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28 January 1998 AGE-NC Project No. 96-0232 Page 2 of 6 as micrograms per liter) in later ground water sampling events. • In March 1995, four 12,000-gallon gasoline USTs and one 12,000-gallon diesel UST were removed from the site. Hydrocarbon petroleum impacted soil was encountered from the sidewalls of the excavation and in ground water filling the excavation pit. During excavation activities, ground water monitoring well MW-3 was destroyed. • In July 1996, ground water monitoring well MW-4 was installed at the site. Samples collected from MW-4 detected dissolved MTBE at a concentration of 0.74 ppb. Ground water samples collected from MW-I and MW-2 detected MTBE at concentrations up to 310 ppb. • In August 1996 and May 1997, ground water samples collected from MW-1, MW-2 and MW-4 detected MTBE at concentrations up to 440 ppb; total petroleum hydrocarbons quantified as diesel (TPH-d) was detected from MW-2 at concentrations up to 110 ppb. • In August and September 1997,five soil probe borings(P1 through P5) were advanced at the site. MTBE was detected in one soil sample collected (P-1 at 35 feet) at a concentration of 140 ppb (reported as micrograms per kilogram). MTBE was detected in ground water samples collected from P1 and P5 at concentrations up to 410 ppb. 2.3. REGIONAL GEOLOGIC SETTING According to Geologic Map of California (1966, California Department of Conservation Division of Mines and Geology), the site area is within the Great Valley Geomorphic Province (GVGP) of California, a large,elongate, northwest trending structural trough. The GVGP is subdivided into two major divisions designated the Sacramento and San Joaquin Valleys, which have been filled to the 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 western and eastern boundaries of this province are comprised of the California Coast Range and the Sierra Nevada, respectively. Based on a 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 described as within the Guard-Devries-Riblancho (GDR) association. The GDR soils are located within basin rims along the 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. Advanced GeoEnvirenmental,Ine.