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1 it <br /> 8 November 1995 <br /> AGE-NC Project No 95-0130 <br /> Page 2 of 7 <br /> 23 REMAINING UNDERGROUND STORAGE TANKS <br /> E <br /> One fill port is located on the north side of the gas station office It is possible that an additional UST <br /> are located in this area The fill port is blocked at depths of approximately eighteen inches below <br /> surface grade, so the volume of the tanks can not be determined A plan to remove the remaining <br /> USTs is presently being considered by the property owners <br /> 2 4 INITIAL INVESTIGATION <br /> On 28 and 29 December 1994, five soil borings (B I - 135) were drilled at the site B 1, B3, and B5 <br /> were advanced to depths of 35 feet below site grade(bsg) near the area of the former USTs Borings <br /> B2 and B4 were advanced to depths of 50 and 65 feet, respectively Significant levels of hydrocarbon <br /> impaction were detected in soil samples Borings B 1 through B5 were backfilled with Portland <br /> cement <br /> On 8 and 9 January 1995, four additional soil bonags (MWA, MW-2, MW-3 and B6) were drilled <br /> at the site Bonng B6 was dnlled to a depth of 65 feet Wells MW 1 through MW-3 were advanced <br /> to depths of 80 feet bsg and completed as 2-inch diameter ground water monitoring wells Boring <br /> B6 was backfilled with Portland cement Hydrocarbon-impacted soil and ground water were detected <br /> in samples from each monitoring well <br /> 25 REGIONAL GEOLOGIC SETTING <br /> The subject site is located in the eastern portion of the Great Valley geomorphic province of <br /> California The valley is a nearly flat, elongate trough trending northwest and southeast for <br /> approximately 450 miles The valley is enclosed by the igneous Sierra Nevada to the east and the <br /> sedimentary and metamorphic Coast Ranges to the west The surficial and upper several hundred feet <br /> of subsurface layers consists of a great thickness of predominantly unconsolidated alluvial and <br /> floodplain deposits(dominantly sands, silts, and clays) of Quaternary age, which are derived from the <br /> predominantly granitic mountains of the Sierra Nevada Beneath the upper sedimentary deposits he <br /> a thick sequence of marine deposits of Mesozoic age These units are further underlain by a pre- <br /> Jurassic complex of igneous and metamorphic basement rock <br /> 2 6 GROUND WATER DEPTH <br /> The water-table surface beneath the site occurs at a depth of approximately 60 feet below surface <br /> grade (bsg) The uppermost ground water flows in an unconfined aquifer With depth, the aquifer <br /> becomes more confined due to the presence of mixed heterogeneous layers of sediments <br />