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Underlying unit 1 is a silt bed that is approximately 8 feet thick This unit has <br /> significantly lower permeability than unit 1, and most of the downward-leaching <br /> hydrocarbons lodged in unit 2 when they encountered this zone of reduced permeability <br /> This explains why no hydrocarbons were detected in any of the permeable sand beds that <br /> underlie unit 2 Further, our monitoring data indicate that groundwater has not risen <br /> above unit 6, hence, it has not been in contact with any residual hydrocarbons that might <br /> still be present in unit 2, and therefore has not become impacted or spread the <br /> hydrocarbons to other lithologic units <br /> 4.0 ESTIMATE OF HYDROCARBON MASS <br /> The original mass of hydrocarbons that were released is unknown However, an estimate <br /> of the mass that was removed from the tank cavity can be made using the estimated <br /> volume of the original stockpile and averaging the TPH-d concentrations of the twelve <br /> stockpile samples that were collected on December 1, 1995 <br /> [TPH-d] = 2101 mg/kg volume = 300 cubic yards <br /> [2101] x 10"6 x 300 yds3 x 1 2 tons/yd3 x 20001bs/ton = 1,512 lbs <br /> This amount is the less than the total that was removed, because it does not include the <br /> • amount that was removed by overexcavation Because no samples were collected from <br /> the 300 cubic yards of overexcavated soil, the mean concentration of diesel in that soil <br /> cannot be determined <br /> The mass of diesel that remains in the soil is difficult to estimate, because only one of the <br /> twenty-five soil samples that were collected during the drilling investigation contained <br /> hydrocarbons, therefore, no map showing the lateral extent of contamination was made <br /> The amount remaining in the soil consists of two parts that which was taken from the <br /> original stockpile and returned to the excavation, and that which was never removed from <br /> the overexcavated area The volume of contaminated soil that was never removed is <br /> conservatively estimated to be a circular area measuring 40 feet in diameter by 3 feet <br /> thick The total mass remaining is estimated as follows <br /> volume taken from stockpile x mean concentration x 12 tons/yd3 x 2000 lbs/ton <br /> volume not excavated x concentration in sample HP-1-25' x 12 tons/yd3 x 2000 lbs/ton <br /> = 100 yds3 x 170 + 13 x 10"6 x 12 tons/yd3 x 2000 lbs/ton= 22 lbs <br /> 2 <br /> +Tc20 ft,2 P ftx 170 x 10"6 x 12 tons/yd3 x 2000 lbs/ton = 62 lbs <br /> 27 ft3/yd3 <br /> `> = 84 pounds <br /> 4 <br />