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Union dee Company - Union Street Facility Quarterly Groundwater Sampling Repot, December, 1992 Page 2 r January 18 and 24, 1990. Of the six samples, four did not show any contaminants. However a sample from the north wall of the excavation (wN-3) had 285 mg/kg of oil and grease and a second, from the southern portion of the bottom of the excavation (FS_ 1) returned 200 mg/kg of oil and grease and 57 µg/kg of ethyl benzene. These two samples were a distance of twenty feet from the bottom of the tank where analysis of a sample (A5961) showed 12,000 mg/kg from beneath the.tank. ._ An estimate was made of the residual contamination 't this location based on the sampling of the tank excavation and attertuation.of contamination from the bottom of the tank to the w' limits of the excavation. From sample P85961 to sample FS-1, the oil and grease contaminant level dropped from 12,000 mg/kg to 200 mg/kg, or 590 mg/kg per foot: At this rate, the contaminant level -- would fall to zero one half foot below the bottom of the excavation. However, for purposes of calculation, a more conservative approach was taken. It was assumed that the contamination extended over an area of ten by ten feet on the bottom of the excavation `- and pinched out in the form of a wedge at a depth of thirty feet, five feet below the bottom of the excavation. The volume of contaminated soil would be: 1/2 X 10' X 10' X 5' X ,. 1/27 = 9.26 cubic yards. Since the contamination levels drop to zero at the zero line, the average contaminant levels would be approximately one half that of the maximum or 100 J mg/kg oil and grease and 28 gg/kg ethyl benzene. M Likewise, for the WN-3 sample area, it was assumed that the contamination extended over an area of six by six feet on the side of the excavation and pinched out in a wedge five fee- from it. This distance is halfway between the excavation and the monitoring well boring, MW-1. The volume of contaminated soil in this area would be 1/2 X 6'X 6'X 5'X 1/27= 3.33 cubic yards with a contaminant level of 142 mg/kg oil acid grease. Calculating a weighted average ..of the contamination in these two areas, the total estimated contaminated soil remaining on the margins of the tank #3 excavation is 12.6 cubic yards with a contaminant level o€111 mg/kg of oil and grease and 20 gg/kg of ethyl benzene. No estimate was made of the volume of soil contaminated by toluene reported from the MW-1 boring. This is because none of the soil samples on the bottom and sides of the tank #3 excavation had any detectable toluene_ Consequently, this contamination is presumed to be from another source.