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Report: Groundwater-quality Monitoring—July 30,2003: 7500 West Eleventh Street, Tracy, CA. Page 12 The samples recovered from Monitoring Wells MW-5 and MW-6 also contained hydrocarbon compounds in the diesel range, at concentrations of 68 gg/L and 130 gg/L, respectively. However, the hydrocarbon detected did not match the laboratory's diesel standard. Similar material has previously been detected at generally similar concentrations in samples recovered from Well MW-6 during earlier monitoring rounds. No sample of groundwater was recovered from Monitoring Well MW-10 during the July 30, 2003 monitoring round. However, a sample was recovered from Monitoring Well MW-9, which is also close to on the eastern perimeter of the principal plume of affected groundwater. For the first time since it was installed, a sample recovered from Monitoring Well MW-9 contained a trace of hydrocarbon compounds that, when quantified as TPH(d) were measured at a concentration of 72 gg/L. Again, those materials did not match the laboratory's diesel standard. SJC plans to investigate further the issue of whether or not the material detected in the sample from Well MW-9 is a false positive for petroleum hydrocarbons during the next round of groundwater-quality monitoring at the subject site. 2.41.3 Analytes of Concern in Secondary Groundwater Plume As is also shown on Figure 2, a secondary plume of diesel and gasoline emanates from an area close to the southeastern corner of the former truck service bay that was located at the rear of the Casa Mendoza restaurant on the 7500 West Eleventh Street property. Monitoring Wells MW-12 and MW-12A are located in this plume. In compliance with r=`l SJCEHD guidance (San Joaquin County Environmental Health Department 2003), sample recovery from those wells is now being performed on a semi-annual basis so that no samples were recovered from either of those wells during the July 30, 2003 round of groundwater-quality monitoring. The results of analyses of samples of groundwater in that area recovered during earlier monitoring rounds are compiled in Table 2. s 2.4.2 Groundwater Quality in the Deeper Confined Auuifers Also in compliance with SJCEHD guidance, no samples were recovered from Monitoring Wells MW-3A, MW-313 or MW-12A during the July 30, 2003 monitoring round. Those wells are screened in aquifers that lie beneath the first clay aquitard present beneath the 7500 West Eleventh Street property. Samples are now being recovered from those deep wells on an annual basis only. s� SJC # �r