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06 April 1998 AGE-NC Project No 96-0229 Page 4 of 5 E ground water is located near MW-1 and MW-2 The approximate known extent of dissolved TPH-g is depicted in Figure 4 The dissolved plume is undefined to the northeast, north, northwest, east and southeast • Benzene was detected in samples collected from wells MW-1 through MW-6, concentrations reported in MW-1 through MW-3 and MW-6 were above the State of California Maximum Contaminant Level (MCL) for drinking water of 1 0 ppb The greatf st concentrations of dissolved benzene is located near the area of MW-1 and MW-2 The approximate known extent of dissolved benzene is depicted in Figure 5 • Concentrations of toluene were above the MCL of 150 ppb in wells M W-1 through MW-3 and in MW-6, concentrations of ethylbenzene were above its MCL of 700 ppb in wells MW- 1 through MW-3,concentrations of xylenes were above the MCL of 1,750 ppb in wells MW- 1 and MW-2 • Utilizing EPA methods 8020 and 8260, concentrations of MTBE were detected above the laboratory reporting Iimits in samples from wells MW-I through MW-6 The approximate known extent of dissolved MTBE is depicted in Figure 6 • Two quarterly ground water monitoring and sampling events have been performed from wells MW-I through MW-6 From November 1997 to February 1998, substantial decreased concentrations were reported in TPH-g, BTE&X and MTBE from wells MW-3, MW-4 and MW-6 Furthermore, ground water flow direction in the area of wells MW-1 through MW-6 ' shifted from a northeast flow direction in November 1997 to a northvA est flow direction in February 1998 Based on available data,the decrease in reported concentrations from MW-3 ' and MW-6 in addition to the shift in ground water flow direction could indicate a shift in the dissolved petroleum hydrocarbon plume from northeast to northwest ' During ground water monitoring activities, the lowest concentrations of dissolved oxygen were measured from MW-2,MW-3 and MW-6(dissolved petroleum hydrocarbon core-area), which is suggestive of aerobic biodegradation of hydrocarbons occurring in the core-area ' (Table 2) Based upon dissolved oxygen measurements collected from MW-1, it appears that aerobic biodegradation is limited in that screened area (48 to 53 feet bsg) 5.0 RECOMMENDATIONS ' Based upon data collected during February 1998, AGE recommends • Preparation of a work plan addendum should be submitted for installation of additional ground water monitoring wells east, southeast, north, northwest and northeast of the site in ' Advanced GeoEnviron mental.Inc 1