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3. ENSR presented data and evaluations suggesting that the source of <br /> petroleum hydrocarbon impacts to groundwater in wells U-20, U-21, and <br /> Hydropunch HP-1 may not be the former Unocal service station. <br /> Instead, the source may be the former Chet's Auto Repair Shop or the <br /> "Vintage Car Wash Site". This suggestion was based on the historical <br /> trends of detected concentrations of benzene and TPHg in offsite wells <br /> U-11, U-12, and U-13, which are located immediately upgradient of U-20, <br /> and U-21; as well as concentrations and trends for onsite and <br /> immediately downgradient wells U-3, U-8, U-9, U-10, NP-1-22, NP-1-65, <br /> NP-1-111, NP-2-22, NP-2-60, and NP-2-116. <br /> The EHD believes that Chet's Auto Repair Shop is near closure and that only <br /> insignificant concentrations of petroleum hydrocarbons had been detected in <br /> soil and groundwater beneath the facility. In addition, the EHD indicated that <br /> the location of the former USTs for the facility were near the intersection of <br /> American Street and Miner Avenue. Therefore, the EHD does not believe the <br /> Chet's Auto Repair Shop is the source of petroleum hydrocarbons detected in <br /> groundwater in samples collected from U-20 and U-21. <br /> The EHD noted apparent differences in the presence/absence of fuel <br /> oxygenates in groundwater samples collected from wells U20, U-21, <br /> hydropunch samples HP-1 and wells screened at similar intervals upgradient <br /> of wells U-20 and U-21. The EHD suggested that Chevron and ENSR further <br /> evaluate chemical profiles of groundwater samples collected from selected <br /> wells to assess the source of petroleum hydrocarbon constituents is wells U- <br /> 20 and U-21. <br /> 4. Chevron and ENSR believe that an evaluation of historical trends in <br /> groundwater concentrations of benzene and TPHg before beginning <br /> ozone sparge, during sparging, and after sparging in selected wells <br /> from the shallow, intermediate, and deep saturated zones indicate that <br /> natural attenuation of petroleum hydrocarbons is occurring at the site <br /> and is successfully decreasing concentrations of these constituents in <br /> groundwater. Historical trends were evaluated using: <br /> • Excel Trend Analyses <br /> • Mann-Kendall Analyses <br /> In addition, mass-balance calculations for 2000 (beginning of ozone <br /> sparge) and second quarter 2007 (post ozone sparge) were presented, <br /> which indicate that the remaining mass of benzene and TPHg has <br /> decreased significantly during that period. <br /> Additional mass-balance calculations are being prepared for the years <br /> 2005 and 2006 to evaluate mass balance trends after termination of <br /> ozone sparging to the present. <br /> The EHD's position regarding the effectiveness of natural attenuation is that it <br /> must be proven using trend plots and degradation analysis that show <br />