ARCHIVED REPORTS_REPORT ON SOIL VAPOR SAMPLING AND HUMAN HEALTH RISK ASSESSMENT DEC 2010

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AECOM Report on Soil Vapor Sampling and Human Health Risk Assessment 1-2 Soil types encountered beneath the Site during previously completed subsurface investigations consist of interlaced silts, clays and sands. Soil beneath the Site consists of clayey and sandy silt from the surface to approximately 40 to 45 feet below ground surface (bgs).The silts are underlain by a layer of fine to coarse sand from approximately 45 to 60 feet bgs.This sand layer contains thin layers of silty day, each approximately 6 inches thick. The sand layer is underlain by clayey silt and silty day to the total explored depth of approximately 103 feet bgs. Three groundwater zones or hydrostratigraphic units have been identified beneath the Site. Groundwater is first encountered at approximately 24 feet below mean sea level (approximately 36 feet bgs).The depth ranges for the three hydrostratigraphic units are: Unit A/B—30 to 70 feet bgs Unit C—85 to 90 feet bgs Unit D—97 to 103 feet bgs Historically,the depth to groundwater at the Site has ranged from approximately 36 to 60 feet bgs, with the highest groundwater consistently observed during the first quarter of each year. The water table elevation has risen by 20 feet in the last 10 years. Data from groundwater monitoring events indicate that groundwater flow is predominantly to the east.The groundwater gradient is relatively flat and typically varies from 0.001 feet per foot(ft/ft)to 0.005 tuft. During the August 27,2010, groundwater monitoring and sampling event, depth to groundwater ranged from 28.75 feet bgs to 34.90 feet bgs. Based on the August 2010 data, the groundwater flow direction was generally to the north by northeast at a variable gradient of 0.002 to 0.005 ft/f: in the hydrostratigraphic Unit AB,while flow in hydrostrabgraphic Unit C was to the north at an average gradient of 0.002 ft/ft. Groundwater flow in hydrostratigraphic Unit D was to the east by northeast at an average gradient of 0.002 to 0.003 f tft. December 15,2010 60146576-M10