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PRE-TANK REPLACEMENT Site Assessment Work Plan 4315 Waterloo Road Stockton, CA. Aegis Project No. 89-047 Page 9 valley, dips toward the west and is nearly flat lying near the center of the valley. The unit thickens to approximately 600 feet at its western margin (DWR 1967). 2.9.2 Laguna Formation The Pleistocene Laguna Formation unconformably overlays the Mehrten Formation. The Laguna Formation consists of heterogeneous fluvial continental sediments of non-volcanic sands and gravels. Included within the Formation is the Arroyo Seco gravel, a thin unit south of the Mokelumne River consisting of weathered sand and cemented gravels. The Laguna Formation outcrops along the eastern margins of the valley, dips toward the west and is nearly flat lying beneath the center of the valley. The unit thickens in a westerly direction to a approximately 1000 feet beneath the Stockton area (DWR 1967) 2.9.3 Victor Formation/Recent Alluvium The Victor Formation/Recent Alluvium occurs near the center of the valley and is generally coarser grained than the underlying Laguna Formation. The formation consists of heterogeneous deposits of sand and gravels with discontinuous lenses of silt and clay. The Formation dips gently and thickens to the west. At it's western margin the Formation is approximately 100 feet thick (DWR 1967). 2.9.4 Structure San Joaquin County lies near the northwest trending axis of a regional syncline with a gently dipping east flank and a fairly steeply dipping west flank. Evidence has been found of extensive folding and faulting of the basement rocks and pre-Pleistocene marine sediments. There is no evidence of faulting or folding in the continental sediments of the valley except along the western flank. (DWR 1967) 2.10 Regional Hydrogeology Three water-bearing formations have been identified beneath the subject area including the Mehrten Formation, the Laguna Formation and the Victor Formation. Ground water occurs in the Mehrten Formation under semi-confined conditions. Permeability in the Mehrten Formation ranges from high to low. Wells that penetrate the Mehrten Formation yield as high as 1000 gallons per minute (gpm). Ground water occurs within the Laguna and Victor Formations under both unconfined and semi-confined conditions. Permeability in both formations varies greatly. Wells penetrating the Victor formation generally also penetrate the Laguna Formation. Yields as high as 1500 gpm have been reported from wells penetrating the Laguna Formation (DWR 1967).