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20 March 1996 <br /> AGE-NC Project No 95-0130 <br /> Page 2 of 23 <br /> approximately eighteen inches below surface grade, so the volume of the tank can not be determined <br /> A plan to remove the abandoned USTs is presently being considered by the property owners <br /> 23 INITIAL INVESTIGATION <br /> On 28 and 29 December 1994, five soil borings (B1 - B5) were drilled at the site Borings B1, B3, <br /> and B5 were advanced to depths of 35 feet below site grade (bsg) near the area of the former USTs <br /> Borings B2 and B4 were advanced to depths of 50 and 65 feet bsg, respectively Petroleum <br /> hydrocarbons were detected in soil samples collected from each boring <br /> On 08 and 09 January 1995, four additional soil borings (MW-1, MW-2, MW-3 and B6) were drilled <br /> at the site Boring B6 was drilled to a depth of 65 feet bsg Bonngs MW-1 through MW-3 were <br /> advanced to depths of 80 feet bsg and completed as 2-inch diameter ground water monitoring wells <br /> Hydrocarbon-impacted soil and ground water were detected in samples collected from each <br /> monitoring well Table 1 summarizes the analytical results of all boring and monitoring well soil <br /> samples <br /> I2 4 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 lying, 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 sedimentary layers consist of a great thickness of predominantly unconsolidated alluvial <br /> and floodplain deposits (dominantly sand, silt, and clay) of Quaternary age, which are derived from <br />' the predominantly granitic mountains of the Sierra Nevada Beneath the upper sedimentary deposits <br /> he 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 5 GROUND WATER DEPTH AND GRADIENT <br /> The water-table surface beneath the site currently occurs at a depth of approximately 55 feet below <br /> surface grade (bsg) The uppermost ground water flows in an unconfined aquifer With depth, the <br /> aquifer becomes semi-confined due to the presence of heterogeneous layers of sediments <br /> Review of ground water surface elevation contour maps published by the San Joaquin County Flood <br /> Control and Water Conservation District suggests that regional ground water flow is towards the <br /> northeast Conversely, previous ground water monitoring events have shown that the local ground <br />