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• MODEL DESIGN <br />I) Model Boundary <br />The model outline was defined by three hydrologic boundaries: the Trout Creek <br />outcrop to the southwest, south, east, and north; the Yampa River to the north; and <br />an azbitrarily chosen boundary to the north and northwest which was located far from <br />both mine sites yet within the regional hydrologic and drainage basins (see Figure 1) <br />(Driver et a1, 1984). <br />2) Model Grrd <br />The grid was developed to maximize definition around areas of highest pumping <br />stress. Grid blocks with 1/8 mile sides were used in areas of anticipated high <br />gradients (i.e. pumping wells). Farther away from the pumping wells, the grid <br />coarsens to a maximum size of 2 miles per grid block side. The grid lines are not <br />• oriented parallel to north (Figure 1). <br />3) Boundary Conditions <br />The model uses three types of boundary conditions: I) a NO-FLOW boundary was <br />used immediately outside the Trout Creek outcrop area and also along the north- <br />northwestern edge of the model to approximate a flow line (i.e. no flow across the <br />boundary), 2) a CONSTANT-HEAD boundary was used where streams with year- <br />round flows intersect a Trout Creek Sandstone outcrop, and along the north- <br />northwestern model boundary where the Trout Creek Sandstone is downgradient and <br />overlain by the Williams Fork Formation, Lewis Shale, and Lance Formation, and 3) <br />a RECHARGE boundary which was applied to the recharge nodes located along the <br />Trout Creek outcrop areas (Figures 1 and 2). <br />• <br />3 <br />