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Report—Remedial Action Feasibility Page 19 Cutter Lumber—Western Lift Site GPE Project No 102 3 August 1,2003 ' 8.0 MASS BALANCE CALCULATIONS 8.1 Contaminant Distribution Over the past 15 years, the groundwater surface in this area has risen from a depth of over 40 feet bgs, to a recent high of less than 25 bgs At the time of the most recent groundwater monitoring event, the level was about 31 feet bgs The most intensely contaminated soil lies from 35 to 45 feet bgs in an area near the onginal point of release A cross-section depicting the mayor soil lithologic units is attached as Figure 4 Although the groundwater level has risen, the bulk of the contaminant mass has remained at the lower levels, meaning it lies from about 5 to 15 feet below the groundwater surface 8.2 Residual Contaminant Mass Loading ' The residual contaminant mass may be divided into three main types The first is the vadose zone contamination that is resident in the soil above the water table The mass body roughly takes the shape of an inverted cone with the apex dust under the point of ' release, and the opening at the groundwater surface For purposes of mass calculations, it was assumed that the water table lies 30 feet below grade The second (and largest) category is the mass of gasoline bound to the soil below the groundwater surface, and the third (and most mobile) type is the gasoline dissolved in the groundwater ' Geo-Phase Environmental has generated an estimate of the residual mass of each of these categories (see Appendix D for details) Based on this estimate, there are about 2,510 pounds of gasoline in the vadose zone, 11,195 pounds in the adsorbed phase of the saturated zone and about 88 pounds in the dissolved phase Combined, the estimated residual contaminant mass is about 13,800 pounds (less the 210 pounds removed in the pilot test) This figure equates to a bit less than 3,000 gallons assuming an average ' density of 0 9 2937 Veneman Ave,#13240 Geo-Phase Environmental Inc Phone(209)569-0293 Modesto, CA 95356 Fax(209)569-0295