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Site Characterization Report: 7500 West Eleventh Street, Tracy, CA. Page 20 f 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 ,I 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. Figures 10, 11 and 12 are hydrostratigraphic sections along the lines A-A', B-B' and C-C' shown on Figure 9. The hydrostratigraphy shown in the cross sections is an interpretation of the stratigraphic data from the boring logs that divides the sediments into two classes: the permeable sands and silts and the relatively less permeable clays and silty clays. This presentation makes it possible to reduce the details of the stratigraphy to a tractable degree of complexity by distinguishing between the different soil types based on the properties that are of importance to the understanding of the distribution and transport of fuel hydrocarbons in the subsurface. Based on pump tests that SJC has conducted in similar strata at other locations in Tracy, it is estimated that the clays and silty clays beneath the 7500 West Eleventh Street site have a hydraulic conductivity on the order lof 10-4to 10"6 cm./sec., while the silts and sands have permeabilities in the range of 10 to 10" cm./sec. (The San Joaquin Company Inc. 1994.) r , As is shown on Figure 10, a stratum of sand some 10-ft. thick appears to be present along the length of Section A-A', which, as is shown on Figure 9, passes from a point to the south of the Casa Mendoza restaurant and from there through the area of the site where the dispenser islands were located and continues across West Eleventh Street. It is recognized that his stratum may not, in fact, have the degree of continuity that is shown by the interpretation presented on Figure 10. However, a reasonable interpretation of the s= data available from logs of push-probe and well borings along Section C-C' (Figure 12), strongly suggests the presence of the same stratum of sands and silts at similar depths to } those at which this stratum of permeable materials was encountered in borings along Section A-A'. A similar zone of sand and silt is seen on Section B-B' on Figure 11. However, along that section, which passes through MW-2 in the west and MW-4 in the east, there is a distinct discontinuity in the sand facies in the vicinity of borings PP-3 and MW-3. Detailed study of the water table elevations presented in Table 3 reveals that the relative depth to groundwater in MW-3, compared to that in MW-2 and MW-4, is higher than might be expected, based on an assumption that the groundwater flow regime flows through an arealy homogeneous stratigraphy. This is compatible with the hydrostratigraphy shown on Section B-B' on Figure 11, where zones of relatively permeable sands and silts are separated by a zone of impermeable clays located in the vicinity of MW-3. As was discussed in Section 4.1 above, funding restrictions on subsurface investigation to I the east of Chrisman Road did not permit the hydrostratigraphy of those areas and its potential influence on groundwater flow directions and the distribution of petroleum hydrocarbons on the eastern side of the 7500 West Eleventh Street site to be fully sic 1 .E ,J