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• <br />• <br />Former Chevron service station #9-5775, 301 West Kettlemaa Lane, Lodi, California <br />4.2 Chemical Concentrations vs. Time and Distance <br />4.2.1 Chemical Concentrations vs. Time <br />Page 6 <br />January 11, 2002 <br />Plots of Chemical Concentration vs. Time can be found on Figures G-1 through G-22 within <br />Attachment G. Concentrations of TPHg and benzene are shown to decrease over time in both <br />on-site and off-site wells. Concentrations of MtBE were found to decrease in on-site and off- <br />site wells. <br />4.2.2 Chemical Concentrations vs. Distance <br />Concentrations of MtBE are shown to decrease in A -Zone wells in Figures H-1 through H-4 <br />from the source to the southeast, southwest, west, and south, respectively (Attachment H). <br />Concentrations of MtBE are below laboratory detection Iimits to the south and southwest of <br />the site (Figures H-5 and H-6). <br />5.0 CONCLUSIONS AND RECOMMENDATIONS <br />• With the exception of EX -1, concentrations of hydrocarbons both on and off-site <br />continue to decline. <br />• Concentrations of hydrocarbons off-site are below both primary and secondary <br />maximum contaminant levels (MCLs). <br />• The GWE and SVE systems continue to effectively remove hydrocarbon mass <br />from the subsurface. <br />• No additional assessment activities are currently planned. <br />• SECOR recommends the continued operation of the GWE and SVE systems and <br />the continuation of quarterly monitoring and sampling during the 1" quarter 2002. <br />I:1Chevron1957751Periodic MonitoringlQuarterly Progress Reportsl4thQuarter 200114th Quarter -2001 Progress Report.doc <br />