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URS ' September 19, 2003 Page 2 I1.2 Previous Investigation and Remedial Action Environmental work completed at the site to date includes the following • August—September 1995 EMCON witnessed the installation of six vertical soil borings (B-1, B-3, B-4, B-5, B-7, and B-8) and two 45° angle borings (B-2 and B-6) Three borings (B-1, B4, and B-5) were converted to monitoring wells (MW-1 through MW-3, respectively) Three borings (B-3, B-7, and B-8) were converted to dual completion vapor extraction wells (VW-1 through VW-3, respectively) (Delta, 2001) • October 1996 EMCON witnessed the installation of one vertical soil boring (B-9), subsequently converted to a monitoring well (MW-4) (Delta, 2001) • June—July 1999 Two 10,000-gallon, and one 8,000-gallon double-walled fiberglass I gasoline USTs, associated dispensers, and piping were removed In the same UST cavity two 20,000-gallon USTs, associated dispensers, and piping were installed The excavations were backfilled with pea gravel Analysis of soil samples taken from the UST replacement activities indicated total petroleum hydrocarbons as gasoline (TPHg), benzene, and methyl tertiary butyl ether(MtBE) contamination in the soil (Pinnacle, 1999) See. lF,vet 4 -6,r soil resvll-s a.nj 1eeA&Ia August 1999 The IT Group (IT) conducted a soil and groundwater assessment consisting of two cone penetration test (CPT) borings (CPT-2 and CPT-3) on the eastern edge of the site and one CPT north of the site (CPT-1) Benzene and MtBE were detected in groundwater samples from CPT-2(Delta, 2001) ' • September 2000 Delta conducted a soil vapor extraction test at the site Three separate 4- hour tests were conducted Soil vapors were extracted from well VW-1A, VW-2A, and VW-3A The tests indicated that extraction of soil vapors from these wells at 9 to 10 cubic ' feet per minute (efm), with 50 to 100 inches of water column (" WC), resulted in a radius of influence between 34 and 45 feet The initial hydrocarbon mass extraction rates were calculated to be between 1 10 and 3 88 pounds of TPHg per day (Delta, 2000) ' 2001 Delta installed three monitoring wells (MW-5, MW-6, and MW-7) MW-5 is east of the site on American Street MW-6 and MW-7 are in Oak Park, north of the site on the other ' side of Alpine Street(Delta, 2001) 1.3 Regional Geology and Hydrogeology ' The site is located in the northern portion of the San Joaquin Valley The San Joaquin Valley is part of the Great Valley Province, which is a northwest-southeast trending asymmetrical structural trough The San Joaquin Valley is a mayor California groundwater basin bordered by the Sierra Nevada Mountain ' Range to the east, the North Coast Mountain Range to the west, the Tehachapi Mountains to the south, and the Sacramento Valley to the northwest The elevation of the site is approximately 15 feet above mean sea level K 1WprocessV5 BP ARCo1021861CAP Aug03\02186 CAP03 doc