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.. will be a District-owned maintenance center as long as the aqueducts that it services <br /> are in use. Therefore, no future residential land use is anticipated for the site. <br /> The aquifer that is impacted by petroleum hydrocarbons is 10 to 24 feet below <br /> ground surface. This shallow aquifer is not likely to ever be used as a drinking water <br /> resource since its proximity to the surface allows for pollution from surface sources <br /> such as industrial and domestic sewage, stormwater infiltration, septic tank discharge <br /> and the nearby contaminant plume from the McCormick-Baxter Superfund site. <br /> t.. <br /> 3.2 Model Description <br /> To assess the remote possibility that the shallow aquifer may be used as a <br /> drinking water resource in the future, Black & Veatch performed a fate and <br /> transport analysis to assess the risk of offsite migration of the hydrocarbon plume. <br /> The American Petroleum Institute's (API) Decision Support System (DSS) for <br /> Exposure and Risk Assessment was used to model the fate and transport of <br /> �— petroleum hydrocarbons at the Stockton Service Center site. The model was used <br /> to determine BTEX concentrations at a receptor point at the property boundary after <br /> a finite duration. <br /> DSS, prepared in May 1994 by Du Pont Environmental Remediation Services <br /> and Woodward-Clyde Consultants under contract to API, was developed to assist <br /> environmental professionals in estimating human exposure and risk from <br /> contaminated sites. 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 /> 3.3 Input Parameters <br /> AT123D requires the input of several variables to simulate the fate and transport <br /> of the chemicals. 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 /> 9687 Fast Bay Municipal Utility District <br /> 40429700.fcr 12 Final Closure Report <br /> V <br />