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r r Historical Conceptual Model Background & Site °i g r AGE-NC Project No 01-0827 Page 6 of 9 rthan B9, had more significant detections of hydrocarbons TPH-g and BTEX compounds were also detected in soil samples collected from boring B13 and B15 r • The most significant concentrations were detected above a depth of 25 feet bsg, indicating that ground water was likely at 25 feet to 30 feet bsg during the release period and/or the sandto silt contact at 30 feet bsg mitigated the vertical migration of the hydrocarbon release r • From the former UST area, the lateral extent of TPH-g-impacted soil appears to extend approximately 20 feet towards the west, less than 10 feet towards the east, which is the r down-gradient ground water flow direction, also less than 10 feet towards the south and up to a distance of 30 feet towards the north,under the building at the site The lateral extent of petroleum-impacted soil appears to be moderately defined in all directions by borings B4/11, rB5, B7 and boring B15 • The vertical extent of TPH-g-impacted soil is defined towards the west (B7), east (B 11), south(B5)and north(B 13 and B 15),where TPH-g-impacted soil appears to extend to at least r40 feet bsg (Figures 3 and 4 depict selected soil sample locations and the estimated extent of TPH-g concentrations adsorbed in soil in cross sectional view) r • The majority of the US F removal samples collected in 1993 were non-detect far petroleum hydrocarbons, however, the initial soil sample collected in April 1987 indicated that the source of the release was the decommissioned/removed UST#1 at the northern edge of the rcommon UST excavation, near vapor well VW-2d • The rise in the water table,over the monitoring period, from a depth of 45 feet to 25 feet bsg r currently has created a smear zone to a depth of 25 to 40 feet bsg and contributed to the dissolved contamination at the site The contribution of impacted silt and clay, surrounding the coarse sand and gravel encountered from 25 to 30 feet bsg 1 r5 0 GROUND WATER MONITORING RESULTS Between December 1998 and December 2004, 35 ground water monitoring events were performed r on monitoring well MW-1,while a total of 18 monitoring event were conducted at vapor well VW-2, to measure dissolved petroleum hydrocarbon concentrations in ground water and to characterize ground water flow direction and gradient on the site rSince initiation of ground water monitoring in December 1998,ground water elevations have ranged at the site from approximately 9 to 34 feet below mean sea level The ground water flow direction rat the site had been dominantly east to northeast, at the former UST area Laboratory analysis of shallow ground water monitoring well samples(between 25 and 30 feet bsg) r Advaneed GeoEnvlronmental,Inc r