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i ' <br /> ground water, resulting in significantly accelerated degradation <br /> of fuel hydrocarbons. This process has not been used in the <br /> United States, probably because of restrictions on injection of <br /> nitrate (a Primary Drinking Water Standard compound) into ground <br /> water. <br /> 3.2.2 Transport of Fuel Constituents <br /> 3.2 .2.1 Model Selection <br /> Several models are available to estimate transport in ground <br /> water. As in any environmental modeling, a number of assumptions <br /> are required to develop solutions. The approach used in this <br /> risk assessment was: <br /> 1. Define Objectives (often the most crucial part of the <br /> modeling)--In this case the objectives was to determined <br /> if the concentrations of dissolved hydrocarbons beneath <br /> �- the site can produce an increase of dissolved <br /> hydrocarbon concentrations at the nearest domestic <br /> wells. <br /> 2. Select Modeling Approach and Develop Model Input Dat: --A <br /> variety of ground-water transport models are available. <br /> These vary in complexity from simple models to estimate <br /> dilution to highly sophisticated computerized models <br /> which estimate the effects of mLltiple fate and trans- <br /> port processes on contaminant concentrations. All - <br /> models necessarily abstract and simplify real environ- <br /> mental processes. Generally, the sophisticated models <br /> provide more accurate estimates of environmental con- <br /> centrations if the contaminants and the hydrogeological <br /> environment are well characterized. If many of the data <br /> - - input values are unknown and must be estimated, the <br /> 18 <br />