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G¢olagkj,Tech4laIu Page 12 Site Conceptual Model Report(2006 Revision) Valentine(Castle Motors) Project No 723 2 June 16,2006 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 generally be adsorbed to soil particles Groundwater plume dimensions are defined by quarterly sampling of the monitoring wells at the site It is extremely difficult to characterize the mass of soil gas at a site and it is generally not done 2.6.1 Soil Contaminant Mass Estimate Calculations GTI completed soil mass balance calculations for the site in our January 21, 2004 "Additional Site Characterization Report, Soil and Groundwater Investigation" SCM Those calculations are reiterated herein since the extent of the soil plume was defined at that point The mass of contaminant in the soil was determined by calculating the volume of each soil ' zone- shallow & deep as shown in Figures 10 and 11 The volume of each zone was multiplied by soil bulk density values from literature to obtain the pounds of soil in each zone The pounds of soil, in each zone, is then converted to kilograms and then multiplied by Mthe contaminant load The historical soil data was then used to calculate the contaminant load in the shallow and ' deep zones For the shallow zone, the soil samples' TPH-G values in borehole SB-B were averaged within the vertical profile to compute a value for the zone A value of one-half the detection limit was used for non-detect samples (0 5 mg/kg) For the deep zone, the soil ' samples' TPH-G values in boreholes SB-B, and MW-1 through MW-6 were averaged within the vertical profile to compute a value for the borehole The average borehole values were then used to complete the contour map presented in Figure 11 The average (median) values ' between the contours (i e - 500 mg/kg, 1500 mg/kg, etc ) were then used in Table 1, Appendix E, to approximate the mass of the zone intervals ' A mass balance of 11,545 kg (25,457 lbs or 4,106 gal) of TPH-G contamination in soil was calculated ' 2.6.2 Groundwater Contaminant Mass Estimate Calculations Figures 12, 13 and 14 illustrate the April 17, 2006 groundwater TPH-G plumes for the shallow, intermediate and deep aquifer horizons, respectively The total mass of TPH-G ' contaminant in the groundwater was determined by calculating the volume of each set of contaminant contour lines for each of the three aquifer horizons CAD software was used to determine the area within each contour line in Figures 12, 13 and 14 The volume of water within each contour interval was determined by multiplying the area of each contour by the vertical height of each aquifer horizon This volume was then multiplied by the porosity of the soil [from literature assume 45% for clay/silts, 30% for sands - Driscoll, 1989] to obtain