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4.0 CONCLUSIONS AND RECOMMENDATIONS <br /> Based on the results of the confirmation soil sample analyses, the soil vapor extraction system <br /> appears to have been effective in remediating the impacted vadose zone soil near the former <br /> USTs Except for some remaining gasoline hydrocarbons in soil at a depth of approximately 10 feet <br /> bgs, the vadose zone was almost completely remediated This effectively removes the source of <br /> any continued impact to groundwater beneath the site <br /> Based on the results of the discrete groundwater sample analyses, the vertical extent of impacted <br /> groundwater is mostly delineated Concentrations of MTBE decline significantly with depth and are <br /> not present in the 125-foot groundwater-bearing interval Only 1,2-DCA was detected in the 125- <br /> foot groundwater sample, at a concentration of 0 5 µg11, which is equal to the laboratory detection <br /> limit <br /> At the water table, concentrations of gasoline hydrocarbons in groundwater continued to decline In <br /> VW-2, the closest well to the source, concentrations of TPH-as-gasoline, MTBE, TBA and 1,2-DCA <br /> have declined steadily over the past seven quarters <br /> In the deeper groundwater monitoring wells with screen intervals below the water table (MW-1 <br /> through MW-5), groundwater samples were collected for the first time since November 1997 In the <br /> three wells nearest the source (MW-1, MW-2 and MW-3), concentrations of gasoline constituent <br /> compounds decreased significantly since November 1997 In the downgradient deeper wells (MW- <br /> 4 and MW-5), MTBE was detected for the first time <br /> For the second consecutive quarter, MTBE and 1,2-DCA were detected in downgradient well MW- <br /> F As shown on Figure 7, the lateral extent of MTBE in groundwater is not defined in the <br /> downgradient direction <br /> Based on these conclusions, Ramage Environmental recommends the following course of <br /> environmental corrective action <br /> 1 Continue to monitor and sample groundwater from all nine monitoring wells and Irrigation Well <br /> #2 on a quarterly basis The next quarterly groundwater monitoring is scheduled for November <br /> 2000 The next groundwater monitoring report should be published during January 2001 <br /> 2 Following completion of the next quarterly groundwater monitoring report, meet with RWQCB <br /> staff to review the project status and determine a strategy to pursue regulatory closure <br /> 11 <br /> 1 <br />