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I <br /> 3.3 Site Geology <br /> The subject site is located in the northern San Joaquin Valley on the eastern margin of the San <br /> Joaquin Delta The site is essentially flat and is at an elevation of approximately 15 feet above <br /> mean sea level Based on lithologic data collected from a total of eight monitoring wells and <br /> numerous soil borings, the sediments beneath the site consist of unconsolidated sand, silt and <br />' clay These sediments are distributed in vertically and laterally semi-continuous zones which <br /> suggest deposition in delta channel and flood plain environments <br />' The eight monitoring well logs developed over the course of the Phase I and Phase II <br /> investigations provide a relatively detailed view of the site's subsurface geologic conditions and <br /> form the basis for two geologic cross sections The locations of geologic cross sections A-A' <br />' and B-B' are shown on Figure 4 and the cross sections are presented as Figures 5 and 6 <br />' The cross sections indicate the presence of at least four lithologic zones within approximately <br /> 80 feet of the surface These zones are as follows <br />' Zone Sediment Type Approximate Depth Thickness <br /> to Top of Zone <br />' (ft , bgs) <br /> I sandy silt (fill) surface 0 - 8 <br /> 2 silt and clay surface - 8 5 - 36 <br /> 3 sand with some gravel 13 - 36 10 - 255 + <br />' 4 silt and clay 29 - 50 34 + <br />' Because zones 3 and 4 have been the water bearing units beneath the site, these will be the focus <br /> of the following discussion <br />' Zone 3 vanes in thickness from 10 to at least 25 5 feet thick throughout the site area This sand <br /> zone contained the water table aquifer to 1989 and was the focus of the Phase I groundwater <br /> investigation Interpretation of cross sections indicate that this zone may split laterally into two <br />' regions which suggests that the sand zone may be a remnant channel deposit <br /> This sand zone is underlain by zone 4 which consists primarily of silt with thin clay lenses and <br />' an occasional sand layer This zone is at least 34 feet thick and appears to be laterally <br /> continuous throughout the site area The contact between the upper sand zone and the lower silt <br />' zone appears to be either channelized or gradational, with no indication of a barrier clay <br /> separating the two zones <br /> RPr5/SFC Wp 10 06MI192 <br />