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Evaluation of Natural Attenuation_ 7500 West Eleventh Street,Tracy, CA. Page 26 concentrations of TPHg, TPHd, BTEX, and MTBE against distance down-gradient. As is shown on those Figures, the rapid reduction of concentrations of the analytes with =' distance down-gradient from well MW-7 results in a highly non-linear curve, even when i plotted in the logarithmic format. However, all the slopes of the linear regression line remain as would be expected - negative - and the RZ values are greatly improved I compared to those computed for the simple plots of analyte concentrations versus distance. The distributions of concentrations of analytes of concern with distance down-gradient ?- that are formally detailed above can be easily seen by an experienced engineer by simple inspection of the data in Table 2 without benefit of sophisticated log-normal plots. Such simple observations and are clear primary evidence of the progress of natural attenuation at the site. Although the relationship between concentrations of analytes of concern and distance in the orthogonal direction to the axis of the plume is not direct evidence of natural attenuation, to complete the discussion, Figures 18 and 19 are presented to show the distribution of BTEX concentrations versus distance along sections B-B' and C-C" (see Figure 3 for locations). As can be seen in those Figures, the concentrations of the analytes are at a peak where the axis of the plume crosses the sectional lines and fall away to east and west as the plume perimeter is approached. 7.1.2 Attenuation of Analytes of Concern with Time In addition to demonstration that concentrations of analytes of concern are reduced with distance from the source of the release, the evidence that those concentrations have a downward trend with time is also of primary importance for demonstrating that natural attenuation is in progress in the source area of a site. Figures 20-23 arep lots of the natural logarithm of, respectively, TPHg, TPHd, BTEX, and MTBE in groundwater with time at monitoring well MW-3. As is shown on Figure 2, Monitoring Well MW-3 is a few feet down-gradient from the pump island area of the former fueling station that was located on the 7500 West Eleventh Street site. That area was the principal location at which fuels were leaked into the subsurface. Groundwater in L- that well is representative of the conditions at the source of the plume of contaminants -a emanating from the site. Examination of Figures 20-23 reveals that while there is a strongly declining trend in the concentrations of analytes of concern in groundwater at Monitoring Well MW-3, there is considerable fluctuation of the concentrations around the trend line. As will be discussed in greater detail in section 8.1, those fluctuations occur due to the rise and fall of the groundwater table through the zone of soil which was most severely affected by the release of the fuels. That zone, known as the "smear zone," was generated as the groundwater table rose and fell and fuel hydrocarbons were adsorbed onto the soil articles dining the period following the release of fuels to the subsurface. However, as is p g p g demonstrated by the linear regression lines shown on the Figures, those fluctuations are sic