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Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, CA. Page 39 f solids, electrical conductivity and chloride. The relationship between those parameters. and the concentration of BTEX compounds aloe section line C-C' which extends from`1 p g Monitoring Well MW-5 on the western periphery of the primary plume of affected groundwater to Monitoring Well MW-S, located beyond the eastern periphery of the plume, are shown on Figures 51-57, respectively. In each case, the selected parameters show a strong correlation (or inverse correlation) with the concentration of BTEX compounds in the groundwater. These observations show how those geochemical parameters are modified within the interior of the contaminant plume compared to j. conditions beyond its periphery. In each case, there are significant anomalies in the area :3 around Monitoring Wells MWFP-1 and MW-7. Those anomalies reflect the strongly- reducing(anaerobic) conditions prevailing in that area of the plume. The fact that the cited parameters show distinct variation along section lines that pass from the western periphery of the plume across its long axis and eastward to areas ' unaffected by the release of fuel hydrocarbons provides strong support to the primary evidentiary findings that are discussed in Sections 7.2.1 and 7.2.2 of this report that natural attenuation processes are vigorously active at the Navarra Site (American Society r: for Testing and Materials 2004). 7.3 Summary of Evidence of Active Bioremediation --i 1 Following are the principal findings developed from the field and laboratory program implemented at the Navarra Site in February and March 2005 that was designed to assess whether natural attenuation is reducing contaminant loads in the subsurface beneath the site. : � • Analysis of the time history of the concentrations of analytes of concern in groundwater and the distribution of those analytes with distance from the source of the release of fuel hydrocarbons provides convincing evidence that natural attenuation processes are actively decreasing the contaminant load in the groundwater. :: • The primary evidence for natural attenuation is robustly supported by secondary evidence developed from analyses of key geochemical parameters obtained from SJC's field and laboratory test program. • Based on the observed decay of concentrations of analytes of concern with time, it is estimated that groundwater in the area around the source of the fuel hydrocarbon release will be free of BTEX compounds some eight years from the present time (March 2006) and that other components of gasoline and diesel will have been cleared from that area ! some time within the next 16 to 20 years. However, it should be noted that SJC believes that it is unwise to place unwarranted reliance on such 1 time estimates. They do, however, provide useful guidance to assess the general order of magnitude of the time required for natural attenuation alone to remediate fully the plume of contaminated groundwater. Given SJC