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February 14 1997 Page 2 BACKGROUND Site Desci iption The site is a former Chevron retail service station (9-1918) located at the northwest corner of the intersection of East Harding Way and El Dorado Avenue in Stockton, California (Figure 1) The site operated as a retail gasoline station from 1947 to 1988 and consisted of two generations of product facilities (Figure 2) The first generation facilities, based on available data, consisted of four steel underground storage tanks (USTs), (4,000-, 3,000-, 1,000-, and 550-gallon) which were initially abandoned in place In 1984, a second set of USTs were installed and consisted of three fiberglass gasoline USTs (10,000-gallon) located in a common excavation in the central portion of I� the site, and one fiberglass waste-oil UST (1,000-gallon) located behind the former service station building Both generations of facilities have been removed from the site The site is bounded to the east by a Unocal service station, to the west by a residence, to the south by a restaurant and ARCO AM/PM NLni-Mart, and to the north by medical offices Geology /Hydrogeology The site is located in the northern part of the San Joaquin Valley and is underlain by approximately 3,000 feet of alluvial gravel, sand, silt, and clay Groundwater in these alluvial deposits is unconfined to semiconfined (GTI, March 1994) Significant ground- water extraction for irrigation and public water supply has reduced groundwater eleva- tions in Stockton to approximately -30 to -40 feet mean sea level (MSL) Subsurface investigations indicate that the site is underlain by interbedded clayey silts, and silty clays with sand lenses to the total depth explored (70 feet below ground surface [bgs]) Historically, depth to groundwater has ranged from 35 36 feet bgs (Well MW-8, February 7, 1989) to 58 01 feet bgs (Well MW-10, October26, 1992) Quarterly groundwater monitoring data from November 1988 to present indicate that groundwater beneath the site consistently flows in a northeasterly direction at an approximate gradient of 0 002 to 0 003 feet per feet The nearest surface water to the site is Yosemite Lake located at the east end of Smith Canal, approximately 3,500 feet west of the site, and the deep water channel of the San Joaquin River located approximately 4,500 feet south of the site The site lies at an approximate elevation of 13 feet above MSL, topography in the vicinity is essentially flat Regional groundwater flow paths merge toward central and western Stockton due to extensive pumping of water wells in this area (EA Engineering, Science and Technology, Inc [EA], January 1989) ` r 3201337E11CLOSURE3 1