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' benzene, and MTBE isoconcentration maps are presented as Figures 3, 4, and 5, respectively. 4.0 CONCLUSIONS The groundwater samples collected from monitoring well MW-1 contained 18 parts per billion (ppb) methyl-tertiary-butyl ether (MTBE). The groundwater samples collected from monitoring well MW-2 contained 8,600 ppb TPH-G, 490 ppb benzene, 300 ppb toluene, 310 ppb ethyl benzene, 1,300 ppb total xylenes, 5,200 ppb MTBE, 13 ppb tert-amyl- methyl-ether (TAME), and 490 ppb tert-butonal (TBA). Groundwater samples collected from monitoring well MW-4 contained 420 ppb TPH-G, 30 ppb benzene, 50 ppb ethyl benzene, 1.6 ppb total xylenes, 390 p p b MTBE, and 19 ppb TAME. Groundwater samples collected from monitoring well MW-5 contained 280 ppb TPH-G, 17 ppb MTBE and 13 ppb TBA. No other hydrocarbons were detected above laboratory reporting limits in a n y of the groundwater samples collected. There appears to be a decreasing trend in hydrocarbon and oxygenate concentrations detected in the groundwater samples collected from monitoring wells MW-1 and MW-5. Concentrations of all compounds detected in groundwater samples collected from all other monitoring wells are consistent with previous results. All the compounds analyzed in monitoring wells MW-3 and MW-6 were below laboratory detection limits. The only compound detected in monitoring well MW-1 was MTBE. 5.0 RECOMMENDATIONS ASE recommends the subject site remain on a quarterly sampling schedule. The next scheduled sampling is scheduled for October 2002. ASE will also be performing monthly groundwater sampling as indicated in the Remedial Action Plan (RAP) prepared by ASE dated March 26, 2002 to monitor the effectiveness of the ozone sparging system which was installed during the month of August 2002_ The ozone sparging system will continue to operate during the next quarter Singh's Unocal Service Station Quarterly Report — July 2002 Sampling -3-