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Report: Groundwater-quality Monitoring—January 20,2003: 7500 West Eleventh Street, Tracy, CA. Page 12 Monitoring Well MW-12 is located, as shown on Figure 2, in the area that is interpreted to be the source of the secondary plume of affected groundwater found beneath the 7500 West Eleventh Street property. Analysis of the sample of groundwater recovered from. 1' Monitoring Well 12 on January 20, 2003 contained, for the first time since water quality in that well was fist tested in April 2002, no detectable concentrations or any other analytes of concern except diesel at a concentration of 98 gg/L, that concentration having fallen to the lowest value since groundwater sampling from the well was initiated. j 2.4.2 Groundwater Quality in the Degper Confined Aquifers No detectable concentrations of any analytes of concern that are "components of fuel =s hydrocarbons were detected in samples of groundwater recovered on January 20, 2003 from any of the monitoring wells screened in the aquifers that lie beneath the first clay aquitard that is present beneath the 7500 West Eleventh Street property at a depth of approximately 20 ft. (See Figures 3 through 5 for hydrostratigraphy.) This was also the case, with the exception of 0.60 µg/L of MTBE apparently present in the sample recovered from Well 3A on April 11, 2002 , which result proved to be spurious, when those wells were sampled on April 11, July 29 and October 25, 2002. However, as had been the case on October 25, 2002 when groundwater in Monitoring i Well MW-3A was similarly affected, the samples recovered from Monitoring Wells MW-3B and MW-12A on January 20, 2003, contained a compound that was NOT a fuel hydrocarbon, at concentrations of 87 µg/L and 100µg/L respectively. Although it was not a fuel hydrocarbon, it included components with molecules having carbon-chain lengths within part of the same range that includes compounds that are components of diesel. As was discussed in the Groundwater-Quality Monitoring Report for October 25, 2002, at that time, SJC and STL carefully evaluated possible sources of the unknown analyte found in samples recovered from Monitoring Wells MW-3A, MW-3B and MW-12A, but could find no plausible explanation that related to the discharge of fuel hydrocarbons that occurred on the 7500 West Eleventh Street site or that might have been introduced by a field or laboratory error (The San Joaquin Company Inc. 2002a). As noted in Section 2.3, in anticipation of the need to further evaluate the unluiown analyte previously found in samples of groundwater from the deeper aquifers beneath the G' site, when the groundwater sample was recovered from Monitoring Well MW-12A on January 20, 2003, a second sample of water from that well was recovered and designated Sample No. MW-12AS. When the results of the primary analyses of the samples recovered on January 20, 2003 were available and the same unknown analyte that included carbon-chains in the diesel range was again found in the samples from Monitoring Wells MW-3B and MW-12A, the laboratory was directed to analyze Sample r _,I No. MW-12AS for diesel, but, in that instance, to prepare the sample with silica gel before the analysis was made. As is recorded in the Certificate of Analysis for Sample No. MW-12AS, which is included in Appendix B that analysis found no detectable concentrations of petroleum hydrocarbons in the diesel range in the sample. is i sic