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CAP Addendum:Former Fue)WKg Station, 7500 West Eleventh Street, Tracy, CAMW Page 6 Lacking other evidence, we will now assume that the mean concentrations of the fuel hydrocarbons varies linearly between Monitoring Wells MW-7 and MWFP-3 and is uniform across the estimated 80 ft. width of the area affected by LNAPL. Then, from the values cited in the table above, we estimate that the mean concentration of fuel hydrocarbons in that portion of the area affected by LNAPL is 279.71 mg/Kg. Similarly, the mean concentration in the area affected by LNAPL that extends from Monitoring Wells MW-13 to MW-7 is 283.68 mg/Kg. The thickness of contaminated soil in the area around Monitoring Well MW-7 is approximately 15 ft. extending from a depth of 5 ft. below the ground surface {BGS} to a depth of 20 ft. BGS, which is at the bottom of the shallowest aquifer beneath the 7500 West Eleventh Street Site and to which the contaminant plume is confined. The distance between Monitoring Wells MW-7 and MWFP-3 is 37 ft. Therefore, the volume of contaminated soil in the sub-area between those wells is 44,400 ft3. Assuming that the soil has a unit weight of 120 lb/ft3, the mass of that volume is 5,328,000 lbs. or 2,416,740 Kg. With an estimated mean concentration of fuel hydrocarbons of 279.71 mg/Zg in that soil mass, the estimated mass of fuel hydrocarbons present is 659,986 grams. The distance between Monitoring Wells MW-13 and MW-7 is 78 ft. Applying the above process of calculation to the soil mass in the LNAPL-affected area between those two wells yields an estimated mass of fuel hydrocarbons in that sub-area of 1,445,279 grams. Summing the masses in each of the two sub-areas yields the total estimated mass in soil in the area under consideration. 1, TOTAL ESTIMATED MASS OF CONTAMINANTS IN SOIL=2,105,265 GRAMS {7 . f 2.2.1.3 Total Mass of Contaminants in Area Affected by LNAPL '37 d It is important to recognize that the concentrations of analytes measured in soils includes { both the analytes that are present in the water in the soil pores as well as those that are adsorbed onto the soil surface. However, due to the fact that the LNAPL affects a very thin layer of soil, i.e., based on the estimate made in Section 2.2.1.1 above, not more than 0.05 ft., it cannot be assumed that the soil samples recovered from the borings necessarily included soil from that thin layer. Although it may yield a conservative estimate, it is prudent to sum the estimated mass of analytes of concern in the LNAPL and the mass of contaminants estimated to be present in the soil to derive an estimate of the total mass of contaminants present in the subsurface. As noted in Section 2.2.1.1 above, the estimated mass of petroleum hydrocarbons in the LNAPL is 3,179,746 grams and, from the calculations made in Section 2.2.1.2, the estimated mass of fuel hydrocarbons in the affected soil is 1,445,279 grams:Summing those amounts yields: ESTIMATED TOTAL MASS OF CONTAMINANTS=4,625,025 GRAMS -+� LL �,,,-�►, 4