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1 ' was generally detected from wells MW-1 through MW-4 at concentrations as high as 430 (µg11). Low concentrations of MTBE were generally detected from wells MW-1 and MW-2 at concentrations as high as 21 µg/l. ' 5.0. REGIONAL GEOLOGIC SETTING The property is situated within the Great Valley Geomorphic Province of.California, a large, ' elongate, north-northwest trending, asymmetric structural trough. The Great Valley Province has been filled with thick sequences of sediment ranging in age from Jurassic to Recent,creating a nearly flat-lying alluvial plain, extending from the Tehachapi Mountains in the south to the Klamath ' Mountains in the north. The western and eastern boundaries of this province are comprised of the California Coast Range and the Sierra Nevada, respectively. Rocks composing the basement ' complex of the province have not been completely defined but are believed to be metamorphic and igneous in origin. The northern and southern portions of the Great Valley Province have been designated the Sacramento and San Joaquin Valleys, respectively. ' Based on the General Soil 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 Jackson-Hallenbeck-Stockton (THS) association. The JHS soils are located within basins, and generally consist of moderate to poorly drained fine textured soils. The soils are generally derived from both marine and non-marine sediment, and are generally formed in alluvium derived from ' mixed rock sources. The Modesto,Riverbank,Turlock Lake Formations and overlying recent alluvium are the principal ' source of domestic ground water in the 13,500 square-mile San Joaquin Valley Ground Water Basin (Basin 5-22). This basin is drained primarily by the San Joaquin River. The nearest surface water feature in the vicinity of the property is the Stockton Diverting Canal located approximately 2,500 ' feet north of the site. ' 6.0. GROUND WATER DEPTH AND GRADIENT ' Regionally, the estimated depth to ground water at the site is between 50 feet below surface grade (bsg)based on information from Lines of Equal Depth of Water Wells Spring 1999, published bythe San Joaquin County Flood Control District and Water Conservation District (SJCFCD). Based on ' review of a regional ground water elevation reap published by the SJCFCD, the prevailing ground water flow direction in east Stockton is generally toward the southeast,although flow directions may be both locally and seasonably variable. ' Locally since February 2000, depth to ground water measurements have varied in monitoring wells at the site between 45 feet and 57 feet bsg. Ground water flow direction was consistently inferred ' towards the southeast at a gradient of 0.001 ft/ft. ' Advanced GeoEnvironmental,Inc. 1