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