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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 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 areas while clay,silt, and sand are dominant in the interfan areas. These units range in thickness from a thin veneer on the east side of the basin to over 150 feet near the center of the basin. Groundwater occurs unconfined within these units. Well yields to 650 ±gpm are reported. Because these units are limited in thickness,most wells penetrate them in order to tap deeper aquifers in the area. Average specific yields in the 10-to 200-foot depth range vary from about 7 to 15 percent within the boundaries of the Tuolumne River Storage Unit(Davis et al. 1959). The average specific yield for fresh water bearing units in the San Joaquin County Groundwater Investigation area as defined in(DWR 1967)is 7.3 percent. The Victor Formation as defined in(DWR 1967)is correlative with these units. Flood Basin Deposits. This unit is exposed in the Delta area of the San Joaquin Valley. These deposits are basinward, fine-grained forms of the Laguna,Riverbank,Modesto,and Recent formations and,therefore,range in age from Pliocene to Recent. They are generally much finer grained with a higher percentage of fine sand and clays than their depositional equivalents to the east and west. Occasional gravel beds occur along the present waterways and are probablyrepresentative ofthe type of underlying lithology distribution. This unit ranges in thickness from 0 to 1,400 t feet. Groundwater in this unit occurs under unconfined to confined conditions.The unit,in general,has low permeabilities and may create semi-confined to confined conditions when interfingered with the Alluvium and Modesto/Riverbank Main\DTnvironmentai%STE\Workplan\Workplanl]192007.wpd 14