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CLOSURE REQUEST 5-31-94 . Clay layers are found throughout the sedimentary section. These Clay layers will tend to slow the downward migration and recharge of ground water to the underlying formations . The recharge of ground water will be accomplished by downdip flow from the outcrop areas to the east along the foothills of the Sierra Nevada Mountains, by slow leakage from the overlying clay layers, and by more rapid movement vertically beyond the limits of the limited area lens type clay layers . Ground Water Occurance In The Stockton Area Ground water production in this region is predominantly from the sands and gravels associated with the Laguna and Mehrten formations which are non-marine (continental) sediment deposits of P1io-Pleistocene age. Underlaying the Mehrten formation is the Valley Springs formation which are non-marine (continental) sediments that unconformably overlie the undifferentiated marine Markley-Nortonville formation. Ground water recharge to the above mentioned formations and to the younger formations, surface to near surface deposits situated above them (the Quaternary age Alluvium fan and basin deposits and the Pleistocene Victor formation) , is predominantly vertical leakage and lateral movement downdip along the less permeable layers . The Mehrten formation can be found outcropping to the east of Stockton in the foothills of the Sierra Nevada Mountain Ranges . Ground water recharge of this formation is predominantly from the exposures in the foothills with some vertical leakage from the overlying formations The Laguna formation, which overlays the Mehrten formation does not outcrop in the foothills of this designated area, but outcrops further north and east of Lodi Ground water recharge for the Laguna formation is also predominantly from its exposures in the foothills and seepage from the Mokelumne and Calaveras Rivers . The overlaying Victor Formation and Alluvium deposits are recharged from surface exposures, leakage from the Mokelumne and Calaveras Rivers, and vertical leakage from the surface Basin and Interfan soil layers in the area The direction of free ground water movement in this region is to the west and towards south Stockton from the foothills of the Sierra Nevada Mountain Ranges, and towards the east from the Sacramento - San Joaquin Delta At present, intrusions of more saline waters from the Delta are impairing the quality of the ground water in the Stockton area . LOCAL GEOLOGY AND HYDROGEOLOGY The excavation created during tank removal revealed a relatively flat lying sequence of clay, clayey sandy silt, and silty sands to a depth of 14 feet below the surface . Over excavation of the UST area revealed a silty clay from the 14 foot depth to the base of the excavation at 26 feet below the surface . Exploratory borings investigated the subsurface on and off site to 74 feet below the surface. These borings revealed a similar sequence of pagel5