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I20 October 200 • AGE-NC Project No 95-0111 ' Page 6 of 8 y volume ofpetroleum impacted soil beneath the site is approximately 7,453 cubic yards(Appendix D) IA volume-mass calculation using the data collected during investigation of the former UST area, yields approximately 24 gallons of TPH-g and 400 gallons of TPH-d in the remaining impacted soil at the site(Appendix D) An average concentration of 34 mg/kg of TPH-g and 152 mg/kg of TPH-d ' was used to calculate the total volume of remaining hydrocarbons in impacted-soil ' 5 2 HYDROCARBON-IMPACTED GROUND WATER ' Between January 1994 and June 2003, thirty-two ground water sampling events were performed at the site Based on analytical data collected from the site and Information provided by the EHD for surrounding UST sites, AGE has developed the following model for petroleum hydrocarbon- impacted ground water • Relatively high concentrations of TPH-d were detected Intermittently in wells MW-4 and I MW-5 between 1994 and June 1999 However, detected concentrations in the same wells have declined significantly and have stabilized over the past two years Relatively low concentrations of TPH-d were also detected in wells MW-1, MW-2, MW-6 and MW-7 ' TPH-d was not detected in wells MW-3 and MW-8, located up- and down-gradient, respectively, of the UST area ' o TPH-g was detected at relatively low concentrations in wells MW-1,MW-2,MW-4,MW-5 and MW-7 The detected concentrations of TPH-g have been sporadic since monitoring began has not been detected in the eight monitoring wells during the five most recent ' monitoring events Benzene has been detected in wells MW-1 through MW-7 at various concentrations Detected concentrations have decreased significantly during the ground water Investigation ' and has not been detected in the eight wells during the eight most recent monitoring events Toluene, ethylbenzene and/or total xylenes were detected in wells MW-I through MW-8 at ' various concentrations With the exception of monitoring well MW-2,the three analytes have not been detected in ground water samples since December 2000 Relatively low concentrations of toluene and ethylbenzene were detected in samples collected from well ' MW-2 as recently as December 2002 • The fuel oxygenate methyl tertiary-butyl ether(MTBE)has been detected in the eight wells ' at various concentrations during the ground water investigation Based on the distribution of MTBE detections In the eight monitoring wells and on information provided by the EHD, two MTBE plumes have been identified at and near the site The on-site plume is centered ' near the former UST area(MW-1) and extends to the west(MW-7) and east(MW-2) of the UST area MTBE has not been detected in monitoring wells MW-3 and MW-4 during the IAdvanced GeoEnvironmental Inc. I