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VVZ1 I N C • 10.0 CONCLUSIONS AND RECOMMENDATIONS The conclusions of this Problem Assessment Report are as follows 1 Approximately 1 ,620 cubic yards of soil in the vadose zone were initially impacted by petroleum hydrocarbon contamination resulting from a single underground storage tank Areal extent of the vadose zone plume is approximately 1,750 square feet The majority of the demonstrated vertical extent of the soil petroleum hydrocarbon contamination in the vadose zone extends from seven to twelve feet bgs to approximately 17 to 18 feet bgs However, a single soil sample was found to contain trace concentrations of petroleum hydrocarbon contamination below the vadose zone at a depth of approximately 40 feet bgs 2 The groundwater petroleum hydrocarbon contamination plume has an areal extent of approximately 18,000 square feet and extends approximately 70 to 100 feet from the tank site Groundwater gradient over time has been predominantly to the southeast and to a lesser degree to the southwest A non-preferential gradient or "mound" has been present during approximately one-third of the monitoring events at the site Groundwater surface elevations have been rising since May 1992 3 Analyses of groundwater samples from the petroleum hydrocarbon plume have indicated that the plume has been stable over the past six or more years Potential for movement of the plume off the Sebastiani property is negligible Analytical laboratory reports indicate that petroleum hydrocarbon concentrations in groundwater samples collected from all of the monitoring wells have attenuated dramatically over time All monitoring wells have been reported to contain groundwater with hydrocarbon constituent concentrations below analytical method detection limits since May 1994, with the exception of monitoring well MW-1 Monitoring well MW-1 contained trace concentration of benzene (4 7 ,ugll) during the last monitoring event 19150011 005 42