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cialogteal reduda ficG Page 13 Revised Site Conceptual Model Report Project No.507.2.jl April 3,2008 a� contaminant mass may continually decrease at a site while characterization progresses outward. `With sites where samples are collected over a long period of time, it is generally not possible to estimate how much concentrations have decreased in, areas where samples were collected early in the investigation. The contaminant plumes at the site'consist of three phases: 1) adsorbed to the soil particles, 2) dissolved in the groundwater and 3) vapor in the pore spaces of the soil. Of these,the bulk of petroleum hydrocarbons will gefierally be adsorbed to soil particles. Groundwater plume i dimensions are defined by quarterly sampling of the monitoring wells at the site. It is extremely difficult to characterize the mass of soil vapor at a site and it is generally not done. GTI has attempted to calculate the' mass of TPH-G in both the soil and groundwater at the isite. The discussions of how the plume dimensions were modeled are included below. Appropriate figures are included to aid in the demonstration of plume dimensions. 2.6.1 Soil Plume The TPH-G soil plume was modeled as a tv_ertical..irregular.ellipsoid extending from the surface to 22 feet bgs; see Figure Gl in Appendix G for plan view. Figure G2 in Appendix G j shows the soil plume as projected on to cross section B -B . ❑ The lateral extent of the soil plume was determined by non-detect points in soil borings in conjunction with non-detect OVM readings and field observations (odors). This core area r L 0 is centered near the southeast korner of the Hesston Shop which may have been the dispenser island location. ❑ The vertical exteirit of-the soil`plume waschosen as 22�feet-bgs This depth was selected based on the elevated TPH-G cariceiitratians in boring MW-9 in the 10, 15 and 20 foot samples. At levels of 25 feet and below the TPH-G concentration attenuate one or more orders of magnitude in all boreholes across the site. To determine the contaminant load for the soil plume, the analytical results for all the soil samples that fell into that zone were averaged using a geometric mean. For non-detect points a default of 1.mg/kg was used. Table G1 in Appendix G includes the soil.data utilized in mass calculations. The total mass of contaminant in the site's soils was determined by multiplying the area of l the irregular ellipsoid (determined by CAD software) by the height of 22 feet to obtain the i volume of the impacted soils. The',total volume was then multiplied by a bulk density value of 94.6 lb/cubic feet (average drydensity for MW-7 @ 20', MW-10 @ 25' and MW-10 @30') to obtain the pounds of soil in each zone. The pounds of soil in each zone was then converted to kilograms and then multiplied by the contaminant load. The average (geometric mean)concentration of TPH-G in the zone was used as the contaminant load. f Table G2 in Appendix G contains the contaminant soil.mass estimate calculations for the site. The calculations show that there is approximately,1,658 gallons (or 4,661,kg) of TPH-G}in the soils at the site. Utilizing the geometric mean to calculate the mass negates the data being J