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Z1 INC <br /> Toluene concentrations were reported to range from below analytical method detection <br /> limits to 3 7 µg11 and had an estimated average concentration of 1 2 µg11 Xylene <br /> concentrations were reported to range from below analytical method detection to 37 µg11 <br /> and had an estimated average concentration of 13 5 µg11 Total petroleum hydrocarbon as <br /> gasoline concentrations were reported to range from below analytical method detection <br /> limits to 100 4g11 and had an estimated concentration average of 60 0 µg11 <br /> The groundwater treatment system continued in operation from September 1992 until <br /> approximately June 1, 1994 when the system was shut down indefinitely Immediately prior <br /> to shutdown the system had developed mechanical problems with a series of valves <br /> County Public Health was notified on June 14, 1994 of Sebastiani Vineyard's intention to <br /> Keep the system shut-off for approximately three months in order to evaluate the effect of <br /> nonoperation Groundwater monitoring well petroleum hydrocarbon constituent <br /> components in all monitoring wells were reported at concentrations below analytical method <br /> detection limits or at trace concentrations during the quarterly monitoring periods after <br /> cessation of the groundwater treatment system No significant difference was found in <br /> petroleum hydrocarbon concentrations in groundwater samples collected from the <br /> monitoring wells between the pre- and post-shutdown of the groundwater treatment system <br /> Accordingly, Sebastiani elected to not restart the groundwater treatment system <br /> 83 Estimated Performance of Remedial Action <br /> Estimated System Performance <br /> An estimated volume of 1,620 cubic yards of petroleum hydrocarbon-contaminated soil is <br /> still in place within the vadose zone immediately below and adjacent to the former <br /> underground storage tank site <br /> Evidence that active biodegradation is continuing to occur in the vadose and shallow <br /> groundwater zones is shown in Exhibit 12 Exhibit 12 depicts dissolved oxygen <br /> concentrations in groundwater samples collected from the monitoring wells during August <br /> 19150011005 36 <br />