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Extended plume Definition:-7500 W 11th St.,Tracy,CA. page 13 , ' From the ground surface to depths varying between 1 ft. and, locally, up to 7.5 ft., the whole of the site is overlain by fill material consisting of sands, gravels, concrete debris, highway paving and base course materials, together with disturbed and remolded native soils. Beneath the fill material to a depth of some 11-15 ft. beneath the ground surface is a layer of relatively impermeable clays and silty clays, the upper 5-10 ft. of which are dark gray in ' color,while the deeper of those clays are typically light brown. Below those clays is the top of a zone of the subsurface consisting of sands and silts that have a lenticular structure, with varying degrees of interconnectivity between the lenses of those materials. That zone extends to depths varying between approximately 11 and 24 ft. Below this are other light brown clays and silty clays that appear to be arealy continuous beneath the areas of the property and adjacent land that were explored. When the sands described above were encountered in borings, they were generally composed of fine to medium-course, sub- rounded particles with some silt. They are, locally, very loose. As is shown on Figure 3, the sand stratum beneath the near-surface clays on the 76500 West Eleventh Street property appears to be present along the length of Section A-A', which starts south of the Casa Mendoza restaurant and continues from there through the area of the site where the dispenser islands were located, then across West Eleventh Street and northward to at least the area around the intersection of Carmelo Avenue and Chrisman Road (see Figure 2 for locations). It is recognized that this stratum may not have been created during a single steady state depositional period. This interpretation is supported by significant variation in the depth to the top of the first aquifer beneath the surface. It is also recognized that, given the complexity of the depositional environment, the aquifer deposits may not have the degree of continuity that is shown by the interpretation presented on Figure 3. However, for the purposes of interpreting the stratigraphy in the context of contaminant transport, a reasonable interpretation of the data available from logs of push-probe and well borings along Section A-A' (Figure 3) suggests that that stratum of sands and silts is essentially continuous and is the same stratum as is seen at generally similar depths in the.other cross sections that have been drawn through the area. See, for example, Section B-B' and C-C' on Figures 4 and 5, respectively. Beneath the sand stratum found in a zone between some 10 ft. and 25 ft. BGS in the area of the Casa Mendoza Restaurant and northward from there to the area around MW-13 , there is another clay zone with its top is at depths varying between approximately 25 ft. and 28 ft. BGS. Where borings were drilled to sufficient depth (i.e., on the 7500 West Eleventh Street property itself) that clay layer was found to have a thickness of some 10 ft. It is underlain by another sand stratum in a zone between approximately 28 ft and 33 ft. BGS. Beneath that sand lies a third clay stratum with a thickness of about 5 ft., and beneath that, with its top approximately 35 ft. BGS, is a thick.sand stratum down to at least the maximum depth of exploration(50 ft. BGS). The stratigraphy described above is typical of the alluvial fan upon which Tracy and the surrounding area is situated. { SJC