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Site Characterization Report: 7500 West Eleventh Street, Tracy, CA. Page 28 <br /> ?+ Eleventh Street <br /> property Yat various times over the 70-year histor 'of the former fueling <br /> station. <br /> Superimposing data from soil and groundwater analyses from push-probe borings and <br /> monitoring wells located along the line of section C-C', which runs along the north side <br /> of West Eleventh Street, yields a zone of the subsurface affected by a primary plume of <br /> gasoline and diesel similar to that seen in Section B-B'. That distribution is shown on <br /> Figure 16. In that case, the maximum thickness of the zone of affected soil that is located <br /> near PP-5 and MW-7 is some 12 ft, and the maximum depth to which fuel hydrocarbons <br /> k were detected is approximately 20 ft BGS. <br /> The concentrations of diesel and gasoline in the sample of groundwater recovered from <br /> MW-7 were 1,300 µg/L and 1,400 pg/L, respectively, with MTBE at a concentration of <br /> 22 µg/L. The concentration of diesel in the sample of groundwater recovered from PP-5 <br /> was 560 pg/L and the concentration of gasoline in that sample was 1,100 pg/L. MTBE, at <br /> a concentration of 22 µg/L, was also recovered from MW-7. The presence of those <br /> relatively high concentrations of diesel and gasoline at that location suggests that MW-7 <br /> { is near the central axis of a plume of hydrocarbons that have migrated across West <br /> Eleventh Street. <br /> When considered relative to the location of the center line of the plume of diesel and <br /> gasoline shown near MW-3 on Section B-B' on Figure 15, the presence of the relatively <br /> high concentrations of hydrocarbons in groundwater recovered from MW-7 is consistent <br /> with a plume of fuel hydrocarbons migrating down the groundwater gradient in a <br /> direction of flow that is approximately 20 degrees east of north. This direction of <br /> t<:� <br /> migration of fuel hydrocarbons is as would be expected given the geometry of the <br /> groundwater table contours shown on Figure 9. <br /> The western limit of the primary plume of diesel and gasoline along Section C-C', as <br /> ;t shown on Figure 16, has been interpreted to be located at a point between MW-5, from <br /> which the groundwater sample contained no components of petroleum hydrocarbons, and <br /> MW-6. As is cited in Table 2, of the 13 components of fuel hydrocarbons for which the <br /> groundwater sample recovered from MW-6 were analyzed, only MTBE, at a <br /> _r concentration of 150 pg/L, was detected. Despite the lack of other components of fuel <br /> hydrocarbons in the groundwater sample recovered from MW-6, the primary plume of <br /> mixed gasoline and diesel is interpreted to extend some distance to the west of that boring <br /> because MTBE was detected in groundwater from that well. MTBE was not generally <br /> added to gasoline in California until the late 1970's, but it is known to migrate more <br /> rapidly through a soil of a given intrinsic permeability when compared to other fuel <br /> hydrocarbon components. Thus, its singular presence in the groundwater sample <br /> .', recovered from MW-6 can be accounted for by its more rapid migration from a source on <br /> the south side of West Eleventh Street compared to other components of fuel <br /> hydrocarbons released at that same source. <br /> As was the case for Section B-B', the assumed distribution and limit of the plume of <br /> ' gasoline and diesel that is shown to the east of MW-7 that is shown on Section C-C' is <br /> sic <br />