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C+e WCCOUnt. Also, Bt is assumed tha$ the regional groundwater gradient flow is assumed to be in the direction of the nearest supply well. This wou9d u0so fend to overestimate the likelihood of contaminants reaching the well. CD.© P3VD0IUUE 04OQ� C»'Q�� G �Oa�M I� 0M 3001 Me fotao mass of gasoline (TPHg) and benzene in soil is Based upon ca volume weighted average of contaminants in soil as exhibited in the concentration contour cross sections (See Azures 4 cand 5 gor cross secenanz o� 4PHO cond L-anzene conours to so59 �ppmj, respe¢�ibe9y). `i°he total mass calculation was based upon the average dry density multiplied by the average chemical concentration between concentration contours multiplied by the volume within a cylinder. Average dray density (site specific data) = T.85 g/cm(cm)cm = 52.x} Ing/f$(ft)ft Gnaample: Volume of cylinder (nrrh) = 3.T 4 (TO f$X 10 ft) 6 ft = 9,884 cubic ft 1,884 cubic ft X 52.4 Kg/cubic ft = 98,722 Kg of soil within cylinder Volume of soil has can average concentration of 2.0 ppm benzene 98,722 (fig N 2.0 mg/Kg = 197A4-4 Doug = 9 97 Kg (mass of benzene) The mass of each cylindrical volume with an assigned average chemical concenv ration was then summed to obtain the overall mass. 'rhe total mass of IrPmg in soil = 67 8T 2 kilograms and the wall mass of benzene in soil = 895 kilograms. Yhese waff Mass 708Ue$ have been jpraygdled, ant Phe Gfeadoaa®d @c aard!'s request' however Phenr redeyanna:e go ME fprD§eCq has -flag been defin a aftdJ by Board sftff. 7.0 COMCILUBOOMS Gasoline constituents were not identified in soil from a depth of 60 to 100 feet bgs. Gasoline constituents were not leachable to groundwater from a depth of 60 to 100 feet bgs. An 18 foot thick silt layer between 42 and 60 feet bgs appears to preventing the vertical migration of contaminants to groundwater. The 4.3 ppb identified in groundwater may be from leaching through the vertical soil profile via another migratory pathway, by cross contamination of the bailer during its decent down the center of the hollow- stem auger, or by vapor phase migration. Regardless of the concentrations identified, an estimation of the length of time at which benzene in groundwater at the site could reach the nearest supply well would be 205 years. 8.0 REC0NMEMDDQ4lOMS Close the site with respect to the hydrocarbon contamination associates) with p y the former underground storage tanks.