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