ARCHIVED REPORTS XR0011739 - Laserfiche WebLink

Home Browse Search

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