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r ' 1995 November 22, _ Page 4 Chevron) at the northeast corner of I lth and Central Avenue during the period of approximately 1913 to at least 1962 Regional Geology The site Iles is located in San Joaquin County, in the northern portion of San Joaquin ' Valley San Joaquin Valley is a broad, alluvium filled basin bound to the east by the Sierra Nevada Mountain Range and to the west by the Coast Ranges The total thickness of the alluvial deposits in the valley is estimated to be thousands of feet The alluvial sediments generally consist of poorly sorted fine sand and`silt with less extensive lenses of medium to coarse-grained sand and gravel Quaternary lacustnne deposits also occur in the central ' and western portions of the valley, and consist of clay, silt and sand In the site vicinity, 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 sands and gravels 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 groundwater flow The nearest surface water course to the site is the Tom Paine Slough, located approximately 2 mules to the north and northeast SCOPE OF WORK ' The scope of work for this investigation included the follow • 'Nme exploratory bonngs�designated U-A through U-G, were drilled on site to ' total depths ranging from approximately 6 to 26 feet bgs on May 18 and ,19, 1995 The boring locations were selected based on PACIFIC'S current understanding of both the former station layout and local groundwater flow direction The borings were drilled to assess subsurface conditions near the former UST complexes, product island, product lines, and waste oil storage tank Borings were also drilled in the southern portion of the site to characterize tbackground soil quality The borings were generally drilled to depths of 15 to 20 (5 to 10 feet below the groundwater surface) However,three borings, designated U-E, U-F, and U-G, were drilled to depths greater than 20 feet bgs to further assess soil hthology • Three groundwater monitoring wells, designated U-1 through U-3, were installed ' on site on May 18 and 19, 1995 The wells were constructed with 2-inch diameter PVC casing, with a screened interval from 6 to 20 feet bgs The well locations were selected to monitor groundwater conditions near potential source areas, The wells were generally placed in anticipated downgradient direction 3100971 AIREPOR'i l _