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12 December 1995 <br />' AGE-NC Project No 95-0134 <br /> Page 5 of 6 <br />' benzene, toluene and xylenes would be below the state drinking water standards of 680, 100, and <br /> 1,750 ppb, respectively <br />' Using degradation rates, the initial receptor well concentrations were similar to those obtained in the <br /> first model However, in this model concentrations would decrease more rapidly Ethyl benzene, <br />' toluene and xylene would decrease to below detection limits after 4 years, 3 years and 9 years, <br /> respectively Appendix C includes SESOIL input data and output graphs <br />' 5.0. CONCLUSIONS AND RECOMMENDATIONS <br />' 5 1 HYDROCARBON-Rv PACTED GROUND WATER <br /> The non-detection of petroleum hydrocarbons in water samples during the last four monitoring events <br />' indicates that the hydrocarbon impact in ground water has been reduced to non-detectable levels The <br /> results of vapor extraction activities are postulated to positively influence the vicinity of the <br /> monitoring wells Combined with natural degradation, this situation is most likely responsible for the <br /> decrease of petroleum hydrocarbon concentrations to below detectable limits We believe that no <br /> further monitoring of ground water is necessary for this site <br />' <br /> 52 HYDROCARBON-"ACTED SOIL <br />' The PHS-EHD stated in the 05 October 1994 letter that "soil contamination remaining is a threat to <br /> ground water and further remediation is required " A soil interval containing elevated concentrations <br />' of hydrocarbons was indeed encountered between 30 to 40 feet in the confirmatory borings drilled <br /> on 14 June 1994, TPH-g was detected in bonngs CB-1 and CB-2 at concentrations of up to 2,200 <br /> ppm However, benzene was not detected in any of the samples analyzed, and only low <br />' concentrations of toluene, ethylbenzene and xylenes were detected Therefore, we believe that the <br /> majority of the most volatile and concurrently the most toxic spectrum of hydrocarbons have been <br /> remediated <br /> A fate and transport modeling program was used to predict the mass of contaminants that would <br /> eventually reach ground water The results of the model program indicate that the remaining <br /> hydrocarbon compounds are not likely to pose a threat to ground water and would naturally degrade <br /> over time <br /> Again,we believe that further remediation activities are not necessary for this site and therefore we <br /> recommend preparation of a closure summary report <br /> I <br />