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Report: Groundwater-quality Monitoring—October 27, 2003: 7500 West Eleventh Street, Tracy, CA. Page 12 L, that conclusion. As is shown on Figure 2, Monitoring Well MW-5 is located at the extreme western perimeter of the plume of affected groundwater. The sample recovered on October 27, 2003 from Monitoring Well MW-6, contained diesel-range petroleum hydrocarbons at a concentration of 55 pg/L, which is a reduction from the 130 µg/L of organic compounds in the diesel range detected in July. However, it is not possible to assign all of that reduction to an actual decrease of diesel in the sample compared to its predecessor because the sample recovered on July 30 was not pre-treated with EPA Method 3630, while the sample recovered on October 27, 2003 was. Because it was appropriately pre-treated, the result from the October 27 sampling round is considered to be a positive indicator of the presence of diesel-range petroleum hydrocarbons in the groundwater at the location of Monitoring Well MW-6. Gasoline, at a concentration of 130 µg/L was also detected in the sample recovered from Monitoring Well MW-6. It had previously been detected in groundwater in that well, but had been absent since July 29, 2002. The only other analyte of concern detected in the well on bm October 27, 2003 was MTBE at a concentration of 130 µg/L. On the eastern perimeter of the plume, diesel-range petroleum hydrocarbons at a -- concentration of 89 pg/L were detected in the sample recovered on October 27, 2003 from Monitoring Well MW-9. This finding supports the tentative conclusion drawn from the July 30 sampling round that diesel range petroleum hydrocarbons had, by that time, �- made an initial appearance in Monitoring Well MW-9. The result from July 30 was considered to be uncertain with respect to the presence of diesel because the sample had not been pre-treated with silica gel and a false positive result could not be ruled out. The sample recovered from Monitoring Well MW-10 contained no detectable concentration of diesel but did contain gasoline-range compounds at a concentration of 200 µg/L, which is the highest concentration reported in a sample from that well to date. Similarly, the 'W concentration of MTBE, the only other analyte of concern detected in MW-10, reached a new high of 130 µg/L. These findings suggest that the plume of affected groundwater may be expanding, to some degree, towards the east, but there is presently insufficient data to confirm such a trend. 2.4.1.3 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 to the rear and slightly to the east of the Casa Mendoza restaurant on the 7500 West Eleventh Street property. The sample recovered from Monitoring Well MW-12 on October 27, 2003 contained 130 �tg/L of petroleum hydrocarbons in the diesel range, 120 µg/L of gasoline and 3.6 pg/L of benzene. These concentrations are at the lower end of `~ the range of concentrations previously detected in samples from that well, which concentrations have exhibited a declining trend since July 29, 2002. `r 2.4.2 Groundwater Quality in the Deeper Confined Aquifers In compliance with SJCEHD guidance for the groundwater monitoring program at the 7500 West Eleventh Street site, a sample was recovered from Monitoring Well MW-3A sic