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05/08/2001 17:16 20946711"1 AGE STOCKTON PAGE 03 HYDROCARBON-IMPACTED SOIL MASS ASSUMPTIONS AND CALCULATIONS MID VALLEY TRAILER SALES 2461 North Wilson Way, Stockton, CA Assumptions: An estimate of the vertical and areal extent of impacted soil surrounding the former UST and dispenser areas was made from samples collected from borings Bi through B4. The radius of the area of impacted soil is approximated by the distance between Bi and B3 at 9 feet. The vertical distribution is limited to the soil between 7 feet bsg(base of excavation)and 20 feet bsg as determined by soil samples collected in December 1999. The hydrocarbon plume below boring Bi is modeled to have the shape of two cones with a common base. The average concentration of Vi is based on the following assumptions:The concentrations detected in samples from boring Bi are the maximum concentrations in the plume as Bi penetrated the soil directly underlying the former UST dispenser. The samples collected at 15 feet bsg in borings B3 and B4 have essentially non-detectable concentrations of TpH-g,and therefore define the shape and maximum extent of impacted soil.The average concentration in the Bi samples collected between 8 feet and 15 feet bsg is 6,620 mg/kg. The average concentration for the whole plume in this depth interval is assumed to be slightly more than the average"core"concentration(6,620 g/kg)and outer plume margin concentration (NDS 1.0) and is assumed to be approximately 4,000 M&g. The average concentration of V2 was approximated using these assumptions(380.5 m&g)as the average "core" concentration, and (ND< 1.0) as the outer plume margin concentration. The average concentration was thea,reduced to a ratio by dividing the contaminant mass per unit weight by the unit weight; i.e. 4,000 mg/kg= 4,000 mgacs/ 1,000,000 mg/kg=0.004 These assumptions ofthe hydrocarbon plume were calculated from soil sample data collected in September 1997 and December 1999. Figure l shows the modeled plume dimensions, and concentrations. Volume of Impacted Soil In Each Cone:4 Va =1/3TErz(h,)- 1/3-Ar2(h2) V2 = 1/3nr2 (h3) V, =1/3,n(9ft)2(13 ft) - WTE(4ft)2 (5 ft)= 1018.92 ft3 V2 =1/3n(9ft)2 (5 ft)=424.121}3 .Mass of,impacted Soil:-4 M,=V(125 lb/ft) K, = 1018.92 ft3 x 125 lb/ft'= 127,365.00 lbs M12=424.12 ft' x 125 lb/ft3=53,015.00 lbs Mass of Gasoline:4 Mg=(lbs soul)x avg concentration Mgt = 127,365.00 lbs (.004)=509.5 lbs gas MZ =53,015 lbs (.0004) =21.21 lbs gas Total Mass of Gasoline = 530.71 pounds of gasoline Advanced GeoEvviroomenta(,Jac.