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i <br /> Evaluation of Natural Attenuation: 7500 West Eleventh Street, Trac}; CA. Page 51 <br /> around Monitoring Well MWFP-1 and MW-7, where relatively <br /> hydrocarbons in <br /> high concentrations of components of fuel h <br /> g p Y <br /> groundwater produce reducing conditions, anaerobic natural <br /> attenuation processes are active. <br /> • Based on the observed decay of concentrations of analytes of <br /> concern with time, it is estimated that groundwater in the area <br /> around the source of the fuel hydrocarbon release will be free of <br /> BTEX compounds some eight years from the present time (March <br /> 2006) and that other components of gasoline and diesel will have <br /> been cleared from that area some time within the next 16 to 20 <br /> ... years. However, SJC believes that, while such statistical data are <br /> generally useful for assessing the practicality of passive site <br /> remediation by natural attenuation, it is unwise to place <br /> unwarranted reliance on their predictive precision. <br /> • With the exception of Monitoring Well MWFP-1, which is <br /> redundant, the groundwater-quality monitoring well array installed <br /> at the Navarra Site is statistically efficient as a system for <br /> monitoring the geometry of the primary plume of contaminated <br /> groundwater affecting the site and the changes in the <br /> concentrations of analytes of concern within it. <br /> ;';t': • A Monitoring and Remediation Optimization System (MAROS) <br /> analysis of the groundwater-quality monitoring program at the <br /> r' Navarra Site, using methodologies that are advocated by the US- <br /> - { EPA has been completed. The results include recommendations to <br /> reduce the frequency of sampling groundwater in the majority of <br /> wells at the Navarra Site to once per year or once every two years. <br /> There are exceptions in the cases of Monitoring Wells MW-3, <br /> MW-7 and MW-13, from which samples should continue to be <br /> recovered for analysis on a quarterly schedule until such time as <br /> the amplitude of oscillations of concentration of some analytes of <br /> concern about the trend lines that they have exhibited have <br /> declined. <br /> • Groundwater monitored for analytes of concern in six potable <br /> water supply wells located on and in the vicinity if the 7500 West <br /> Eleventh Street property remains free of detectible concentrations <br /> of analytes of concern, as it has since potable water monitoring <br /> was initiated at the site in April 2004. <br /> 4 <br /> sic <br /> t_ <br />