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i f 22 June 1998 AGE-NC Project No 95-0173 Page 4 of 5 VW-1, AW-1 and AW-2) was estimated to be toward the southeast (Figure 3) • The site gradient was calculated at 0 0035 Rift, or approximately 19 feet per mile (Figure 3) • Due to Increasing ground water elevation at the site, the upper surface of the ground water zone is 2 feet to 2 5 feet above the slotted screened Intervals in wells MW-1 through MW-4, AW-1B and AW-2B, therefore, samples collected from these wells may not be completely Irepresentative of dissolved petroleum hydrocarbon concentrations in shallow ground water • Laboratory data Indicates that the greatest concentrations of TPH-g Impacted ground water are located near AW-1, AW-2, VW-1 and MW-3 The approximate known extent of dissolved TPH-g is depicted in Figure 4 The dissolved plume is poorly defined toward the northeast, east and southeast • Benzene was detected in samples collected from wells MW-1 through MW-4, MW-6, MW- 9,VW-I and AW-1B at concentrations above the State of California Maximum Contaminant Level (MCL) for drinking water of 1 0 ppb The area with the highest concentrations of dissolved benzene is located near the area of VW-1, AW-2 and MW-3 The approximate known extent of dissolved benzene Is depicted in Figure 5 lie • Concentrations of toluene were above the MCL of 150 ppb in wells MW-2 through MW-4, VW-I and AW-I B • Concentrations of ethylbenzene were above Its MCL of 700 ppb in wells MW-2 through MW-4 and VW-1 • Concentrations of xylenes were above the MCL of 1,750 ppb in wells MW-2 through MW-4, VW-1 and AW-113 • Utilizing EPA Method 8020, concentrations of MTBE were detected dust above the laboratory reporting limits in the sample from well AW-2B, analysis of the sample by EPA Method 8260 could not confirm the presence of MTBE in the sample MTBE was not detected in any other samples I • From October 1997 to April 1998, decreasing concentrations of petroleum hydrocarbons have been reported from AW-213 • Diir1ilg grniind crater monitoring activit!es� the to-7-st concentrations of dissol-ed oxyc ZD were measured from MW-2, MW-3, VW-1, AW-1 and AW-2 (dissolved petroleum hydrocarbon core-area), which is suggestive of aerobic biodegradation of hydrocarbons occurring in the core-area (Table 2) 0 Advanced CeoEni ironmental,Inc I I