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Evaluation of Natural attenuation: 7500 West Eleventh Street, Tracy, Ca. Page 25 As. is shown on Figure 10, the distribution of TPHg compared with distance down adient had a marked peak where the concentration is as high as 15,000 L which peak occurs at Monitoring Well MW-7. Thereafter, the concentration of TPHg falls rapidly to 86 µg/L at well MW-14 and, at the down-gradient distance of well MW-16, it has fallen to an undetectably low concentration. (Note: 4n the plot shown in Figure 10, undetectable concentrations of the analyte of concern are shown as 50 µg/L, which is the � method detection limit(NML) of the analytic procedure used by the Iaboratory.) Up the groundwater gradient from MW-7, the concentrations of TPHg are also lower than f the 15,000 µg/L that was detected in the groundwater in that well. That distribution reflects the extensive groundwater remediation that was achieved in the source area of the -' fuels released that was achieved by pumping LNAPL at the time the tanks were exhumed from the 7500 West Eleventh Street site. As is demonstrated by the distribution of TPHg that is shown in the figure, the effect of that remediation was to reduce the concentration of TPHg at the location of well MW-3 to 2,300 µg/L and to 4,000 µg/L at well MW-13. Up-gradient of well MW-3 (located at the zero distance coordinate on Figure 10), the concentration of TPHg in groundwater falls away and is non-detectable in well MWA, r='i which was installed so as to be up-gradient from the southern boundary of the primary plume of affected groundwater. Due to the distribution of the concentrations of TPHg in the down-gradient direction, with its high peak at monitoring well MW-7, the linear regression line, shown on Figure 10, has a significantly negative slope. However, the regression line's low fraction of total variance of TPHg between the regression line and the data points is indicated by the low value of 0.1182, computed for the "goodness of fit"parameter, R2. The down-gradient distribution of the concentrations of TPHd and BTEX, shown on Figures 11 and 12, is similar in characteristic to that shown on Figure 10 for TPHg for distance down-gradient. The distribution of MTBE, as it is graphed on Figure 13, appears �M to be different from the distribution of other analytes of concern due to the fact that no MTBE was detected in the sample from monitoring well MW-7. However, as has been previously discussed by SJC (The San Joaquin Company Inc. 20014, 2002c), MTBE is actually present in samples recovered from that well at significant concentrations, but those concentrations cannot be detected in the laboratory due to the masking effect of the high concentrations of TPHg, TPHd, and other interfering analytes. Because, as is illustrated in Figures 10-13, the concentrations of analytes of concern often vary by orders of magnitude over the distance from the source at which the fuels were released to the down-gradient limit of the plume, it is preferable to plot the relationship between the natural logarithm of contaminant concentrations with distance down- gradient, rather than plotting the simple concentrations with distance down-gradient. In addition, at the 7500 West Eleventh Street site, the beneficial effects achieved by the remediation of groundwater at the area around the source was such that in the context of an assessment of natural attenuation, the area around monitoring well MW-7 now acts essentially as the source of contamination for locations down-gradient from that point. Accordingly, on Figures 14-17, we have plotted the natural logarithm of the sic