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' Ms Diane Hinson <br /> September 14, 1994 <br /> Page 2 <br /> MASS BALANCE CALCULATIONS <br /> 1 <br /> EMCON has calculated the mass of TPHG in the soil at both ARCO service stations The <br /> ' calculations are based on the estimated lateral and vertical extent of TPHG-impacted soil <br /> EMCON estimated the lateral and vertical extent of TPHG-impacted soil by evaluating the <br /> analytical results of soil samples collected during drilling of the soil borings (soil analytical <br /> results are included in Appendix A) Soil samples were collected during the soil boring program <br /> at 5 foot intervals (soil borings are included in Appendix B) <br /> ' To determine the mass of TPHG underlying each site, EMCON divided the entire vadose zone <br /> for each site into 5 layers of either 5 or 10 feet in depth EMCON then developed <br /> ' isoconcentration contour lines for TPHG as developed for each soil layer based on the soil <br /> analytical results Figures 1 through 5 present the isoconcentration contour lines of TPHG at <br /> different depths below ground surface (bgs) for the ARCO Lodi site Figures 6 through 10 <br /> present isoconcentration contour lines for TPHG at different depths bgs for the ARCO Stockton <br /> site <br /> EMCON then calculated the mass of TPHG contained in the sod interval underlying each <br /> r isoconcentratton contour between successive figures <br /> ' The following assumptions were made to simplify the calculations <br /> 1 TPHG concentration remains constant within a given soil interval <br /> ' 2 Soil type and density remains constant within a given soil interval <br /> ' EMCON calculated TPHG-impacted soil volumes using the following formula <br /> Volume of = area between successive x depth of soil interval <br /> TPHG-impacted isoconcentration <br /> ' soil contour line <br /> EMCON calculated the mass of petroleum hydrocarbons (lbs) in each soil interval was calculated <br /> ' as follows <br /> Mass of TPHG = Volume of TPHG- x Average x Concentration <br /> ' in Soil Interval Impacted Soil Bulk Density <br /> ' EMCON determined the total mass of TPHG at each site to be the sum of the masses calculated <br /> for each soil interval Appendix C includes the calculations of total mass of TPHG for both <br /> ARCO service stations For consistency, EMCON employed identical procedures and <br /> ` assumptions for both sites <br />