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1 <br /> • <br /> t 15.0 RATIONALE FOR "NO ADVERSE IMPACT" <br /> ' The majority of the source of continued ground water contamination was removed from <br /> the site following the removal of approximately 1,522 cubic yards of gasoline <br /> contaminated soil. The addition of ORC to the completed excavation has aided the <br /> ' passive degradation of contaminants. The contaminant levels present in monitoring well <br /> MW-1A have declined significantly since the remedial excavation was completed <br /> indicating that natural degradation processes are operating effectively. It is believed that <br /> ' over time continued degradation will reduce these levels to below analytical detection <br /> limits. <br /> ' The clay content of the soil at this site provides a significant adsorptive capacity Iimiting <br /> contaminant migration. This has been demonstrated by high soil contaminant levels and <br /> comparatively low ground water contaminant levels in the area of the former UST. <br /> ' During the remedial excavation it was noted that contaminant levels decreased sharply <br /> over a short distance, both vertically and laterally. Thus, residual contaminants are riot <br /> ' believed to pose a serious migration threat. <br /> The reductions in ground water contaminant levels during the remediation of this site <br /> ' . have been dramatic. Following the remedial soil excavation, reductions of 54% in <br /> gasoline, and 80% in benzene levels were observed. Since the September 1998 sampling <br /> event, further reductions of more than 77% for both gasoline and benzene have been <br /> ' observed. Additionally, the reduction in toluene is more than 96%, ethyl benzene is 83% <br /> and total xylenes is more than 99%. The total reduction of gasoline and gasoline <br /> constituents following active and passive remediation to date range between 89% and <br /> ' 99%. <br /> 1 <br /> 1 <br /> 1 <br /> American Geological Service, Inc. <br /> CA95DE-029 18 <br />