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Parsons Engineering Science, Inc f dark gray clayey silt with moderate to well sorted, fine sand lenses The reported hydraulic ip conductivity values associated with these materials ranges from 10' to 10' cm/sec (Freeze and Cherry, 1979) The Central Valley is drained by two large river systems the Sacramento River (northern part of the valley) and the San Joaquin River (most of the southern part of the valley) The two rivers meet about 13 6 miles northwest of the facility l The closest surface water features are the easternmost reach of the Port of Stockton, approximately 1,000 feet north-northwest of the facility and the Morahan Slough, approximately 1,300 feet southwest of the facility Historically, groundwater was encountered at a depth of 25 to 35 feet below land surface, however, recent drought conditions have caused the water table to be lowered to as much as 44 feet below ground surface Groundwater flow direction in recent months has been toward the southwest under a hydraulic gradient of less than 0 002 ft/ft Surface Water and Groundwater Use The City of Stockton obtains its municipal and industrial water supplies from a combination of treated surface water and groundwater supply wells located in residential areas in and around Stockton According to the State of California, Department of Water Resources well inventory records, no domestic or potable water supply wells are located within I mile of the Greyhound facility ti Previous Site Investigations In May 1991, Greyhound removed one 12,000-gallon UFST and associated piping from the site (Figure 2) Soil samples were collected to assess the potential for environmental contamination associated with the normal operation of the UFST (Groundwater Technology, Inc 199 1) Samples collected at the base of the tank pit excavation contained total petroleum hydrocarbons as diesel (TPH-D) at concentrations as high as 4,200 rrulligrams per kilogram (mg/kg) In addition, soil samples collected from the piping trench contained TPH-D up to 11,000 mg/kg Based on these analytical results, the San Joaquin County, Public Health Service, Environmental Health Department (PHS-EHD) requested additional investigation at the site to assess the extent of soil and groundwater contamination In September 1991, a total of five soil borings (including; B-1 and B-2 on Figure 2) were advanced by Parsons ES to characterize the vertical and horizontal extent of hydrocarbon contamination at the site [Engineering-Science, Inc (ES), now known as Parsons ES, 1991] Three of the borings were completed as monitoring wells (MW-1, 2, and 3 on Figure 2) All ' soil and groundwater samples collected during the investigation were submitted for analysis for total petroleum hydrocarbons as gasoline (TPH-G), TPH-D and aromatic hydrocarbons [benzene, toluene, ethylbenzene, and total xylenes (BTEX)] TPH-G was not detected in any of the soil samples TPH-D and ethylbenzene were detected in three of the five samples collected [MW-2, 520 mg/kg and 0 0072 mg/kg respectively, MW-3, 500 mg/kg and 0 024 mg/kg, respectively, and B-I, 6,800 mg/kg and PARESSYR01 VOLT R IWP1727607 80969127607R03 WW6 3