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Extended Site Characterization Report: 7500 West Eleventh Street, Tracy, CA. Page 53 Although remediation of floating product and high concentrations of analytes of concern in the groundwater along the West Eleventh Street frontage of the subject property has proved effective in reducing the concentrations of analytes of concern in that area, as of 'f April 11, 2002, floating product remains present on the north side of that street in the area around Monitoring Well MW-7, in which well on that date the apparent floating product thickness was 0.3 ft. Fortunately, at that time, such product was not detected to the north :., of that area in the down-gradient portions of the primary plume. >; The maximum depth to which groundwater is affected in the primary plume along West Eleventh Street is limited to some 25 ft. BGS by a clay aquitard that protects,groundwater quality in underlying sand aquifers within which no analytes of concern have been -� detected. As is shown on Figure. 13, the primary plume where the groundwater is affected by diesel- and gasoline-range hydrocarbons is bounded by a narrow fringe in which only =_3 MTBE is detectable. This finding is consistent with the well-known propensity for that fuel oxygenate to migrate through the subsurface at a faster rate than is the case for other components of fuel hydrocarbons. 9.3 Secondary Plume As is also shown on Figure 13, a second distinct plume of diesel and gasoline was also identified in the subsurface beneath the site. That plume appears to have originated at a source located near the southeast corner of the truck service bay located to the rear of the Casa Mendoza restaurant. Groundwater recovered from Monitoring Well MW-12, which is screened from a point above the water table to a depth of 21 ft. BGS and is located near the apparent source of that secondary plume, is affected by diesel-range hydrocarbons at a concentration of 280 4g/L and gasoline-range hydrocarbons at a concentration of 570 µg/L, with no detectable concentrations of any of the BTEX compounds or fuel oxygenates. No analytes of concern were detected in the sample from Monitoring Well MW-12A, located in the same area, but screened between 30 ft. and 35 ft. BGS, below a clay aquitard that separates shallow groundwater affected by components of fuel hydrocarbons from deeper groundwater that remains unaffected by analytes of concern. 9.4 Hydrocarbons in Shallow Soils 1 hi addition to the primary and secondary plumes of diesel and gasoline in the subsurface, the shallow soils in essentially all of the areas investigated by the site characterization program contained low concentrations of a variety of petroleum hydrocarbons, the presence of which can be attributed to minor leaks and spills that occurred over the more - than 70 years during which the fueling station operated, other leaks and spills that occurred along the shoulders of West Eleventh Street and Chrisman Road and to pieces of bituminous macadam that had been incorporated into fill material used to bring the bases of those two streets to their presents grades. (See Figure 12.) Other fuel hydrocarbons in the subsurface in the neighborhood of the subject property are the result of leaks that occurred from underground storage tanks located in the neighborhood. sic