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Ms Margaret Lagorio Project 20805-152 008 <br /> October 12, 1998 <br /> Page 3 <br /> TPHG AND BTEX MASS CALCULATIONS <br /> The TPHG and total BTEX mass calculations are based on estimated lateral and vertical <br /> extent of impacted soil The lateral and vertical extent of impacts were evaluated using the <br /> soli sample analytical results obtained from the former UST cavity, former piping <br /> trenches, and exploratory borings TPHG and total BTEX isoconcentration maps were <br /> prepared for four depth intervals based on the distribution of petroleum hydrocarbon <br /> impacts <br /> • 10 to 25 feet bgs for evaluation of near surface impacts, generally related to the <br /> dispensers and piping <br /> • 26 to 45 feet bgs for evaluation of impacts related to the piping trenches and UST <br /> complex <br /> • 46 to 65 feet bgs for lateral and vertical evaluation of impacts extending off-site <br /> from beneath the area between the dispensers and the UST complex <br /> • • 66 to 85 feet bgs for evaluation of the impacts extending off-site <br /> The interval from 86 feet to 115 feet bgs contained very low detected petroleum <br /> hydrocarbon concentrations in localized areas and this mass was not calculated <br /> The area enclosed within each isoconcentration line was determined using a planimeter <br /> and the volume was determined by multiplying the area by the thickness of the soil zone <br /> Isoconcentration maps are shown on Figures 5 though 12 <br /> In order to simplify the calculations, the following assumptions were made <br /> • TPHG or total BTEX concentrations within a given interval remained constant <br /> using the isoconcentration line value <br /> • the isoconcentration line location did not vary with depth within a given interval <br /> • soil type and bulk density remained constant with a given interval <br /> • the average concentration with each isoconcentration line was calculated using <br /> detected concentrations <br /> • the volume of soil excavated during UST removal and replacement and the <br /> is volume occupied by the USTs was estimated and the average concentration of <br /> SACK 1LSGVNDQTR--116100MASS DOC-96bntt 1 <br />