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1VVZ 0 . A S S O C I A T E S I N C using the dispersion package, and biodegradation of dissolved hydrocarbons was modeled using the chemical reaction package The transport model was created for volatile organic ' hydrocarbons for a 30-year period at 3-year intervals Additionally, the laboratory data was used to assign values for the concentration of recharge �. flux This model shows the fate and transport of the TPHg contamination in the soil as it is c", ' dissolved into groundwater Information regarding assumptions and input parameters is given 0411 listed in Table 6 The model predicts TPHg contamination concentrations contributed by ' residual hydrocarbon left in since the removal of the USTs A distribution coefficient (K.) was calculated from the following equation �s ' q Kd = Kogyo, S�V ' Where Ko, is the soil organic carbon - water partition coefficient A Ko, value of 58 9 liters per kilogram (1/kg) was obtained from the USEPA, Soil Screening Guidance User's Guide, Table C-1, Chemical-Specific Properties used in SSL Calculations This was converted to a value of 2 08 ft3/kg f,, is the mass fraction for a geologic unit that consists of organic carbon A fo, value of 0 01 was used which is a typical value for silty sand (Bennet & Zheng, Applied Contaminant Transport Modeling, 2002) A Kd value of 2 08x10-2 ft /kg was ' calculated 1 USEPA, Calculation and Use of First-Order Rate Constants for Monitored Natural e� Attenuation Studies, Table 2, indicates that biodegradation rates of benzene, toluene, ethylbenzene, and xylenes (BTEX) have half-lives of 70 to 700 days A value of 315 days (the ;(Je Sf ,o b` ' median value) was used for the first-order reaction rate Converting this value into units of „art 1/time in seconds produces 3 67x10 s sec' The model was calibrated by comparing TPHg isocontours generated from November 2003 groundwater laboratory data and TPHg isocontours generated by the model and adjusting the horizontal hydraulic conductivity of the soil until the positions of the two sets of isocontours ' were in approximate agreement A hydraulic conductivity value of 5x105 feet per second (4 32 feet per day) was selected This value falls within the range of values that are typical of silty sands (Freeze and Cherry, 1979) Subsequent to the calibration of the flow model, the ' transport model was calibrated by adjusting the concentration of recharge flux until the longitidal extents of the two sets of TPHg isocontours were in approximate agreement A value of 85,000 micrograms per cubic foot (ug/ft3) was selected for the concentration of ' recharge flux This corresponds to a contaminant dissolution rate of 2 69x10-3 micrograms per second per square foot (ug/s/ft2) based on a recharge rate of 3 17x10.8 cubic foot per square foot per second (ft3/fell), which corresponds to an average annual rainfall of 12 inches w Q7142WepoMNclosure doe 18 r