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Geologfcal7'ecwcsinc. Page 10 <br /> Site Conceptual Model <br /> Project No. 989.2 <br /> December 17,2003 <br /> S. Temperatures are usually between 68 and 75° Fehrenhiet and slightly elevated in the <br /> impacted area. <br /> 4.6 Contaminant Mass Estimate Calculations <br /> ' 4.6.1 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 /> ' • The upper 22 feet of soil is composed of clays and silts <br /> • The plume is defined by MW-5 (north), MW-3 (south), control points 25-feet east and <br /> west of MW-4 <br /> ' • 2,850 cubic feet of contaminated soil was removed during the excavation of the former <br /> USTs <br /> • TPH-G concentrations have attenuated since the investigation began in 1994 as described <br /> in Section 3.2.1 <br /> • Zone of impacted soil is 10 feet thick <br /> Some basic assumptions were used to determine the size and extent of MtBE soil plume: <br /> • Used the area of an ellipse that encompassed the Surfer® generated contour intervals <br /> • Concentrations of MtBE used to generate contour map are the average MtBE <br /> concentrations detected during soil analyses between 5 and 25 feet bgs <br /> • The plume was divided into two areas; the core containing MtBE concentrations in <br /> ' excess of 14 ug/Kg and the ring containing concentrations less than 14 ug/Kg <br /> • The highest concentration of MtBE detected in both areas was used in calculations <br /> • Zone of impacted soil is 15 feet thick <br /> Groundwater Calculations <br /> Some basic assumptions were used to determine the size and extent of the TPH-G <br /> groundwater plume: <br /> • Groundwater plume assumed to be twice the area and thickness as the soil plume <br /> • Assumed volume of the groundwater plume exceeds the prediction generated by Surfer® <br /> ' • Plume is 20 feet thick <br /> • The plume was divided into three areas; the core containing concentrations >140 ug/L, <br /> ' ring 1 containing concentrations between 60 and 139 ug/L, and ring 2 with <br /> concentrations < 60 ug/L <br /> Some basic assumptions were used to determine the size and extent of the MtBE groundwater <br /> plume: <br /> • The area of a circle encompassing each area generated by Surfer® was used in the mass <br /> estimate calculations <br /> • The entire screen interval was used as the plume thickness for estimated mass <br /> j The highest concentrations were used in the calculations <br />