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' MASS CALCULATIONS <br />' <br /> As required b the PHS/EHD Checklist of Required Data for the Closure q Y <br /> of UST Sites, estimates of the mass of total petroleum hydrocarbons as <br /> gasoline (TPH-g) remaining in soil and ground water at the site are <br />' presented below Estimating the mass of hydrocarbons in soil and/or <br /> ground water based on data from field investigations is an inexact science at <br /> best, even when an investigation has been designed with this intent The <br />' estimated masses reported for this site have been calculated using available <br /> data, which were not collected for this purpose Therefore, the values <br /> calculated below are only as reliable as the associated assumptions As a <br /> result, the actual mass of hydrocarbons remaining at the site may vary <br /> significantly from the masses presented below <br /> Soil samples have been collected from borings B-2, B-5, B-6, and B-7 and <br /> from monitoring wells MW-1 through MW-3 at varying times and depths <br /> during investigation of the site Soil samples were collected from boring <br /> B-2 in 1997, from B-5 and B-6 in 1998, and from B-7 in 1999 The <br /> monitoring wells were installed and soil samples were collected in 1999 <br />' Samples were collected at depths ranging from 115 to 415 feet bgs ERM <br /> used the concentrations of TPH at the boring and well locations to <br /> • estimate the limits of impacted soil <br /> Mass of Residual Gasoline to Soil <br />' Based on site sampling results, the lateral extent of residual gasoline <br /> in soil is estimated to be confined to an approximately 15-foot radius <br />' (see attached Figure B-1) The vertical extent of residual gasoline was <br /> estimated to be a 25-foot-thick zone between the depths of 10 and 35 feet <br /> below ground surface (bgs) The upper limit of 10 feet was chosen <br /> rbecause when the UST was removed, the upper 10 feet of soil was <br /> excavated and clean backfill was imported The lower limit of 35 feet bgs <br /> was used because stabilized ground water elevations occur at <br /> approximately 35 feet bgs <br />' A uniform TPH-g concentration was assumed over the entire impacted <br /> area This concentration was determined by averaging the TPH-g <br /> concentrations found in the two borings (B-2 and B-5) located in the <br />' vicinity of the former UST Although there is no other basis for <br /> establishing a numerical value, this average is conservative for the <br />' following reasons <br /> . The average concentration found in the immediate source area is <br /> applied over the entire impacted area, <br />' ERM B-1 9343 50/i1PRR/77/06/02 <br />