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Extended Site Characterization Report; 7500 West Eleventh Street, Tracy, CA. Page 39 li maximum thickness of the zone of affected soil is located between MW-6 and MW-7 and is approximately 21 ft BGS. .: The concentrations of diesel and gasoline in the sample of groundwater recovered from MW-7 on May 5, 2000 were 1,300 µg/L and 1,400 µg/L, respectively, with MTBE at a concentration of 22 µg/L. The concentration of diesel in the sample of groundwater recovered from PP-5 was 560 µg/L and the concentration of gasoline in that sample was 1,100 µg/L. No detectable concentrations of any analytes of concern were detected in Monitoring Well MW-8, which is located to the east of Chrisman Road and was installed �'==1 in March 2002. f> i When considered relative to the location of the center line of the plume of diesel and gasoline shown near MW-3 on Section B-B' (Figure 15), the presence of the relatively high concentrations of hydrocarbons in groundwater recovered from MW-7 is consistent ' with a plume of fuel hydrocarbons migrating down the groundwater gradient in a direction of flow that is approximately 20 degrees east of north. This direction of migration of fuel hydrocarbons is as would be expected given the geometry of the groundwater table contours shown on Figure 8. The western limit of the primary plume of diesel and gasoline along Section C-C, as r ' shown on Figure 16, is interpreted to be located close to Monitoring Well MW-6, from which the groundwater sample recovered on May 11, 2000 contained no components of petroleum hydrocarbons. As is cited in Table 4, of the 13 components of fuel hydrocarbons for which the groundwater sample recovered from MW-6 on that date were analyzed, only MTBE, at a concentration of 150 [.g/L, was detected. Despite the lack of r. other components of fuel hydrocarbons in the groundwater sample recovered from MW- 6, the primary plume of mixed gasoline and diesel is interpreted to extend some distance to the west of that boring because MTBE was detected in groundwater from that well. Although it had been used as a gasoline additive prior to that time, gasoline formulated i with MTBE was not generally distributed in California until October of 1992 (California Environmental Protection Agency 1997), but it is known to migrate more rapidly through soil of a given intrinsic permeability when compared to other fuel hydrocarbon components. Thus, its singular presence in the groundwater sample recovered from MW- 6 in May 2000 can be accounted for by its more rapid migration from a source on the south side of West Eleventh Street compared to other components of fuel hydrocarbons released at that same source. As was the case for Section B-B', the assumed distribution and limit of the plume of gasoline and diesel that is shown to the east of MW-7 on Section C-C' is based on consideration of plume symmetry and the assumed general continuity of the hydrostratigraphy from MW-7 eastward to MW-8, together with information regarding j soil and groundwater chemistry from samples recovered from Monitoring Well MW-9. No concentrations of any of the analytes of concern were detected in the samples of groundwater recovered in April and May 2000 from MW-5 or PP-6, which are located on the shoulder of West Eleventh Street in front of the lot at 7601 West Eleventh Street. i --� sic