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1 16.0 RATIONALE FOR "NO ADVERSE IMPACT" ' The majority of the source of ground water contamination was removed from the subsurface and remediated on site. The reductions in ground water contaminant levels during the remediation of this site have been dramatic. The total reduction of gasoline ' and gasoline constituents following active and passive remediation to date range between 89% and 99%. Following the remedial soil excavation, reductions of 54% in gasoline, and 80% in benzene levels were observed. Since the September 1998 sampling event, further reductions of more than 77% for both gasoline and benzene have been observed. Additionally, the reduction in toluene is more than 96%, ethyl benzene is 83% and total xylenes is more than 99%. ' This source consisted of approximately 1,522 cubic yards of gasoline contaminated soil. The addition of ORC to the completed excavation has aided the passive degradation of tcontaminants. The contaminant levels present in monitoring well MW-lA have declined significantly since the remedial excavation was completed indicating that natural degradation processes are operating effectively. It is believed that over time continued ' degradation will reduce these levels to below analytical detection limits. ' The clay content of the soil at this site provides a significant adsorptive capacity limiting contaminant migration. This has been demonstrated by high soil contaminant levels and comparatively low ground water contaminant levels in the area of the former UST. During the remedial excavation it was noted that contaminant levels decreased sharply over a short distance, both vertically and laterally. Thus, residual contaminants are not believed to pose a serious migration threat. ' A downhole video log of the South Well was performed to attempt to determine the condition of the well casing and its well seal. The inspection found that a steel insert had ' been installed in the upper 58 feet of the well limiting the inspection, but indicating that that portion of the well casing was in poor condition. The shallow depth to ground water in the well, 14 feet bgs, in conjunction with the poor condition of the casing, indicates ' that a good seal was not present. In order to prevent this well from becoming a potential conduit for ground water contamination, the South Well was abandoned in accordance with San Joaquin County and California water well standards. A replacement well was ' installed to the south of the remedial excavation area, and includes a 200 feet thick well seal. Therefore, the South Well has been removed as a potential conduit, and the new t well is located and sealed such that it will not act as a conduit for vertical contaminant migration. American Geological Service,Inc. ' CA95DE-029 18