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i 3.3 Site Hydrogeology The San Joaquin Valley comprises the southernmost portion of the Great Valley Geomorphic Province of California(Figure 1). The Great,Valley is a broad structural trough bounded by the tilted block of the Sierra Nevada on the east and the complexly folded and faulted Coast Ranges on the west. The Eastern San Joaquin Subbasin is defined by the areal extent of unconsolidated to semi consolidated sedimentary deposits that are bounded by the Mokelumne River on the north and northwest; San Joaquin River on the west;.Stanislaus River on the south; and consolidated bedrock on the cast(Figure 2) . The Eastern San Joaquin Subbasin is bounded on the.south, southwest, and west by the Modesto, Delta-Mendota, and Tracy Subbasins,respectively and on the northwest and north by the Solano; South American, and Cosumnes Subbasins. The Solano i and South:American are subbasins of the Sacramento Valley Groundwater Basin. The Eastern San Joaquin Subbasin is drained by the San Joaquin River and several of its major tributaries namely, the Stanislaus, and Calaveras, and Mokelumne Rivers (Figure 2). The San Joaquin River flows northward into the Sacramento and San Joaquin Delta and discharges into the San Francisco Bay. Annual'precipitation within the Subbasin ranges from about 11 inches in the southwest to about 25 inches in the northeast. Water Bearing Formations Water bearing formations of significance in the Eastern San Joaquin Subbasin consist of the(1)Quaternary-age alluvium and Modesto/Riverbank Formations,(2)Flood Basin Deposits, (3) Laguna Formation, and (4) Mehrten Formation. The Mehrten Formation is considered to be the oldest fresh water-bearing formation on the east side of the basin, even though I the underlying Valley Springs Formation produces minor quantities (DWR, 2003). Information on water bearing units and groundwater,conditions was taken primarily from (DWR 1967). Alluvium and Modesto/Riverbank Formations(Undifferentiated). These units are exposed within the subbasin along a band approximately 15 miles wide that extends from about Stockton eastward. These units are Recent to Late Pleistocene in age and consist primarily of sand and gravel in the fan Environmenta]\STEIPARSCR09302008.wpd 14 i