ARCHIVED REPORTS_XR0003066 - Laserfiche WebLink

Home Browse Search

28 February 2005 AGE-NC Project No 99-0624 Page 4 of 12 by low concentrations of petroleum hydrocarbons, soil is impacted at the UST location and in the areas of well MW-2 and MW-6 (Figure 4) At the UST location, low concentrations of mainly MTBE were detected at shallow depths (<45 ft bsg) surrounding the UST location, but extend to a depth of 55 feet bsg in the area of boring P-1,MTBE was also detected below 45 feet bsg in the area of well MW-2,at a maximum concentration of 36 mg/kg Total petroleum hydrocarbons as gasoline (TPH-g) was detected in one deep soil sample (P 1-50') at the UST location,two deep soil samples (MW2-45'and MW7-50') at the MW-2 location and one shallow sample (MW6-25') at the eastern site boundary, at a maximum concentration of 12 mg/kg (MW-7-50') Low concentrations of benzene, toluene, ethylbenzene and xylenes (BTEX) were detected in soil samples collected from borings P-1,MW-1 and MW-2 at depths between 45 and 55 feet bsg,possibly denoting a smear zone of the former water table The plume of impacted soil at the former UST release area has an estimated north-south dimension of 45 feet and east-west of dimension of 55 feet (Figure 4), the impacted soil in the area of well MW-2 has a diameter of approximately 25 feet,and impacted soil at well MW-6 is interpreted to be approximately 5 feet in diameter Impacted soil is not defined in the area between the USTs and well MW-2 The vertical extent of impacted soil is defined in the tank pit area, however the vertical extent has not be defined in the area of MW-2 Analytical results of soil samples are included in Table 1 Between November 1999 and November 2004, nineteen ground water monitoring events were performed at the site to monitor dissolved petroleum hydrocarbon concentrations in ground water and to characterize ground water flow direction and gradient on the site Since initiation of the ground water monitoring program,ground water elevations have ranged from approximately 9 to 17 feet below mean sea level, and the ground water flow has been consistently towards the south to southeast at a low hydraulic gradient TPH-g,MTBE and BTEX compounds have been detected in water samples collected from wells MW-1 through MW-7 at concentrations as high as 43,000 µg/l TPH-g, 1,000µg/1 benzene, 100 pg/1 toluene,58 pg/I ethylbenzene and 1,700µg/1 xylenes Whereas the BTEX concentrations have been declining at the site,MTBE concentrations generally have been increasing, during the more recent ground water events, MTBE appears to make up most if not all of the TPH-g hydrocarbon components(Table 2) At the present time,the vertical and lateral extent of dissolved BTEX contamination appears to be defined, but the extent of MTBE contamination is not defined (Figure 4) Based on AGE's interpretation of the available data,the initial release(s)of petroleum hydrocarbons occurred from the former dispensers and USTs prior to their removal in 1998 Once released to the subsurface, the contaminants migrated through the clayey silt/silt material to a vertical depth of approximately 45-55 feet bsg, at which point the contaminants encountered saturated fine grained soil (HU1) and began to migrate laterally as well as vertically, lateral migration was dominantly towards the south and-southeast, in the direction of ground water flow Advanced GeoEnvironmental,Inc