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1 ' Historical Background & Site Conceptual Model AGE-NC Project No 01-0827 ' Page 8 of 9 ' Occasional occurrences of DIPE, TBA, and/or TAME detections have no longer been regularly documented The lateral extent of the dissolved petroleum hydrocarbon plume is defined toward the east by well VW-3, west by well MW-5, south by well MW-3 and north by well MW-4 The plume is locally undefined northeast of the well RW-1,due to low detection in samples from that well The dissolved ' petroleum hydrocarbon plume is currently vertically defined near the former UST release and vertically at location of the highest soil contamination by well MW-6, screened from 60 feet to ' 65 feet bsg ' 6.0. SITE CONCEPTUAL MODEL Based on the data collected to date from soil borings and ground water monitoring wells,AGE has ' developed a site conceptual model for the release,migration and distribution of the contaminants in the subsurface as described below ' • Release(s) of petroleum hydrocarbons occurred from the on-site USTs The only hydrocarbon impacted UST removal sample was collected from UST#1, collected in April 1987 The UST #1 removal reportedly occurred at the location of vapor well VW-2d, which coincides with the ' significant petroleum hydrocarbons detected in soil from boring B10 samples collected at 15 feet bsg The high detections of petroleum hydrocarbons in soil samples from boring 1316, indicates a migration of hydrocarbons west of the former UST 91 release location Once released to the ' subsurface, the contaminants migrated vertically into the sand/gravel near 30 feet bsg or Hydrogeologic Unit 2 The petroleum hydrocarbons then dissolved into the ground water and migrated laterally at the water table level near 35 feet bsg to the northeast The migration of hydrocarbons was limited vertically, in the dissolved phase, above 60 feet bsg, or above Hydrogeologic Unit 4 Since initiation of ground water monitoring in December 1988, the depth to ground water began at 30 feet bsg, declined to 45 feet bsg in October 1992, increased and remained near 40 feet bsg and then steadily increased to a depth of 20 feet bsg in June 1998,and has declined in elevation to 25 feet bsg or the current water table level Ground water flow direction has been consistently east to northeast iBased upon laboratory analysis of ground water samples, the current zone of maximum petroleum hydrocarbon-impacted water is limited to Hydrogeologic Unit 2 (well VW-2d)at the former USTs' location The nearest surface water feature to the site is Mormon Slough, located approximately 150 feet Advanced GeoEnvironmenta[,Inc