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I I20 January 2005 S AGE-NC Project No 97-0343 ' Page 6 of 21 ' Vadose Zone-Extending from surface grade to 62 feet bsg,the depth to the water table The vadose zone is predominantly fine grained material (silt/clay)with fine grained soil occurring from surface grade to 62 feet bsg with a fine to coarse grain sand unit occurring between 25 feet and 45 feet bsg Iacross the site The zone is characterized by low pore pressure Hydrostratigraphic Unit 1 (HU 1), at the top of the current saturated zone,extends from 62 feet to 80 I feet bsg and is comprised of silty sand to sandy silt and characterized by predominantly high pore pressure ' Hydrostratigraphic Unit 2 (HU2) extends from 80 to 96 feet bsg, is a mix of coarse grained sands and gravelly sands and silty clay, and is characterized by low pore pressure IHydrostratigraphic Unit 3 (HU3)extends from 96 feet bsg to and possibly beyond 134 feet bsg Silt and clay are the dominant soil type in the interval, though sand is present The interval is characterized by pore pressure variations from moderate to high pressures I Hydrostratigraphic units are depicted in cross sectional views A-A' and B-B' (Figures 4 and 5 ) 5.0. ASSESSMENT OF HYDROCARBON-IMPACTED I C SOIL ' Between October 1986 and July 2003, soil sampling was performed at fifteen locations, six soil borings (B1 through B6), one shallow tank excavation sample (tankpit) and eight ground water I monitoring well pilot borings (MW I through MW8)as part of a subsurface petroleum hydrocarbon investigation Over all 42 soil samples have been submitted for laboratory analysis (Table 6) Petroleum hydrocarbons were not detected in soil samples collected from borings B2,B4,and MW4 through MW8,delineating the maximum potential lateral extent of impacted soil The borings listed above and boring MW-3 were advanced to depths below the vertical extent of impacted soil, defining the vertical extent of impacted soil at these locations The greatest concentrations of hydrocarbons were detected beneath the former UST location Higher concentrations were also detected toward the south,with moderate detections primarily toward the northwest of the UST area I The plume of impacted soil at the site has an estimated north-south diameter of approximately 45 feet, an west-east diameter of approximately 30 feet and appears to extend downward to approximately 55 feet bsg The calculated volume of hydrocarbon-impacted soil is a roximatel I1,600 cubic yards (Appendices B and Q The approximate lateral and vertical extent of impacted -soil illustrate in Figures 6 through 8 Advanced GeoEnviron mental,Inc I