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3.0 Contaminant Fate and Transport <br /> The data collected through investigations and testing indicates that the site poses <br /> mammal risks to human health and the environment because the petroleum <br /> hydrocarbon plume has stabilized and passive bioremediation is occurring at the site <br /> To further support this conclusion, a contaminant fate and transport model was <br /> performed The model was used to simulate the transport of BTEX constituents <br /> through the saturated zone to a downgradient imaginary well at the southwestern <br /> property boundary Contaminant fate and transport model documentation is <br /> presented in Appendix D <br /> 3.1 Model Description <br /> The American Petroleum Institute's (API)Decision Support System for Exposure <br /> ' and Risk Assessment (DSS) was used to model the fate and transport of petroleum <br /> hydrocarbons at the Stockton Service Center The model was used to determine <br /> BTEX concentrations at a receptor point at the property boundary after a finite <br /> duration DSS, prepared in May 1994 by Du Pont Environmental Remediation <br /> Services and Woodward-Clyde Consultants under contract to API, was developed to <br /> assist environmental professionals in estimating human exposure and risk from <br /> contaminated saes DSS includes a fate and transport module that incorporates <br /> ' several previously developed models to calculate the fate and transport of chemicals <br /> through water, soil, and air DSS uses Analytical Transport One, Two, and Three <br /> Dimensional (AT123D) to simulate contaminant fate and transport in groundwater <br /> AT123D can be used alone or in conjunction with other models AT123D was used <br /> alone for the Stockton Service Center site, because contaminants in excess of <br /> detection limits were not detected in vadose zone soils from within the plume <br /> boundaries soil contamination was removed during overexcavation at the time of <br /> tank removal <br /> i3.2 Input Parameters <br /> AT123D requires the input of several variables to simulate the fate and transport <br /> ' of the chenucals These parameters are divided into the following categories model <br /> control, source, media, chemical, and receptor well parameters The values to be <br /> used for these parameters for the Stockton Service Center site were derived from <br /> ' 96117 Fast Bay Municipal Utility District <br /> 40429700 fcr 14 Final Closure Report <br />