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I + - ' 18 December 1997 AGE-NC Project No 95-0173 ' Page 8 of 10 ' 8260 The laboratory reports (Castle Analytical Laboratory Reference No 1316) and chain-of- custody forms are included in Appendix D The soil sample analysis are summarized to Table No 4 I 4.0. SUMMARY AND CONCLUSIONS ' Based on the results of ground water monitoring and sampling activities and IAS well installations performed in October and November 1997, AGE concludes I0 Review of depth to ground water maps dating back to 1971 and published by the San Joaquin ' County Flood Control and Water Conservation Distnct suggest that historical ground water depths for the site area to have been as low as 70 feet bsg • General ground water flow direction at the site was calculated to be toward the northeast at I the site Ground water flow direction has been reported as relatively northeast since April 1997 �. • Laboratory data continues to indicate that the greatest concentrations of dissolved petroleum hydrocarbons are located near VW-1 and MW3 Laboratory data suggest that the impacted ground water beneath the site appears to have taken a relatively elongated oval shape The I dissolved plume is relatively unknown to the east, southeast and northwest The approximate extent of dissolved petroleum Hydrocarbons is depicted in Figure 8 -Approximate Extent of Dissolved Petroleum Hydrocarbons, while the extent of dissolved benzene at the site is depicted in Figure 9 -Approximate Extent of Dissolved Benzene • During ground water monitoring activities, the lowest concentrations of dissolved oxygen were measured from MW3, MW4 and VW2 (dissolved petroleum hydrocarbon core-area), which is suggestive of aerobic biodegradation occurring in the core-area (Table 2) • Concentrations of benzene detected in MW 1 through MW4, MW6, MW9, AW 1 (Interval B)and AW2 (Interval B) were above the State of California Maximum Contaminant Levels (MCLs) for drinking water of 1 0 ppb • Concentrations of toluene, ethylbenzene and xylenes detected from MW1 through MW4, V VVI1, A VV 1 (Intel va, v) and A YY 2 linter V41 Jam) eXl.eedGd the Sldtc of 1�_.dll Ulllla lv1CLJ of 150 ppb, 700 ppb and 1,750 ppb, respectively C Utilizing EPA method 8260,concentrations of MTBE were detected at higher levels than the 35 ,ug/l state action level in ground water samples collected from MW3, MW4, VW1 and AW2 (Interval B) MTBE was not detected from any well locations utilizing EPA method sola Advanced GeoEnvironmentat,Inc