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27 March 2000 AGE-NC Pro feet No, 98-0450 Page 2 of 19 Tlie Great Vallee Province has been filled to its present elevation with trick sequences of sediment rangim, in agc from Jurassic to present day. creating a nearly flat lying alluvial plain extending from the Tehachapi Mountains in the south to the Klamath Mountains in the north. The Valley is enclosed by the 11111cous Sierra Nevada to the east and the sedimentary and metamorphic Coast Ranges to the West. The surficial and upper several hundred feet of subsurf:ace layers consists of a great thickness of predominantly unconsolidated alluvial and flood plain deposits(dominantly sands. silts. and clays) Of Quaternary age, which are derived from the predominantly granitic mountains of the Sierra Nevada. Beneath the upper sedimentary deposits lie a thick sequence of marine deposits of N4e.sozoic age. These units are further underlain by a pre-JuraSSiC complex of igneous and metamorphic basement rock. The Great Valley Province is subdivided into two major divisions designated the Sacramento and San Joaquin Valleys. The Modesto. Riverbank and Turlock Lake Formations and overlying Recent alluvium are the principal soLu-ces of domestic ground water in the 1-31,500-square mile San Joaquin Valley Ground Water Basin (Basin 5-22). This basin is drained primarily by the San Joaquin River. Based on the General Sol] Map from the San Joaquin County Soil Survey. published by the United States Department of Agriculture Soil Conservation Service in 1992. the site area is within the .Iacktone-l-Iollenbeck-Stockton (JI IS) association. 'I he JHS soils are located within basins. and generally consist of moderate to poorly drained line textured soils. The soils are oencrally derived from both marine and non-marine sediment.. and are generally formed in alluvium derived from mixed rock sources. 2.3. GROUND WATER DEPTH AND FLOW DIRECTION Ground water is currently cncoLmtered at a depth of approximately 45 feet bsg and has a dominant southcrly flow direction. First ground water occurs in a generally sandy layer extending from approximately 45 feet to 55 feet bsg. ']'he uppermost ground water flows in an unconfined aquifer. With increasing depth. the aquifer becomes semi-confined due to the presence of tine-grained sediments. The sedimentary units are relatively horizontal and continuous over the site. Relative ground water elevation maps for the site have been included in previous quarterly reports prepared by AGE. Information collected from the site over the past four years indicates that the ground water flow at the site is directed toward the south. The depth to ground water meaSUred in site monitoring wells Lind soil borings ranged from approximately 45 to 55 feet bsg. Advanced(:coVm it-numentai.Inc.