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ATTACHMENT 3 <br /> Estimate Of Hydrocarbon Mass <br /> Remaining In Soil <br /> The mass of hydrocarbons remaining in soil was conservatively estimated to be 656 pounds. The <br /> remaining mass is confined within a 59,976 square foot area measuring 21 feet thick which <br /> includes former tanks #2 and#3 and extends northeast beyond former well MW-1. The residual <br /> hydrocarbon mass estimate was based on the following assumptions: <br /> The areal extent of hydrocarbons remaining in soil is confined to the designated area outlined in <br /> Attachment 4. The southeastern area boundary was determined by non-detect measurements in <br /> soil at EA-6 and beneath tank#1. The boundary represents half the distance between tank#2 <br /> sample locations and tank#1 sample locations. The southwestern area boundary was established <br /> by the non-detect soil samples obtained at MW-3. The northwestern area boundary reflects the <br /> lateral extent which contamination could have migrated from the tank#3 sample location. It is <br /> based on the apparent distance gasoline migrated in soil as inferred from soil samples at well <br /> MW-4. The northeastern boundary extends slightly beyond the location of former well MW-1. <br /> The relatively low detections of soil contamination at former well MW 1 suggest that this location <br /> may have represented the furthest extent of a small contaminant plume.The boundary distance <br /> extends beyond MW-1 to approximate a probable non-detect limit. <br /> -The vertical extent of hydrocarbons is confined between 15.0 and 36.0 feet below surface. These <br /> confining depths are based on the shallowest residual contamination measured at 15.0 feet at well <br /> MW-4 and the vertical extent of contamination defined at 36.0 feet at former well MW-1. <br /> The single detection of 0.013 ppm xylenes at 70.0 feet in boring SB-1 was not used to define the <br /> vertical extent of soil contamination beneath the site. This detection was not considered <br /> representative of actual soil conditions at that depth because of the very low concentration <br /> detected and the lack of other confirming detections at shallower depths. <br /> - 130.0 ppm TPHG(measured at 15 ft. at MW-4) was applied uniformly throughout the estimated <br /> soil volume as a representative residual hydrocarbon concentration. <br /> A soil sample obtained at 18.0 feet beneath former tank#2 detected 1,100.0 ppm TPHG. This <br /> concentration was not used in the determination of remaining hydrocarbon mass as it is assumed <br /> to be remediated. This assumption is based on the fact that the former tank pit was overexcavated <br /> to 18.0 feet and the excavation was left open for nearly six months. It has been assumed that <br /> soil contamination at the excavation bottom was effectively aerated during that six month period. <br /> This assumption is supported by the non-detect sample results obtained beneath tank#2 at 20 <br /> feet in boring SB-1. <br /> - A bulk density value of 1.35 glcc was used to represent the confined soil volume. This is a <br /> published mean value described to be representative of all soil types. <br /> -The actual calculations used to derive this revised mass estimate are provided in Attachment 5. <br /> It should be pointed out that the estimated mass is conservatively high. The TPHG concentration <br /> which was uniformly applied throughout the estimated bulk soil volume represents the largest <br /> concentration detected within the estimated area and was only detected in one sample. The <br />