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Lr 2.2 REGIONAL GEOLOGY AND HYDROGEOLOGY V The Earthgrains facility is situated in the Great Valley geomorphic province, which is bounded ,r by the Coast Ranges to the west and the Sierra Nevada province to the east. The site is also situated within the Eastern San Joaquin Subbasin of the San Joaquin Valley Groundwater Basin. 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 east. Significant water -bearing formations in the Eastern San Joaquin Subbasin i" consist of the Alluvium and Modesto/Riverbank Formations, Flood Basin Deposits, Laguna Formation, and Mehrten Formation (DWR, 2003). La The majority of the groundwater in the basin is characterized by calcium -magnesium bicarbonate or calcium -sodium bicarbonate types. Bicarbonate is the predominant anion in the eastern part of the basin. Reportedly, large areas of chloride type water occur along the western margin of the subbasin along the San Joaquin River. As a result of declining groundwater levels, poor -quality water has been moving east along a 16 -mile front on the eastern side of the Delta. The degradation has been particularly evident in the Stockton area where the saline front was moving eastward at a rate of 140 to 150 feet per year (DWR, 2003). V Each significant water -bearing formation in the Eastern San Joaquin Subbasin is described in the following subsections. V 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 Late Pleistocene to Recent in age and consist primarily of sand and gravel in the fan areas and clay, silt, and sand in the interfan areas. V These units are thin on the eastern side of the basin and are 150 or more feet thick near the center of the basin. Groundwater is under unconfined conditions within these units. Well ,.. yields of approximately 650 gallons per minute (gpm) have been reported. Reportedly, because these units are limited in thickness, most wells penetrate them to tap deeper aquifers in the area (DWR, 2003). L+ Flood Basin Deposits Lr This unit is exposed in the Delta area of the San Joaquin Valley and ranges in age from Pliocene to Recent. These deposits are generally much finer -grained with a higher percentage of fine sand and clay than their depositional equivalents to the east and west. Occasional gravel beds occur along the present waterways. This unit ranges in thickness from zero to L 2 L+