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Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, CA. Page 27 <br /> r secondary to the primary trend of the data,which shows a decline in the concentrations of <br /> el, <br /> analytes of concern over the period of years since the monitoring well at the source of the <br /> plume was first installed. <br /> As can be seen on Figures 20-23 and by the negative components of the linear regression <br /> ! lines drawn through the data points, there is a strongly developed downward trend of <br /> concentrations with time. This trend is clear evidence of processes of natural attenuation <br /> being active in the groundwater beneath the site. <br /> �3 Figures 24-26 are plots of the natural logarithm of,respectively, TPHg, TPHd, and BTEX <br /> in groundwater with time at Monitoring Well MW-12. As shown on Figure 3, MW-12 is <br /> located at the source of the secondary plume of affected groundwater at the Navarra site. <br /> As is the case for the time history of analytes of concern in samples recovered from MW- <br /> 3, the concentrations of TPHg and TPHd in MW-12 have fluctuated around a trend line <br /> . . that has a strongly developed negative slope with time. The equivalent plot for the <br /> concentrations of BTEX with time, shown on Figure 26, does not exhibit a declining <br /> trend in the concentrations of BTEX with time. This is because, as can be seen in Table 2, <br /> concentrations of BTEX in the well were frequently undetectable and when present, were <br /> in very low concentrations so that a mathematical regression through the data does not <br /> produce a meaningful indication of a trend. A plot of the concentration of MTBE versus <br /> j time at MW-12 was not produced because that analyte has never been detected from <br /> samples recovered from that well. However, despite the scattered occurrence of very low <br /> concentrations of BTEX in the well, the strongly declining trend of the concentrations of <br /> TPHg and TPHd in MW-12 do demonstrate that natural attenuation processes are very <br /> well developed in the secondary plume at the site, as well as in the primary plume. <br /> a7.1.2.1 Time Required for Complete Remediation of Contaminants in Source Area <br /> Figures 27-30 show extrapolations of the trend lines through the same plots of the natural <br /> log of analytes of TPHg, TPHd, BTEX, and MTBE versus time in samples recovered <br /> from Monitoring Well MW-3, as are shown on Figures 20-23. The extrapolated trend <br /> lines cross the time axes where the axes of the log of analytes of concern have a value of <br /> ! 1.0, i.e., where the concentrations of the analytes are zero. The extrapolation shown on <br /> Figure 29, where the analytes that are of greatest concern due to their human <br /> carcinogenity or toxicity (i.e., BTEX), predicts that those compounds will have been <br /> reduced to zero concentration approximately fourteen years from the time that <br /> Monitoring Well MW-3 was first installed at the site in May 2000. In other words, that <br /> prediction suggests that the source area of the primary plume will be free of any BTEX <br /> t. _l compounds in groundwater some eight years from the present time (March 2006). <br /> However, SJC recommends that such predictions should not be over-interpreted to <br /> '. indicate that such complete remediation will, in fact, occur at the exact time predicted. <br /> This is because the prediction relies upon solely mathematical extrapolations based on a <br /> simple linear regression. The processes of natural attenuation of analytes of concern are, <br /> . in fact, highly complex and such predictions can be interpreted only as order of <br /> magnitude indicators of the time required for complete remediation of components of fuel <br /> sic <br /> ,. <br />