-6 contained 0 5 ppb ethyl benzene and 0 75 ppb total xylenes • No other hydrocarbons were detected above the laboratory method reporting limits in any of the groundwater samples collected The MTBE concentrations in the groundwater samples collected from monitoring wells MW-1, MW-2, and MW-5 exceeded the California Department of Health Services (DHS) Maximum Contaminant Levels (MCLS) for drinking water No other compound concentrations exceeded MCLS Hydrocarbon concentrations remained relatively stable this quarter, with slight increases observed in monitoring wells MW-1, MW-5 and MW-6, and more notable increases observed in MW-2 There was a slight decrease in the MTBE concentration in monitoring well MW-4, which is at an historic low 5.0 RECOMMENDATIONS ASE recommends continued quarterly groundwater monitoring The next sampling is scheduled for July 2004 ASE also recommends continued operation of the ozone-spargtng system Additionally, ozone-sparge well OS-2, which is located in the former UST area, developed a subsurface seal leak during the previous quarter Very low hydrocarbon concentrations at the time did not warrant replacement of the well However, with concentrations increasing in monitoring well MW-2, which is generally downgradient of OS-2, ASE now recommends replacement and reactivation of the well II Singh's Unocal Service Station Quarterly Report — April 2004 Sampling -3-