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� v G � � r ASSOCIATES I NC J ft/1,000 ft) which is an average value obtained from historical groundwater elevation data The remaining cells values in the IBOUND matrix were set at an initial value of —10 feet with a ' corresponding IBOUND value of I (active cell) The recharge package was used with a recharge flux of 3 17E-08 feet per second (ft/sec) This corresponds to a typical annual rainfall of 12- - -- inches per year No wells were included in the flow model The model was based on natural flow conditions that would be present at the site once the pump and treat system is decommissioned The soil is assumed to be sand and silty sand based on information from soil ' boring previous subsurface investigations A bulk density of 42 47 kilograms per cubic foot (kglft3) and an effective porosity of 0 25 were assigned to the soil These parameters were chosen based on typical values for silty sands Information regarding assumptions and input ' parameters is given listed in Table 6 5.2 Transport model using MT3D—Run Number 1 ' MT3D was used to generate a solute transport model using the flow model created in MODFLOW Advection was modeled using the Advection Package, dispersion was modeled ' 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 10-year period at 1-year intervals In this run of the model, initial TPHg concentrations were interpolated from an isoconcentration map of dissolved TPHg concentrations in groundwater generated from laboratory analytical data from the November 2003 sampling event This model shows the fate and transport of the existing TPHg �5r��tes ' ,S'UUYGe contaminant plume with the source removed Information regarding assumptions and input /'c01a11-tl parameters is given listed in Table 6 s��Id S4 ' The results of this MT3D model run indicates that the groundwater contaminant plume will continue to decrease in size and concentrations of hydrocarbon contaminants will decrease ' significantly over the next several years The model predicts that TPHg concentrations in weld_ 0 Flu MW3 will decrease from the current maximum concentrations of approximately 1,700 ug/1 to concentrations of approximately 3 ugll in 5 years to concentrations of less than 0 0002 in 10 ' years The model predicts similar decreases in TPHg concentrations in wells MWI. and MW4 Figures 12, 13, and 14 show hydrographs of wells MW1, MW3, and MW4, respectively The hydrographs show TPHg concentrations over time predicted with the MT3D ' model These predicted trends in decreasing hydrocarbon contaminant concentrations are in ft«J ti's] accord with trends in TPHg and benzene concentrations over the past 5 years as shown in �Gz,+ laboratory analytical data collected during quarterly groundwater sampling events A map '>W1111401f showing TPHg isocontours protected over time generated with the MT3D model is shown in f ss 4— Figures 15 and 16 VG ' 53 Transport model using MT3D—Run Number 2 MT3D was used to generate a solute transport model using the flow model created in MODFLOW Advection was modeled using the Advection Package, dispersion was modeled w 1271421reportslclosure doc 17