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2 HYDROGEOLOGIC, GEOLOGIC, AND SURFACE WATER CONDITIONS The hydrogeologic, geologic, and surface water conditions for the site apply for both LF- T and LF-2. 2.1 Groundwater Occurrence The first encountered groundwater monitoring zone at the site varies from about 200 to 300 feet below the ground surface. Groundwater elevations generally range from about 36 feet above mean sea level (MSL) on the southern side of the site to about 61 feet MSL on the northern side of the site. The average groundwater elevation at the site is about 45 feet MSL. The uppermost aquifer beneath the site occurs in Mehrten alluvium (e.g., sandy gravel) under confined or partially confined conditions. The overall permeability of these deposits is estimated to be about 9 x 10-3 centimeter per second (cm/sec) based on monitoring well slug testing data for similar alluvial deposits at the North County Landfill about eight miles northwest of the site. The direction of groundwater movement in the uppermost aquifer is to the east-southeast at a velocity of approximately 85 feet per year. Above the water-bearing zone, the soil layers are moderately to well-cemented layers of sandstone and siltstone with claystone and conglomerates. 2.2 Regional Geology The regional geology in the area represents a transition between the Cretaceous - Quaternary age alluvial deposits of the Great Valley flood plain and the Jurassic age metamorphic rocks of the Sierra Nevada foothills. Valley deposits thin out within about a mile east of the site, beyond which the surface geology is dominated by dissected alluvial uplands and exposed, uplifted bedrock characteristic of foothill terrain. Valley deposits in the vicinity of the site include mid-to-late Pleistocene Modesto and Riverbank, Pliocene and early Pleistocene Laguna, and Miocene Mehrten deposits. The Mehrten occurs at the surface throughout the landfill property, except for the northeast corner which is mapped as Laguna. The Laguna soils generally consist of sand and gravel with minor silt and are typically weakly cemented with low to moderate permeability. The Mehrten soils consist of reworked volcanic mudflow deposits containing moderately cemented agglomerate, conglomerate, tuffaceous sandstone, and siltstone. Permeability is generally low. The Mehrten Formation also contains some andesitic mudflow breccias, which are also known as lahars. Both the Laguna and Mehrten can also contain expansive clay soil at the surface. Foothill WQPS Rev Ldoc 2-1