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08 July 1999 AGE-NC Project No. 95-0130 Page 3 of 10 hydrocarbons were detected in soil samples collected from MW-4. Dissolved TPH-g was not detected in initial ground water samples collected from MW-4. However, TPH-g and benzene were detected in MW-4 in later sampling events at concentrations as high as 78µg/1 and 10µg/1, respectively(Tables 2 and 3). • In January 1996,one soil boring(MW-5)was advanced upgradient of the former UST area to a depth of 70 feet. Soil boring MW-5 was completed as a 2-inch diameter ground water monitoring well. Petroleum hydrocarbon impacted soil was encountered at depths of 45 and 50 feet. Dissolved TPH-g was detected in the initial ground water sample from MW-5 at a concentration of 23,000 µg/1 (Tables 2 and 3). • An 8-hour soil vapor extraction(SVE)pilot test was conducted at the site in January 1996. The pilot test was performed to provide vapor extraction data for proper system design of a remediation system. • In March 1996, a Corrective Action Plan (CAP) was submitted to the PHS-EHD, and included details of the SVE pilot test. Recommendations in the CAP included installation of an SVE system to remediate hydrocarbon-impacted soil in conjunction with an in-situ air sparging (IAS) system to remediate hydrocarbon-impacted ground water. • Between January 1995 and June 1999, sixteen ground water monitoring events have been performed at the site to monitor dissolved petroleum hydrocarbon concentrations in ground water and to characterize ground water flow direction and gradient on the site (Table 3). 2.3. REGIONAL GEOLOGIC SETTING The property is situated within the Great Valley Geomorphic Province of California, a large, elongate, 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 Siena 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 (JHS) association. The JHS soils are located within basins, and generally consist of moderate to poorly drained fine textured soils. The soils are generally derived Advanced CeoEnvironmental,Inc.