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Report: Groundwater-quality Monitoring—July 29,2002: 7500 West Eleventh Street, Tracy, CA. Page 11 the sample recovered from Monitoring Well MW-6 on July 29, 2002, although 74 gg/L of that analyte had been detected in the sample recovered from that well on April 29, 2002. Conversely, 220 gg/L of gasoline-range hydrocarbons were detected in the July 29, 2002 sample from that well when none had been detectable in the sample of groundwater recovered from it on April 11, 2002. The sample of groundwater recovered from Monitoring Well MW-2, located on the western periphery of the principal plume of affected groundwater, on July 29, 2002 remained free of any analytes of concern expect for a trace of MTBE at a concentration of 2.5 gg/L. Groundwater recovered on that date from Monitoring Wells MW-8 and MW- 9, which are beyond the eastern lateral extent of the primary plume of affected groundwater, remained entirely free of any detectable concentrations of any analytes of .i concern.. On the northeastern fringe of the primary plume, gasoline-range hydrocarbons were detected for the first time in the sample of groundwater recovered from Monitoring Well MW-10, although it was accompanied by a less than significant reduction in the concentration of MTBE in that sample (58 gg/L compared to the concentration of 110 gg/L measured in the sample recovered from that well on April 11, 2002). Groundwater recovered from Monitoring WeII MW-11 remained free of any detectable concentration of analytes of concern, except for a trace of MTBE at a concentration of 8.3 gg/L, which is only slightly higher than the concentration of 5.1 gg/L detected in the sample of groundwater recovered from that well on April 11, 2002. No detectable concentration of any analytes of concern were detected in samples of groundwater recovered on July 29, 2002 from any of the monitoring wells screened in the aquifers that lie beneath the first clay aquitard that is present beneath the 7500 West :i Eleventh Street property at a depth of approximately 20 ft. (See Figures 3 through 5 for hydrostratigraphy.) This was also the case when those wells were sampled on April 11, . 2002, except that, on that occasion, the sample recovered from Monitoring Well MW-3A appeared to contain a very slight trace of MTBE at a concentration of 0.6 gg/L, which, in SJC's opinion offered at that time, was likely to have been spurious (The San Joaquin k ; Company Inc. 2002a). These findings strongly support a conclusion that the vertical extent to which groundwater under the subject property has been affected by components of fuel hydrocarbons is Iimited to the shallowest of the several sandy aquifers that lie beneath it. Figures 2 through 5 provide an interpretation of the lateral and vertical extent to which the subsurface beneath the site is affected by components of fuel hydrocarbons that was developed from the results of the groundwater-quality sampling conducted on July 29, 2002. 1 SJC