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' 25 February 2005 AGE-NC Project No 01-0827 ' Page 9 of 10 ' 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 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(Appendix A) Groundwater flow direction has been consistently north-northeast ' Based 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 (Figure 5) The nearest surface water feature to the site is Mormon Slough, located approximately 150 feet ' 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 ' on its distance and direction (up-gradient) from the site ' 7.0. SUMMARY AND CONCLUSIONS The key components of Fire Station#2 site conceptual model include • The subsurface soil stratigraphy consisted of fine-grained,sandy silt/clayey silt between the surface/bottom the former UST excavation and generally 20 feet bsg A medium to locally coarse sand and gravel cut/fills were observed at 20 feet to 30 or 35 feet bsg at the former UST excavation A fine-grained silt/clayey silt was observed between a depth of 30 feet and 60 feet bsg and a silty sand was observed at 65 feet bsg (Figures 3 and 4) • Ground water elevations at the site have fluctuated between 9 and 34 feet below MSL since ground water monitoring began in December 1988 The depth to ground water has been a low as 45 feet bsg and as shallow as 20 feet bsg Ground water flow has been consistently east-northeasterly • Distribution of petroleum hydrocarbon contaminants in soil and ground water indicates an initial release/s in the area of the former UST#1,which then migrated laterally and vertically through the upper silts into the more permeable sand layer between 25 and 30 feet bsg IAdvanced GeoEn"ronmentai,Inc