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PRE-TANK REPLACEMENT Site Assessment Work Plan 4315 Waterloo Road Stockton, CA. Aegis Project No. 89-047 Page 8 2.8 Site Hydrology Alluvial deposits in the Stockton area contain horizons of gravel, sand, silt, and clay. Horizons of gravels and sand deposits act as important recharge areas and yield large amounts of water to wells while horizons of silt and clay are low in permeability, and do not yield significant amounts of water to wells. The water bearing formation immediately beneath the site is the Victor Formation. Well logs from water and monitoring wells in the area of the site indicate that ground water beneath the site is between 55 and 65 feet below grade. The local hydraulic gradient is anticipated to be toward the southeast (DWR 1967). 2.9 Regional Geology The site lies within the northern section of the San Joaquin Valley, which is located in the southern portion of the Great Valley Province. The San Joaquin Valley is a structural trough extending south from the Sacramento-San Joaquin Delta to the Tehachapi Mountains. The San Joaquin Valley is bounded by the Sierra Nevada in the east and the Coast Ranges in the west. Cretaceous sandstones and shales overlay pre-Cretaceous ultra basic igneous rocks, granitic rocks, and metamorphosed sedimentary and volcanic rocks. Sediments originating from the Sierra Nevada and the Coast Ranges were deposited in a marine environment during the Cretaceous and the Pleistocene periods. Interruptions in the pre- Pleistocene sedimentary depositional cycle occurred several times as volcanic activity, uplift, faulting and/or folding altered the region. From the Pleistocene to the Present, deposition has been alluvial. The subject site is situated near the axis of the San Joaquin trough. Recent Alluvium and the Victor Formation overlay the Laguna Formation. The Laguna Formation consists of clay and sand units and unconformably overlays the andesitic sediments of the Mehrten Formation. From the Pleistocene to the present, the depositional environment has been alluvial. Several thousand feet of unconsolidated silty, sandy and gravelly sediments lie within the basin (DWR 1967). 2.9.1 Mehrten Formation The Miocene/Pliocene Mehrten Formation is composed of andesitic sand and sandstone units derived from Sierran andesitic flows and agglomerates interbedded with conglomerate tufaceous siltstone and claystone horizons. Beneath the center of the San Joaquin Valley the upper surface of the Mehrten Formation is 800 to 1000 feet below the surface. The formation outcrops along the eastern margins of the