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06 December 2004 AGE-NC Project No.04-1238 Page 2 of 6 as gasoline (TPH-g) were detected at the extent of the northern portion of the common gasoline excavation at a concentration of 460 milligrams per kilogram(mg/kg)in soil.Based on information available at EHD (Cambria prepared Site Summary and Closure Path Recommendations dated 01 November 1996) it appears this soil contamination was never abated. • During excavation activities on 09 July 1993,two grab groundwater samples(water#1 and water#2)were collected from the excavation. TPH-g,benzene, toluene, ethylbenzene and xylenes (BTEX) were detected in ground water samples at maximum concentrations of 34,000 micrograms per liter (µg/1), 3,200 µg/1, 1,600 µg/1, 840 µg/1 and 4,300 µg/1, respectively (American Consulting Remediation and Construction Inc. prepared Tank Removal-Soil Sampling Report dated 10 August 1993).Based on the information available at EHD, it does not appear that the hydrocarbon impact to ground water was addressed adequately in this area. • On 20 January 1993, monitoring wells MW-1 through MW-3 were installed on-site and a quarterly ground water monitoring program was initiated.On 20 March 1995,two additional ground water monitoring wells were installed off-site to determine the lateral migration of petroleum hydrocarbon impacted ground water. • Quarterly ground water monitoring occurred from December 1992 until June 1996. TPH-g, benzene and methyl-tertiary butyl ether (MTBE) were detected in ground water samples collected during monitoring events at maximum concentrations of 2,400 µg/1, 11 µg/1 and 51 µg/1,respectively(Cambria prepared Site Summary and Closure Path Recommendations dated 01 November 1996). 2.2. REGIONAL GEOLOGIC/HYDROGEOLOGIC SETTING The site is located within the northern San Joaquin Valley which comprises part of the Great Valley geomorphic province of California. The San Joaquin Valley is formed by the Great Valley geosyncline,which is a large, elongate,northwest-trending asymmetrical structural trough(basin). It is bordered by the Coast Ranges to the west, the Klamath Mountains and Cascade Range to the north, and the Sierra Nevada to the east. This trough has been filled with sediments derived from both marine and continental sources. The thickness of the sedimentary fill ranges from thin veneers along the valley edges to greater than 20,000 feet in the south central portion of the valley. The sedimentary formations range in age with the older deposits being primarily marine in origin and the younger deposits being primarily continental. Continental-derived sediments were primarily deposited in lacustrine, fluvial, and alluvial environments with sources being the mountain ranges surrounding the valley(Olmsted and Davis, 1961). The site is located on unconsolidated and semi- Advanced GmEnvironmental,Inc.