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referred to as "MODFLOW" is a widely used, highly verified and more or less <br />universally accepted model code for carrying out ground water flow simulations. <br />To assist in model set-up, running and output generation, the MODFLOW pre- <br />and post-processor package known as "Groundwater Vistas" (referred to as <br />"Vistas" for the remainder of this report), published by Environmental <br />Simulations, Inc., was utilized. This package allows for relatively easier model <br />set-up and refinement and offers fairly good simulation output options. The <br />MODFLOW data files are initially generated by Vistas, are accessible for <br />independent adjustment or alteration, and .are automatically imported into <br />MODFLOW once the user is satisfied with the data files. Vistas can then output a <br />variety of maps, statistical analyses, and other output, depending on the type, <br />purpose and nature of the model in question. <br />There are a number of general categories of ground water flow models depending <br />on the nature of the problem and the type of answers being sought. For this study, <br />it was decided to apply what is referred to in the ASTM modeling standards as an <br />"Engineering Calculation" modeling exercise. These types of studies are <br />appropriate where the objective is to isolate the impacts on a flow system from <br />one particular defined set of stresses. An older and frequently used term for this <br />type of simulation is a "difference model" in which the goal is to determine the <br />impacts from a set of stresses separate from other stresses that may be occurring <br />in the model domain. Thus, an engineering calculation model does not account <br />for all stresses on the system, which is useful when there are many stresses or <br />3 <br />Martin and Wood Water Consultants, Inc. <br />