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Site Characterization Report: 7500 West Eleventh Street, Tracy, CA. Page 19 corridor in Tracy [The San Joaquin Company Inc. 1993]) and localized variations in groundwater gradient and flow direction due to the presence of paleogeomorphologic channels filled with sand and other permeable, clastic materials that may cut through the alluvial fan deposits underlying the neighborhood. As is discussed below in Section 4.2, the presence of sands and other high-permeability deposits confined within impermeable clays were found beneath the 7500 West Eleventh Street property by the site characterization program conducted by SJC. Unfortunately, due to funding restrictions, it was not possible to resolve the details of the hydrostratigraphy and groundwater table j elevations beneath and to the east of Chrisman Road. To ensure that the apparent discrepancy between the direction of groundwater flow beneath the 7500 West Eleventh Street property and adjacent areas computed by SJC and the direction of groundwater flow beneath the C&B Equipment property computed by SGI was not due to a surveying error or an error in field measurement made in wells MW-I through MW-7, SJC re-surveyed the elevations of the well casings and re- measured the depths to groundwater in those wells on August 22, 2000. The casing elevations were found to be correct. The measured depths to groundwater are shown in Table 3. Compared to the elevations computed for May 11, 2000, by August 22, 2000, the water table had dropped some 0.75 ft. However, from inspection of Table 3, it can be seen that the direction of groundwater flow on August 22, 2000 was also to the east of north, as it = had been when first computed by SJC. As will be discussed in Section 5.0, this north- - ' northeasterly direction of groundwater flow is compatible with the distribution of petroleum hydrocarbons in the subsurface that was revealed by analyses of soil and groundwater samples recovered from the borings drilled as part of the site characterization program. 4.2 Hydrostratigraphy As noted in Section 1.4, the soils beneath the 7500 West Eleventh Street property and adjacent areas are composed of clays, silty and sandy clays, silt and sands that are ' complexly interbedded. The boring logs prepared when the push-probe borings and the borings for the groundwater-quality monitoring wells described in Sections 3.4 and 3.6 were drilled enabled details of this complex system to be developed. From the ground surface to depths varying between 1 ft and, locally, up to 7.5 ft, the lwhole of the site area 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 there 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 if interconnectivity between the lenses of those materials. This zone extends to depths varying between approximately 11 and 24 ft. sic