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i ISite Background Information Prem Dor Page 4 of 7 STRATIGRAPHY IIn general, silty clay and clayey silt, with lesser, discontinuous layers of sandy material, were encountered from surface grade to approximately 23 feet to 25 feet bsg From between 23 feet and I 25 feet to approximately 29 feet bsg, silty sand and fine-to coarse-grained sand was identified,grain size appeared to increase with depth From 29 feet to approximately 67 feet bsg,silty clay and clayey silt were encountered,with a thin,discontmous layer of sandy material encountered at approximately I34 feet bsg in borings MW-4 and MW-5, a thin layer of sandy sediments and stiff, fine-grained material was encountered at approximately 52 feet bsg,as well From 67 feet bsg to 76 feet bsg,silty sand and sand,underlain by silt,was identified in boring CPT-1,with silty clay,clay,and stiff,fine- grained material encountered from 76 feet to the total depth of boring CPT-1 at 97 feet bsg ' SITE CONCEPTUAL MODEL ' Based on the data collected to date from soil borings and monitoring well installations, AGE has developed a conceptual site model (CSM) for the release, migration and distribution of the contaminants in the subsurface as described below I • The soil profile beneath the site is generally composed of silt with sand and gravel (fill material) from surface grade to 6 feet below surface grade (bsg), clays, clayey silts and clayey sands from ' 6 feet bsg to 21 feet bsg, fine to coarse sand and silty sand from 21 feet to 29 feet bsg (35 feet at MW-2),clay with medium to high toughness and plasticity from 29 feet bsg to 36 feet bsg, silt from 36 feet bsg to 49 feet bsg, and fine sand from 49 feet bsg to 52 feet bsg or beyond ' Since initiation of ground water monitoring in October 2003,depth to ground water measurements have fluctuated between approximately 18 5 feet and 21 feet below the top of the casing(btoc) The ' inferred ground water flow direction generally has been towards the northeast, four times, at gradients ranging between 0 005 ft/ft and 0 022 ft/ft, ground water flow was towards the east- northeast once at a gradient of 0 002 ft/ft ' Releases o p f petroleum hydrocarbons occurred from the former on-site USTs Once released to the ' subsurface,the contaminants migrated through the fine grained unit directly below the UST area to a vertical depth of approximately 20 feet bsg, at which point the contaminants encountered a more porous sand unit and the water table and began to migrate laterally as well as vertically,the direction ' of lateral migration should be towards the northeast, in the preferred ground water flaw direction, however, maps of contaminant distribution suggest a more uniform spread away from the former UST location The vertical migration of soil contamination appears to be slowed by the clay layer ' encountered from 29 to 36 feet bsg except at the location of boring B3, where significant concentrations are detected at this depth • ' .Advanced GeoEnvironmental,Inc. 1