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ATC Associates Inc. f .' TC 1117 Lone Palm Avenue, Suite 201 (:v9% Modesto, California 95351 r — 209-579-2221 I fax: 209-579-2225 Afi60C1ATE 6 INC . -I from within the vadose zone associated with soil borings SBI , SB2, SB3, and SB15 . Although the largest reported value was used for contouring purposes, both of these values were lI considered for estimating residual mass beneath the site. L The average concentration of TPHg detected within vadose zone soil was calculated using all data associated with identified soil borings located within the 1 .0 mg/Kg contour interval and from contour intervals greater than 1 .0 mg/Kg. ATC did not use the contour interval value to J calculate the average concentration. TPHg concentrations were detecte at a range of 4 . 1 milligrams per kilogram (mg/Kg) to 7,800 mg/Kg. ATC used the reported laboratory method detection limit (MDL) where the associated data was not reported at or above the laboratory MDL. The average vadose zone soil concentration was calculated to be 2, 196.91 mg/Kg. The general shape of the isoconcentration lines associated with TPHg in vadose zone soil from 0 f to 25 feet bgs (Figure 3) was elliptical. The thickness of this zone was assumed to be approximately 25 feet. ATC assumes the geometric shape of the TPHg impacted vadose zone ( 1 soil to be cylindrical. As a result, ATC used the formula "Volume = Ir * rl * r2 * t" to calculate the estimated volume of TPHg impacted vadose zone soil, where n = 3 . 14159, r, = longest measured radius of ellipse in feet, r2 = radius of ellipse in feet measure perpendicular to rl , and t f = average thickness of the vadose zone in feet. The estimated volume of TPHg impacted vadose 1 zone soil is approximately 44,296.42 cubic feet (ft3) . The estimated mass was calculated for TPHg within the vadose zone using the following formula: - Mass (lbs) = Volume (ft) * Average weight of soil (Ibs/ft) * 0.4536 (Kg/lbs) * Average Concentration (mg/Kg) * 2.205e-oa Ibs/mg it Based on soil analytical results, the mass of TPHg impacted vadose zone soil is estimated to be approximately 11 , 862.52 pounds (lbs) assuming no degredation. 2.2 Saturated Zone Soil To estimate the lateral extent of petroleum hydrocarbon impacted saturated zone soil, ATC reviewed previous soil boring logs to evaluate which samples were collected from the saturated zone. Typically groundwater has been encountered at a depth of approximately 25 feet bgs,- however, on occasion groundwater has been encountered at depths as shallow as 21 feet bgs and as deep as 29 feet bgs. For all intents and purposes of this report, ATC has defined the saturated zone from 25 to 50 feet bgs based on the general depth of fust encountered groundwater and the - - estimated depth of TPHg impacted saturated zone soil due to attenuating concentration with depth from SB2 (MWI ), SB5 (MW3 ), SB8, AS 1 , and AS2 . Soil samples submitted for chemical analyses associated with the saturated zone were collected from soil borings SBI (VW ] ), SB2 (MWI ), SB3 (VW2), SB4 (MW2), SB5 (MW3 ), SB6 through SB9, SB9A, SB 11 through SB 16, ASB1 , MW4 through MW10, VW3, VW4, ASI , and AS2. The distribution of TPHg in lI S:Environmemal\62596\Reports\Mass Calcs 2009.doc 4 �.J