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z Formations. Occasional pockets of fresh water are found in the Delta deposits, but generally speaking the formation contains poor quality water. This unit is designated as Dos Palos Alluvium by(Wagner and others, 1991). Laguna Formation. The Laguna Formation is Plio-Pleistocene in age and consists of discontinuous lenses of stream laid sand and silt with lesser amounts of clay and gravel. There are no regionally significant fine-grained intervals that could cause water pressure conditions, although the heterogeneous nature of the sediments causes local confinement. From the Mokelumne River area located near Lodi, the formation thickens from approximately 400 feet to approximately 1,000 feet in the Stockton area. Regionally,yields of 1,500 gpm have been reported from highly permeable beds,but average yields are about 900 t gpm. Groundwater occurs under unconfined to locally semi-confined conditions within this unit. Occasional minor perched water zones are encountered in this formation, .particularly in the Mokelumne River area near Lodi_ Mehrten Formation. This 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(Wagner and others,1990). The Mehrten Formation is approximately 400 feet thick in eastern surface outcrops to over 600 feet thick in the subsurface near Stockton. It is reported to be 1,300±feet thick at McDonald Island(Page, 1986). The top of the Mehrten Formation occurs at depths of approximately 800 to 1,000 feet in the Stockton area. Regional studies indicate that Mehrten Formation sands commonly yield on the order of 1,000 gpm from wells.The formation appears to be semi-confined at least locally in the Stockton area,due to the inferred extensive fine-grained beds in its upper part. 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. E M sinlMEnvironmcnta❑1STE\WorkplanlWorkplanl l 192007—pd 15 e