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Extended Site Characterization Report; 7500 West Eleventh Street, Tracy, CA, Page 37 r upper approximate 5 ft is above the water table. The zone reaches a maximum depth of less than 25 ft BGS in the area around exploratory boring PP-3, which is directly down- gradient from the area of the site where fuel leaked from piping that served the former fueling station's dispenser pumps. To the west of PP-3, the thickness of affected soil in the plume reduces progressively until, at MW-2, it pinches out entirely. The affected zone of soil within the plume is shown as having a similar lens-like geometry to the east of PP-3, but conditions in the subsurface to the east of MW-4 were not investigated, due to the prohibition imposed by the USTCF staff on the location of any wells or borings that SJC planned to install to the east of Chrisman Road when the original site characterization program was conducted in 2000. The estimated eastern limit r= of the lens of soil affected by gasoline and diesel shown on Figure 15 is based on consideration of the symmetry that would be a characteristic of plumes of hydrocarbons that progress through uniform, .porous media and of the data from groundwater-quality Monitoring Wells MW-8 and MW-9, which were installed as part of the extended site �=f characterization program that is the subject of this report. F It is recognized that not all of the locations in the subsurface where components of fuel ' = hydrocarbons were detected in samples recovered from the borings are included within the lens-shaped zone shown on Figure 15. Inspection of Tables 3 and 4 reveals that -= hydrocarbons in locations outside of that zone are of various compositions (see notes at i ' foot of Table 3), are all at very low concentrations (less than 4 mg/Kg), and are at shallow depths that are generally above the water table in areas where there is no evidence of the past existence of leakage of fuel to the subsurface from a point source. As was discussed in Section 7.1, these occurrences can be attributed to spills, drips and leaks onto the unpaved surfaces of the 7500 West Eleventh Street property and surrounding area at various times over the 70-year history of the former fueling station and to bituminous macadam included in imported fill. Two wells, designated MW-3A and MW-3B, were installed in March 2002 at the direction of the SJCEHD at depths of 35 ft. and 51 ft. BGS, respectively, in a cluster around Well MW-3, a short. distance down-gradient from the former location of the =� dispenser island. (See Figures 13 and 15.) As had been anticipated by SJC, based on the hydrostratigraphy developed following the first phase of site characterization (The San Joaquin Company Inc. 20018), no traces of =' components of hydrocarbon fuels were detected in samples of soil recovered Wells MW- 3A or MW-3B from depths greater than 15.5 ft BGS. (See Table 3.) With the exception of a trace of MTBE at a concentration of 0.60 µg/L, which may be a spurious result, no =' components of fuel hydrocarbons were detected in the first samples of groundwater recovered from these wells on April 11, 2002. Those results are fully compatible with the conclusions reached by SJC following the first phase of site characterization when it was found that the sand stratum present beneath the site in a zone of varying thickness located between approximately 11 and 24 ft BGS is underlain by a clay aquitard of considerable ' lateral extent. As was noted in Section 6.2 and is shown on Figures 14 and 15, the installation of Wells MW-3A and MW-3B, together with information obtained from SJC