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1. INT'RODUCT'ION Under contract to Chevron U.S.A. Inc.,EA was requested to drill and install two groundwater monitoring wells at former Chevron Service Station(SS)9-7780 located at 3246 East Fremont Street, Stockton,California(Figure 1). Because of extended drought conditions and the use of groundwater in the immediate area,the water table had dropped,leaving all seven monitoring wells dry. During the week of 6 August 1990 EA installed two deeper monitoring wells at the eastern and western boundaries of the site to monitor potential migration of the dissolved con- stituent plume:off the site toward three water supply wells located east and west of the site within a few hundred feet(Figure 2). Three:wells closer to the form tank field were abandoned. 1.1 SITE DESCRUMONi The investigated site is located in the eastern part of Stockton,California.at the intersection of E. Fremont Street and Windsor Avenue,about 1,000 feet west of Highway 99,in an area of mixed residential/commercial and industrial character. The facility has been closed, and the buildings and all installations have been removed. The site surface is covered by unimproved fill and is slightly (1-2 feet)elevated above the surrounding area. The site is on flat terrain approximately 25 feet above mean sea level, near the northern end of the San Joaquin Valley. The topography,typical of the flood plaid of the San Joaquin Valley,is flat, sloping gently to the west toward the San Joaquin River. The nearest surface waters are the Stockton Diversion Canal,about three-quarters of a mile northeast, and the intermittent Mormon Slough,about 1.5 miles south of the site. 1.2 SrM HISTORY When the undergrout.4 storage tanks at the site were removed,soil samples were taken from the excavations for the three fuel tanks and the waste oil tank (BTS 1987). The site was investigated in March 1987 by Groundwater Technology, Inc. (GTI 1987), who installed three monitoring wells (Figure 3) around the location of the former tank field. The boreholes were drilled, and soil samples were taken,on 18 March; groundwater from the wells was sampled initially on 19 March. In August 1987,EA sampled the water in nearby water supply wells for analysis and evaluated those samples and the earlier results(EA 1987). EA installed four additional monitoring wells in February 1988 (Figure 4). The wells have been gauged quarterly and sampled when sufficient water was present to permit sampling. c67/97780/tx 1