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Site Characterization Report: 7500 West Eleventh Street, Tracy, CA. Page 29 ' based on consideration of plume symmetry and the assumed general continuity of the hydrostratigraphy from MW-7 eastward into the unexplored zones beneath and to the east of Chrisman Road. As was the case on the south side of West Eleventh Street, limitations placed on the California-licensed geotechnical engineer in responsible charge of the site characterization program prevented drilling of borings or installation of monitoring wells in that area that would have resolved uncertainties with respect to the eastern limit of the plume and the down-gradient distance to which the plume of fuel hydrocarbons has migrated. As was the case with Section B-B' shown on Figure 15, areas affected by low concentrations of fuel hydrocarbons in shallow soils along the line of Section C-C' are cross-hatched on Figure 16. As was also the case for Section B-B', those traces of petroleum hydrocarbons appear to have a source or sources separate from that of the principal plume of soil and groundwater that migrated across West Eleventh Street from the former fueling station at 7500 West Eleventh Street. Long-time Tracy residents and landowners report that the Fisco Farm and Home Store at 7501 West Eleventh Street was the first structure built on that property, but that, on the vacant lot immediately to its west (see Figure 2 for location), there was a small truck stop and restaurant known as "Demo's," which was demolished in the early 1960's. (Fields 2001,Navarra 2001.) No concentrations of the analytes of concern were detected in the samples of groundwater recovered from MW-5 or PP-6, which are located on the shoulder of West Eleventh Street in front of the vacant lot that had been the site of Demo's truck stop and restaurant. Samples of the shallow soils in those borings and at other locations beneath the north shoulder of West Eleventh Street were, however, affected by very low i concentrations of various types of petroleum hydrocarbons. As with those of similar type =' detected over a broad area of the 7500 West Eleventh Street property, it is likely that their source was various leaks and spills that occurred during an earlier era, when truck and other vehicular traffic used that frontage area when entering to, exiting from and parking at the truck stop formerly located on the now-vacant lot. The distribution of fuel hydrocarbons along section line A-A', running from the property at 24195 Chrisman Road, which is immediately to the south of the subject property, through the central area of the former fueling station, across West Eleventh Street and ? north into the Fisco Farm and Home Store property is shown on Figure 14. The shape of ' the primary plume of gasoline and diesel is, again, similar to the well-known form found on sites where hydrocarbons have leaked from subsurface locations above the r; groundwater table and migrated down the groundwater gradient. The zones of relatively shallow soils affected by traces of disparate hydrocarbons are shown cross-hatched on section A-A' in a manner similar to that employed on sections B- B' and C-C'. This includes a zone around a back-filled pit on the property at 24195 Chrisman Road, from which two 1,000-gal. underground storage tanks that had contained diesel and gasoline were removed in January 1999. (Dietz Irrigation 1999b.) Although soil samples recovered from the bottom of that pit contained diesel at concentrations up to 370 mg/Kg and barely detectable traces of the BTEX compounds, it is not clear sic