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' 12 December 2006 . AGE-NC Prod ect No 01-0827 ' Page 12 of 14 ' 7.0. 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 B16, indicates a migration of hydrocarbons west of the former UST #1 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 lateral 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 2 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 Groundwater flow direction has been consistently east-northeast The hydrocarbon-plume is stable and declining,within the local area of the former UST excavation 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 UST , location Hydrocarbon impacted soil is defined and limited to area within Hydrogeologic Unit 2, greater than 25 feet bsg (CB4) and above 30 feet (MW-6) 'L The nearest surface water feature to the site is Mormon Slough, located approximately 150 feet ff south-southwest and up-gradient of the site The Slough has only wet seasonal water flow It is unlikely that water in Mormon Slough is contaminated by impacted ground water from the site based 'I on its distance and direction (up-gradient) from the site 8.0. SUMMARY AND CONCLUSIONS The key components of Fire Station #2 site Closure Summary Report Advanced GeoEnvnronmental,Inc