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I31 August 2000 ' AGE Project No 96-0232 Page 9 of 25 ' MTBE was detected at concentrations as high as 580 µg/1 The steadily decreasing concentrations of MTBE quantified in ground water samples collected from EW-1 suggest plume control and plume mass reduction in the area of EW-1 A report of findings (Quarterly I Report - Second Quarter 2000, dated 19 July 2000) documenting investigation activities performed in May 2000 was prepared by AGE and submitted to the PHS-EHD and the RWQCB I Additional site background information was prepared by AGE and submitted to the RWQCB in the Problem Assessment Report dated 19 Jude 2000 2 3 REGIONAL GEOLOGIC SETTING The Geologic Map of California, published in 1966 by the California Department of Conservation I Division of Mmes and Geology, shows the site area within the Great Valley Syncline(GVS),a large, elongate, northwest trending structural trough The GVS is subdivided Into two mayor divisions designated the Sacramento and San Joaquin Valleys, which have been filled to the present elevation lie with thick sequences of sediment ranging in age 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 western and eastern boundaries of this province are comprised of the California Coast Range and the Sierra Nevada, respectively Based on 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 Guard- Devnes-Rioblancho (GDR) association The GDR soils are located within basin rims along the I eastern edge of the Sacramento-San Joaquin Delta area, and generally consist of poorly drained, fine to coarse textured soils The GDR soils generally formed in alluvium from mixed rock sources The Modesto,Riverbank,Turlock Lake Formations and overlying recent alluviumm 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 White Slough, located approximately 7,500 feet southwest of the site AGE has been informed that ground water in the area of the Flag City Chevron site below 150 feet bsg is not of beneficial use due to high total dissolved solids (TDS) Advanced GeoEnvironmental,Inc. I