2010-12-17_REVISION - C1981019 (140)

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DRAFT 0 3 MODEL DEVELOPMENT 11 3.1 Choice of modeling code The 3-dimensional finite element code FEFLOW (Finite Element Subsurface FLOW system), version 5.1 (WASY, 2004) was used to develop the numerical hydrogeologic model. FEFLOW has the capability to simulate complex geometry and variably saturated groundwater conditions. It can simulate perched groundwater zones characteristic of the existing conditions in the southern part of the proposed mine and that will develop in the pit walls as mine dewatering occurs. This feature may be used at a later stage of modeling (e.g., during dewatering evaluations) but is not needed for this version of the regional model. 3.2 Finite element mesh The finite element mesh was designed with 16 layers and with about 61,000 triangular elements in each model layer. The size of the elements varies with elements being finer in the vicinity of the proposed mine and along the stream channels and coarser near the model edges and between the stream channels. The size of the elements ranges from about 5,000 ft2 in the finer areas of the mesh to about 100,000 ft2 in the coarser areas. Figure 3.1 shows the finite element mesh used for the regional model. The model mesh is oriented such that the y-axis in the model is perpendicular to the N66°W fracture set and the x-axis is parallel to this fracture set. 3.3 Model layer surfaces The layering used in the regional model to represent the hydrogeologic units is based on geological layer topography from the geological model of the Collom area developed by Kennecott Energy Company. This geological model covers most of the numerical hydrogeological model area. Areas outside of this coverage were extrapolated based on structural features such as the Elkhorn syncline and outcrop areas of the Trout Creek sandstone. The model includes 14 layers representing bedrock units and 2 layers representing valley fill. 11 2572-R3 Water Management Consultants