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II. GEOLOGIC AND HYDROGEOLOGIC SETTING <br />A. GEOLOGY <br />Located along the eastern edge of the San Joaquin Valley, the proposed <br />landfill site is situated within an area of gently rolling topography <br />(see Figure 2). The geology of the area is characterized by. alluvial <br />sediments deposited by westward -flowing rivers and streams emanating <br />from the Sierra Nevada Mountains. <br />The youngest geologic units in the vicinity of the site are semicon- <br />solidated alluvial deposits of the Pleistocene Turlock Lake and <br />Riverbank Formations (Wagner et al., 1981). These formations consist <br />of alluvial deposits of clays, silts, arkosic sands, and gravels. The <br />deposits are structurally undeformed and lithologically gradational, <br />both vertically and laterally. <br />Underlying the Turlock Lake and Riverbank Formations are the Arroyo <br />Seco Gravel (Pleistocene), Laguna Formation (Pliocene), and the <br />Mehrten Formation (Miocene -Pliocene). Differentiation of alluvial <br />deposits in this region into formations is difficult, due to similar <br />origins and common lithologies (Marchand and Allwardt, 1981). <br />During EMCON's assessment of this site for development into a <br />Class III landfill, the geologic units were characterized by reviewing <br />existing geologic information and drilling six borings ranging from <br />40-1/2 to 215 feet in depth. A seismic .refraction survey was conduct- <br />ed to evaluate the rippability of subsurface materials. In addition, <br />an evaluation was completed to determine the Maximum Probable Earth- <br />quake and associated peak ground acceleration. <br />Drilling information indicates that site surficial soils consist of <br />silts and clays to a depth of 2 to 3 feet. These soils are commonly <br />underlain by a thin layer of cemented soil called hardpan. The under- <br />lying Turlock Lake and Riverbank Formations consist of heterogeneous <br />. .0 O <br />