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31 March 2006 <br /> AGE-NC Project No. 98-0471 <br /> Page 12 of 13 <br /> MTBE over time. <br /> • AGE calculated that an estimated 365.6 lbs, or 45.8 gallons, of petroleum hydrocarbons <br /> remain in the predominantly silt and sandy soil, and an estimated 0.0014 lbs, or <br /> 0.002 gallons, of petroleum hydrocarbons remain in ground water in the area of the former <br /> 1,000-gallon gasoline AST. <br /> • Fate and transport study of the contaminated soil plume and the contaminated ground water <br /> plume predicts that concentrations of contaminants in any shallow down-gradient well within <br /> 130 feet of the former AST location will be at non-detectable levels. <br /> • The Saturated Soil To Ground Water model predicts that the benzene source concentration <br /> will be non-detectable at<0.005 mg/kg after 1 year;the toluene source concentration will be <br /> non-detectable at<0.005 mg/kg after 2 years;the ethylbenzene source concentration will be <br /> non-detectable at<0.005 mg/kg after 2 years;the xylenes source concentration will be non- <br /> detectable at<0.005 mg/kg after 2 years; and the MTBE source concentration will be non- <br /> detectable at<0.05 mg/kg after 1 year. <br /> • The liquid phase concentrations for benzene,toluene,ethylbenzene,xylenes,and MTBE over <br /> a 100-year period would range from an initial 9,850 µg/1 benzene to non-detectable levels <br /> (<O.5 µg/1)in 2 years; from an initial 8,110 µg/1 toluene to non-detectable levels(<O.5 µg/1) <br /> in 2 years; from an initial 907 µg/1 ethylbenzene to non-detectable levels (<0.5 µg/1) in <br /> 2 years;from an initial 7,630µg/1 xylenes to non-detectable levels(<0.6µg/1)in 2 years;and <br /> from an initial 33,500 µg/1 MTBE to non-detectable levels (<1.0 µg/1) in 2 years. <br /> 10.0. RECOMMENDATIONS <br /> Based on the results of this investigation, AGE recommends: <br /> • Low-risk site closure as defined in Section 6.5 of Appendix A of the Tri-Regional <br /> Recommendations: 1)The contaminant plume is stable and limited to the immediate area of <br /> the former 1,000-gallon gasoline AST; 2) Trend analyses and numerical modeling indicate <br /> that contaminants of concern are degrading and may achieve water quality objectives within <br /> a reasonable time frame; 3)There are no current or anticipated uses of the impaired water at <br /> the site. <br /> • Abandon monitoring wells in accordance with applicable regulations as part of the site <br /> closure preparations. <br /> Advanced GeoEnvironmental,Inc <br />