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♦ tank was underlain by an undergrorTid concrete pad, which was not removed during excavation. Grwridwater was observed in the excavation at a depth of approximately 6 feet below su.-face during the December 1989 tan];removal activity. ' PACIFIC installed Groundwater Monitoring Wells U-1 through U-3 on October 24 and 25, 1990. Selected soil samples were analyzed for total petroleum hydrocarbons (TPH-gasohne and TPH-diesel), halogenated volatile organics (EPA 8010), and benzene, 'oluene, ethylbenzene, and total xylenes {BTEX compounds}. The hydrocarbon concentrations ranged from none-detected to 1,800 ppm TPH-gasoline, and from none-detected to 710 ppm TPH-diesel at the 5 to 6-1/2 foot interval. The concentrations attenuate to none-detected at the 10 to 11-1/2 foot interval with the exception of 3.2 and 5.2 ppm TPH-diesel in Borings U-1 and U-3, respectively. Details of the first phase of l?ACIFIC's site investigation are described in a Soil and Groundwater Investigation Report dated February 13, 1991. _ Static groundwater occurred at a depth of approximately 9-1/2 feet. below grade in November 1990 with a.calculated northerly flow direction at a gradient of 0.006. # Groundwater samples were analyzed for TPH-Gasoline, TPH-diesel,BTEX compounds, and halogenated volatile organics (EPA 624). Samples obtained from Wells U-1 and ^ U-2 contained 140 ppb and 670 ppb gasoline, respectively. Wells U-3 and WSW-1 contained no detectable concentrations of any of the analyzed compounds. Diesel was not detected in an of the wells sam led � y P 1.2 Regional Hydrogeology i . F The site lies at an elevation of approximately 47 feet above mean sea level and is located in San Joaquin County, in the northern portion of the San Joaquin Valley. The San Joaquin Valley is a broad, alluvium-filled basin bounded to the east by the Sierra Nevada Mountain Range and to the west by the Coast Ranges. The total thickness of ,-" alluvial till in the valley is estimated to be thousands of feet. The alluvial sediments generally consist of poorly sorted silt and fine sand Mih less extensive lenses of medium to coarse-;rained sand and gravel. Quaternary lacustrine deposits also occur in the central and westernportions of the valley, and consist of clay, silt, and sand. In the v]ciniry of the site, surficial deposits are characterized as distal alluvial fan and flood plain deposits. These deposits generally are less than 10 feet thick and consist of silts, fine sands, and less extensive coarse sand and ;ravel lenses. Underlying deposits are referred to as older, alluvium and reach a thickness of up to 400 feet. These deposits consist of interbedded clay, silt, sand. and gravel. Geomorphology and � . . regional drainage patterns suggest a northerly direction of grrundwater clow. 3100101/REPORT-- ? September 30, 1191 u