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I' 12.2 Ground Water Contaminant Mass Calculation The ground water contaminant plume is believed to cover an area of approximately 3,900 square feet, with a vertical thickness of 8 feet. With an estimated soil porosity of 30%, this soil volume contains approximately 70,000 gallons of ground water. Prior to performing the remedial soil excavation, the gasoline level measured at monitoring well MW-1 ranged between 20 mg/L and 61 mg/L. The averaged contaminant level for ' samples collected between August 1994 and June 1998 is 52 mg/L. This average value indicates a gasoline mass of approximately 14 Kg, or nearly 4.8 gallons. This mass is believed to be conservative since the average contaminant level has been applied to the entire plume volume. Thus the actual value was likely to have been lower than that calculated. t Following the remedial soil excavation, ground water contaminant levels at MW-IA, the replacement well for MW-1, have shown a significant decrease. These decreases include a 54% reduction in gasoline levels, and an 80% reduction in benzene levels. Using the ground water analytical data obtained in June 1999, and the same contaminant plume volume, a mass of 1 Kg as gasoline, or approximately 0.35 gallons remain. Once again, this estimate is considered to be very conservative and the actual residual mass is ' believed to be lower than presented here. ' A sharp reduction was noted in ground water contaminant levels immediately following the completion of the remedial excavation. This can be attributed to the removal of the source of continued ground water contamination. Quarterly sampling events conducted in December 1998, and March and June 1999 indicate a continuation of the reduction in contaminant levels at MW-lA. These data indicate that natural degradation is occurring, likely accelerated by the addition of ORC during backfilling of the remedial excavation. Reductions observed between the September 1998 and June 1999 sampling events indicate a decrease of more than 77% in gasoline and benzene levels. These reductions are in addition to the reductions observed immediately following the remedial excavation. ' Since the ORC was anticipated to have been exhausted within six months of its installation, the reductions observed since March .1999 are more likely the result of indigenous organisms, volatilization, or other natural factors. 13.0 SITE REMEDIATION A remedial soil excavation was performed in July and August, 1998. Approximately 1,522 cubic yards of gasoline contaminated soil were excavated and aerated on-site. Following the remedial excavation, Oxygen Release Compound® (ORC) was added to ' the backfill material to aid passive in-situ remediation. A total of 510 pounds of ORC was placed into the excavation below the anticipated high ground water elevation. American Geological Service, Inc. ' CA95DE-029 16