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' to tcalTcdurlcsluc. Page 9 <br /> G� B <br /> Valley wholesale Drug Company <br /> Project No 989 2 <br /> November 8,2004 <br /> 3.6 Contaminant Mass Estimate Calculations <br /> 1 A preliminary contaminant mass estimate calculation was performed to determine the <br /> possible volume of gasoline present on this site (Table 8) These calculations suggest that <br /> ' approximately 56 1 gallons of gasoline are present in the soil and 0 1 gallons in groundwater <br /> A preliminary mass estimate calculation was performed to determine the possible volume of <br /> ' MtBE present on this site (Table 9) The mass estimate calculations also suggest that <br /> approximately 0 041 gallons of MtBE are present in the soil and 0 034 gallons in <br /> groundwater <br /> ' 3.6.1 Assumptions in Mass Estimate Calculations <br /> ' Soil Calculations <br /> Some basic assumptions were used to determine the size and extent of the TPH-G soil plume <br /> • Soil plume was divided into three areas <br /> ' o The Core encompasses concentrations measured between 300 and 500 mg/Kg <br /> o Ring 1 includes concentrations measured between 100 and 300 mg/Kg <br /> o Ring 2 includes concentrations measured between ND<1 and 100 mg/Kg <br /> • The upper 23 feet of soil is composed of clay and silt mixtures <br /> • The plume is defined laterally by MW-5 (north), MW-3 (south), MW-2 (southeast) <br /> control points placed 25-feet east and west of MW-4 <br /> ' • The plume is defined vertically by MW-101 <br /> • 2,850 cubic feet of contaminated soil was removed during the excavation of the <br /> former USTs <br /> ' • TPH-G concentrations have attenuated since the investigation began in 1994 <br /> o Chart 1 indicates TPH-G concentrations from.MW-1 have attenuated 96 7% <br /> o Mass estimate calculations of TPH-G in soil assume an attenuation of 25% <br /> ' since the initial soil samples were collected in 1994 <br /> 0 25% is likely a conservative estimate of attenuation over 10 years <br /> ' • Zone of impacted soil is 10 feet thick <br /> • The conversion factor of 1 gallon of gasoline equals 6 2 pounds was used <br /> t Some basic assumptions were used to determine the size and extent of the MtBE soil plume <br /> • Soil plume was divided into two areas <br /> o The Core encompasses concentrations measured between 8 and 15 }ig/Kg <br /> ' o The Ring includes concentrations measured between ND<l and 8 wg/Kg <br /> • Mass estimates based on the average concentrations of MtBE found in these areas <br /> • Zone of impacted soil is assumed to be 15-feet thick <br /> ' • MtBE concentrations used to generate contour map were average concentrations <br /> detected during analyses of samples collected between 5 and 25 feet bgs <br /> • The conversion factor of 1 gallon of MtBE equals 6 2 pounds was used <br /> f <br />