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C Extended Site Characterization Report,- 7500 West Eleventh Street, Tracy, CA. Page 38 r Wells MW-12 and MW-12A, did permit additional understanding of the hydrostratigraphy beneath the site, but did not add significantly to the previously-derived interpretation of the lateral and vertical extent of the primary plume of fuel hydrocarbons. i During the approximate two-year period from May 2000 to April 2002, groundwater quality beneath the 7500 West Eleventh Street property improved significantly. By April 11, 2002, the concentration of diesel in groundwater in Monitoring well MW-3 had fallen from 2,800 µg/L to 200 µg/L, the concentration of gasoline had fallen from 11,000 gg/L to 330 gg/L and there were commensurate large reductions in the concentrations of the ' BTEX compounds. The concentration of MTBE fell from 10,000 gg/L to 170 gg/L. The concentrations of diesel and gasoline in Well MW-4 remained very low over that period _ and none of the BTEX compound were detected in the samples recovered from that well, except in the case of the sample gathered during the first sampling round conducted on May 11, 2000, The improved groundwater-quality conditions along the West Eleventh Street frontage of the subject property can be attributed to the removal of large volumes of affected soil and groundwater from the site when the underground storage tanks and the mass of piping were exhumed in December 1998. When the initial phase of site characterization was conducted in April and May 2000, it was found to be difficult to select a location for a groundwater-quality monitoring well that would be up-gradient of the area of the subsurface where soil and groundwater is affected by fuel hydrocarbons. This was due to the presence of the secondary plume of `=t fuel hydrocarbons beneath the site and the previously-discussed widespread presence of various petroleum hydrocarbons in the shallow soils. After an intensive exploration using small-diameter exploratory borings, Monitoring Well MW-1 was installed at the location 4 shown on Figure 13 to permit monitoring of groundwater quality beyond the limits of either the primary or secondary plume, = As is shown in Table 3, no analytes of concern were detected in soil samples recovered from the boring drilled to install Monitoring Well MW-1. The first sample of groundwater, which was recovered from that well on May 11, 2000, did contain 82 gg/L 1 of diesel-range hydrocarbons and traces of benzene, ethyl benzene and total xylene isomers at concentrations of 6.1 4g/L, 1.8 gg/L and 2.6 respectively, However, except for the appearance of 270 gg/L of diesel-range hydrocarbons in the sample recovered on j September 11, 2001, which may have been spurious, no analytes of concern have been present in the samples of groundwater recovered from that well since that time, 7.3.2.1 Primary Plume Beneath North Side of Subject Property f i Superimposing data from soil and groundwater analyses from push-probe borings and monitoring wells located along the line of section C-C', which is shown on Figure 16 and 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'. The -_s sic i