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_ _ _ _ a G i. - -� - Of - It tank was underlain by an underground concrete pad, which was not removed during excavation. Groundwater was observed in the excavation at a depth of approximately 6 feet below surface during the December 1989 tank removal activity. F "..s PACIFIC installed Groundwater Monitoring `,Wells U-1 through U-3 on October24 i and 25, 1990. Selected soil samples were analyzed for total petroleum hydrocarbons (TPH-gasoline and TPH-diesel), halogenated volatile organics (EPA 8010), and `-' benzene, toluene, ethvibenzene, and total xylenes (B"I'}=X compounds). The hydrocarbon concentrations ranged from none-detected to 1,800 ppm TPH-gasoline, i 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 �j exception of 3.2 and 5.2 ppm TPH-diesel in Borings U-1 and U-3, respectively. Details of the first phase of PACIFIC's. site investigation a:a described in a Soil and Groundwater Investigation Report dated February U, 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 4.006. 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 s contained no detectable concentrations of any of the analyzed compounds. Diesel was not detected in any of the wells sampled. 's r L: Regional Hydrogeology , i The site lies at an elevation of approximateiy 47 feet above mean sea level and is i -' located in San Joaquin County, in the northern portion of the San Joaquin Valley. The 4 i San Joaquin Valley is a broad, alluvium-tilled basin bounded to.the east by the Sierra Nevada Mountain Range and to the west by the Coast Ranges. The total thickness of alluvial fill in the valley is estimated to be thousands of feet. ne alluvial sediments generally consist of poorly sorted silt and fine sand with less extensive lenses of medium to coarse-grained sand and gravel. Quaternary lacustrine deposits also occur in the i central and western portions of the valley, and consist of clay,silt, and sand_ 3 In the vicinity of the site, surficial deposits are characterized as distal alluvial fan and — - -- --flood plain deposits._These deposits generally are less than-l0 feet thick-and-consist-of ---- --- -, -- -- ? ' . . silts, fine sands, and less extensive coarse sand and gravel 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 LIN { regional drainage patterns suggest a northerly direction of groundwater flow. 3100101/REPORT2 2 September 30,191 t -