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Historical Background & Site Conceptual Model <br /> g p <br />' AGE-NC Project No 01-0827 <br /> Page 9 of 9 <br /> I south-southwest and up-gradient of the site The Slough has only wet seasonal water flow It is <br /> unlikely that water in Mormon Slough is contaminated by impacted ground water from the site based <br /> on its distance and direction (up-gradient) from the site <br />' 7.0. SUMMARY AND CONCLUSIONS <br />' The key components of the Fire Station #2 site conceptual model are as follows <br />' • The subsurface soil stratigraphy consists of fine-grained, sandy silt/clayey silt between the <br /> surface/bottom the former UST excavation and generally 20 feet bsg A medium to locally <br /> coarse sand and gravel cut/fills were observed at 20 feet to 30 or 35 feet bsg at the former <br />' UST excavation A fine-grained silt/clayey silt was observed between a depth of 30 feet and <br /> 60 feet bsg and a silty sand was observed at 65 feet bsg <br /> • Ground water elevations at the site have fluctuated between 9 and 34 feet below mean sea <br /> Ilevel (MSL) since ground water monitoring began in December 1988 The depth to ground <br /> water has been as low as 45 feet bsg and as shallow as 20 feet bsg Ground water flow has <br />' been consistently east-northeasterly <br /> • Distribution of petroleum hydrocarbon contaminants in soil and ground water indicates an <br /> initial release(s)in the area of the former UST#1,which then migrated laterally and vertically <br />' through the upper silts into the more permeable sand layer between 25 and 30 feet bsg <br /> Vertically,except for very low concentrations of BTEX compounds beneath the former UST <br /> location, soil contamination does appear to extend below HU3 and the vertical extent of <br />' ground water contamination is defined <br /> 1 <br /> 1 <br /> I <br /> I <br /> Advanced GeoEnviron mental,Inc <br /> I <br />