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11 August 1997 AGE-NC Project No. 95-0118 Page 2 of 9 • Between 1994 and 1995 six ground water monitoring wells (MW-1 through MW-6) were installed on the site,to delineate the extent of hydrocarbon-impacted soil and ground water in the vicinity of the former UST. Hydrocarbon-impacted soil and ground water were detected in two of the wells. • In January 1996, two borings were advanced.around the former UST area and completed as soil vapor extraction/in-situ air sparging wells. Remedial feasibility studies were conducted in February 1996. In June,1996, a Corrective Action Plan(CAP).was submitted to the PHS- EHD. Recommendations in the CAP included installation of a soil vapor extraction(SVE) system to remediate hydrocarbon-impacted soil and an in-situ air sparging {IAS) system to remediate hydrocarbon-impacted ground water. 2.3. REGIONAL GEOLOGICALIHYDROGEOLOGICAL SETTING According to the Geologic Map of California, published in 1977 by the State of California Department of Mines and Geology, the site area is within the Great Valley Geomorphic Province of California, a large, elongate, northwest trending structural trough. The Valley is a nearly flat, { = 1 elongate trough trending northwest and southeast for approximately 450 miles. The Great Valley Province has been filled to its present elevation 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 Valley is enclosed by the igneous Sierra Nevada to the east and the sedimentary and metamorphic Coast Ranges to the west. The surficial and upper several hundred feet of subsurface 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 Mesozoic 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 sources 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. Furthermore, 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 Jacktone-Hollenbeck-Stockton(JHS)association.The JHS soils are located within basins, and generally consist of moderate to poorly drained fine textured soils. The soils are generally Advanced GeoEnvironmental,Inc.