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• Actual concentrations of TPH-g at the site are not uniform and <br /> decrease laterally and vertically from the source area out to the limits <br /> Of impacted soil, and <br /> • The boring data are over 3 years old and do not account for <br /> biodegradation that has likely reduced the TPH-g concentrations over <br /> this period of time <br /> Using the estimated volume of impacted soil and the average <br /> concentration of TPH-g detected an borings B-2 and B-5, and an assumed <br /> soil density of 50 kilograms per cubic foot, the mass of residual gasoline in <br /> soil can be estimated using the following formula <br /> Mass= (volume of impacted soil)*(average TPH-g concentration)*(soil density) <br />' (17,662 cubic feet)*(0 0091 gram/kilogram)*(50 kilogram/cubic foot) <br />' =8,036 grams <br /> _8 04 kilograms(17 7 pounds) <br /> The calculation indicates that approximately S 04 kilograms (17 7 pounds) <br /> of TPH-g remains in the soil at the site This mass would be equivalent to <br /> approximately 2 to 3 gallons of gasoline Again, this is a conservative <br /> estimate using average values from soil samples collected within the <br /> source area and the average concentration across the entire cross-sectional <br /> Iarea This estimate also does not account for biodegradation that has <br /> likely taken place in the 3 years since the source area soil samples were <br /> Icollected <br /> Mass of Residual Gasoline Remaining in Ground Water <br /> The mass of TPH-g remaining an ground water was also calculated Based <br /> on ground water sampling at the site, the area of impacted ground water <br /> was estimated by developing three ground water contours with TPH-g <br /> concentrations of 100, 1,000, and 1.0,000 micrograms per liter (see attached <br /> Figure B-2) The depth of impacted ground water was assumed to exist <br /> between 10 and 50 feet bgs (40-foot thickness) The volume of ground water <br /> within each of the contour areas was calculated and the mass of TPH-g <br /> remaining at the site was estimated using the following formula <br /> (volume)*(average TPH-g concentration)*(cubic feet to liters conversion factor) <br /> (an estimated porosity of 35%)*(microgram to gram conversion factor) <br /> ERM B-2 9343 50/UPRR/07/11/02 <br />