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ATC Associates Inc. 1117 Lone Palm Avenue, Suite 201 (YA Modesto, California 95351 209-579-2221 ASSOCIATES l ry c . fax: 209-579-2225 saturated zone soil is illustrated as Figure 4. It is important to note that two or more samples were collected from the saturated zone associated with soil borings SB1 (VWI ), S132 (MWI ), S133 (VW2), SB4 (MW2), S135 (MW3), S136, SB7, SB8, S139A, SB10, SB11 , SB12, ASB1 , MW4 through MWIO, ASI , and AS2. Although the largest reported value was used for contouring purposes, all of these values were considered for estimating residual mass beneath the l site. TPHg was detected at concentrations ranging from 0 .28 to 8,700 mg/Kg from soil samples collected from the saturated zone. The laboratory MDLs were used for all data points located within the 1 .0 mg/Kg contour interval and from all contour intervals greater than 1 . 0 mg/Kg to l calculate the concentration of TPHg within the saturated zone. The average saturated zone soil concentration was calculated to be 712.92 mg/Kg. 1 i:_f The general shape of the isoconcentration lines associated with TPHg detected in soil samples collected from the saturated zone (Figure 4) was elliptical . The thickness of this zone, based on the assumed vertical impact, was assumed to be approximately 25 feet. ATC assumes the general geometric shape of the TPHg impacted saturated zone soil to be cone shaped. As a result, ATC used the formula "Volume = 1/3 * 7r * rt * r2 * V to calculate the estimated volume for each contour interval, where n = 3 . 14159, rt = longest measured radius of ellipse in feet, r2 = radius of ellipse in feet measure perpendicular to rl , and t = average thickness of the saturated zone in feet. The estimated volume of TPHg impacted saturated zone soil is approximately 269,973 .85 The estimated mass was calculated for TPHg within the saturated zone using the following formula: Mass (Ibs) = Volume (W) * Average weight of soil (lbs/f') * 0.4536 (Kg/lbs) * Average Concentration (mg/Kg) * 2.205C-16 lbs/mg = Based on soil analytical results, the mass of TPHg impacted saturated zone soil is estimated to be approximately 24,506.94 lbs. -I 2.3 Groundwater ATC estimated the lateral distribution of TPHg in groundwater beneath the site using the groundwater analytical results associated with the third quarter 2008 sampling event, illustrated as Figure 5. TPHg was detected at concentrations ranging from 140 to 42,000 micrograms per liter (ug/L) . Based on the isoconcentration map, the average concentration of TPHg in groundwater was calculated to be 16.513 .33 ug/L. The general shape of the isoconcentration lines associated with the TPHg isoconcentration map (Figure 5) was elliptical. The estimated vertical thickness of the petroleum hydrocarbon impacted groundwater plume was estimated to be approximately 20 feet using the average DTW S:Envim mental\62596\Reports\Mass Calcs 2009.doc 5 i