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Flood Basin Deposits 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 approximately 1,400 feet. Groundwater is under unconfined to confined conditions within this unit. In general, this unit has low permeabilities and may create semi-confined to confined conditions when interfingered with the Alluvium and Modesto/Riverbank Formations. Occasional pockets of fresh water are found in the Delta deposits, but generally this unit contains water of poor quality(DWR, 2003). Laguna Formation The Laguna Formation is Pliocene-Pleistocene in age and consists of discontinuous lenses of stream-laid sand and silt with lesser amounts of clay and gravel. The formation thickens from approximately 400 feet in the Mokelumne River area to approximately 1,000 feet in the Stockton area. Groundwater is under unconfined to locally semi-confined conditions within this unit. Regionally, groundwater yields of 1,500 gpm have been reported from highly- permeable layers, but average yields are approximately 900 gpm(DWR, 2003). Mehrten Formation The Mehrten Formation is exposed in the easternmost part of the subbasin where it forms readily-identifiable, nearly flat-topped hills. The formation is late Miocene to Pliocene in age and is composed of moderately- to well-indurated andesitic sand to sandstone interbedded with conglomerate, tuffaceous siltstone, and claystone. The Mehrten Formation is approximately 400 feet thick in eastern surface outcrops to over 600 feet thick in the subsurface near Stockton. The top of the Mehrten Formation occurs at depths of approximately 800 to 1,000 feet in the Stockton area. The formation is reportedly semi-confined, at least locally in the Stockton area, due to the inferred extensive fine-grained beds in its upper part. Regional studies indicate that the Mehrten Formation sand commonly yields groundwater from wells at 1,000 gpm. The Mehrten Formation is considered to be the oldest fresh water-bearing formation on the eastern side of the basin(DWR, 2003). 2.3 LOCAL GEOLOGY AND HYDROGEOLOGY Historical boring logs indicate that the Site is underlain primarily by clay, silt, sand and gravel layers of varying thickness. During past drilling activities, groundwater was encountered at approximately 40 to 42 feet below-ground-surface (BGS) (Secor, 1996). Limited groundwater monitoring data are available for the Site; however, data from nearby sites indicate that the 3