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F Extended Site Characterization Report, 7500 West Eleventh Street, Tracy, CA, Page 40 Samples of the shallow soils in those borings and at other Iocations beneath the north shoulder of West Eleventh Street were, however, affected by very low concentrations of } various types of petroleum hydrocarbons. As with those of a similar type detected over a broad area of the 7500 West Eleventh Street property and adjacent areas, it is likely that their source was various leaks and spills from vehicles that occurred during an earlier era or from pieces of bituminous macadam incorporated in imported fill. Long-time Tracy residents and landowners report that the now vacant building at 7501 West Eleventh Street, which was previously occupied by the Fisco Farm and Home Store, was the first structure built on that property, but that, on the lot immediately to its west at 7601 West Eleventh Street (see Figure 2 for location), there had been a small truck stop and restaurant known as "Demo's," which was demolished in the early 1960s (Fields 2001, Navarra 2001). Following the initial round of groundwater sampling and analysis that was conducted using Monitoring Wells MW-1 through MW-7, installed in April 2000, five additional sampling rounds have been performed. Those include the round conducted on April 11, 2002, when samples of groundwater were recovered from the newly-installed Wells MW- 3A, MW-3B, MW-8 through MW-12 and MW-12A, as well as the pre-existing Wells MW-1 through MW-7. As can be seen by inspection of Table 4, by May 30, 2001, hydrocarbons in the diesel ,i range at a concentration of 81 µg/L had appeared in the groundwater in Well MW-6. This was not unexpected given the presence of MTBE in samples of groundwater recovered from the well at earlier dates, which is indicative of the leading edge of a plume of fuel hydrocarbons migrating in the direction of that well. Groundwater in Well MW-5, located on the western fringe of the primary plume of hydrocarbon fuels emanating from the 7500 Eleventh Street site, was free of any analytes of concern until December 13, 2001, when TBA at 9.1 µg/L was detected in a sample of groundwater recovered from that well. While no TBA was detected in a sample recovered from the same well on April 11, 2002, it did contain 0.80 µg/L of MTBE. The appearance of those fuel oxygenates in Well MW-5 can be attributed to an ongoing process of lateral dispersion of the components of gasoline in the plume. New information gathered during the second phase of site characterization conducted in March and April 2002 did not require major revision of the interpretation of the subsurface zone within which soil and groundwater is affected by components of fuel hydrocarbons beneath the 7500 West Eleventh Street property itself and beneath West Eleventh Street. Minor revisions that were appropriate are reflected on Figures 13, 14, 15 and 16. There were, however, some noteworthy changes in the concentrations of analytes of concern detected in samples of groundwater recovered from the older monitoring wells located in those areas. When groundwater in Well MW-7 was first analyzed in May 2000, it was affected by jrelatively moderate concentrations diesel- and gasoline-range petroleum hydrocarbons (at concentrations of 1,300 gg/L and 1,400 µg/L, respectively) and MTBE, at 22 µg/L. sic