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Report: Groundwater-gualio,Monitoring—January 20, 2003: 7500 West Eleventh Street, Tracy, CA. Page Ill I Methyl-tertiary butyl ether EFA Method 8260B y Di-isopropyl ether EPA Method 8260B Ethyl tertiary-butyl ether EPA Method 8260B Tertiary-amyl methyl ether EFA Method 8260B 1,2 Dichloroethane EPA Method 8260B STL is certified to perform the specified analyses by the California Department of Health Services (DHS). The results of the analyses of samples of groundwater are presented in Table 2, which also includes the results of analyses of groundwater samples recovered from the previous sampling rounds. 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 January 20, 2003. < 3 2.4.1 Groundwater Qualityin the Near-Surface Unconfined Aq uifer The results of analyses of samples of groundwater recovered on January 20, 2003 show that only the samples recovered from Monitoring Wells 2, 4, 5, 6, 7, 10, 11 and 12 were affected by components of fuel hydrocarbons. Of those, the samples from Wells MW-2 and MW-5, which, as is shown on Figure 2, are located at the extreme western perimeter of the plume of affected groundwater, contained only traces of MTBE at the very low concentrations of 2.5 µg/L and 1.9 µg/L, respectively. The sample from Monitoring Well MW-10, also located on the perimeter of the plume, but on its eastern edge, also contained low concentrations of the fuel oxygenates MTBE and TAME at concentrations of 95 µg/L and 30 µg/L. 1; The groundwater sample recovered on January 20, 2003 from Monitoring Well MW-6, which is situated across the street and down the groundwater gradient from the location at which fael hydrocarbons were discharged to the subsurface beneath the 7500 West Eleventh Street property, contained 88 µg/l, of diesel (but no gasoline), together with TBA at a concentration of 83 µg/L and MTBE at a concentration of 300 µg/L. Those low concentrations of those analytes of concern are similar to the fluctuating concentrations detected in samples recovered from that well during earlier rounds of groundwater- quality monitoring. As it has since groundwater has been investigated in the area at the down gradient fringe of the plume, Monitoring Well MW-11 contained MTBE, but no other analyte of sic