2010-12-17_REVISION - C1981019 (140)

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Model development DRAFT 15 There are two major vertical fracture sets in the Collom area, oriented approximately N66°W and N25°E. Based on the elliptical drawdown response observed during the pilot dewatering test in the F/G units and the transient model calibration to the pilot dewatering test the fracture sets have an influence on groundwater flow and hydraulic response to pumping. These fracture sets result in some horizontal anisotropy in hydraulic conductivity that was included in the model at a ratio of 2:1. 3.6 Boundary conditions The lateral boundaries of the regional model were chosen to be far enough from the mine to minimize their effect on the model output and close enough to keep the model manageable. The lateral boundaries were chosen on the following basis: • The base of the model at the bottom of the Trout Creek Sandstone is set as a no-flow boundary for the model. • The Collom study area is within a relatively well-defined groundwater basin bounded in most areas by the outcrop of the Trout Creek Sandstone. Therefore, the lateral boundary of the model area was chosen as the outcrop of the Trout Creek Sandstone on the north, east, west and portions of the south side of the model. This boundary was treated as a `no-flow' boundary. • On the southeast and southwest margins of the model the Trout Creek Sandstone does not outcrop. In these two areas, the Danforth Hills Anticline forms a structural high, which is used as the model boundary. This boundary was also treated as a 'no-flow' boundary. • At the points where upstream drainage areas enter the model domain, valley fill boundaries are set as a prescribed flux (Figure 3.11). This represents recharge from groundwater and surface water inflow from upstream areas. • Valley fill boundaries in the lower reaches of the major streams are set as constrained constant head nodes (Figure 3.11). This allows the potentiometric head to vary within the valley fill but not to rise above the stream surface. This boundary condition is used to represent areas where valley fill groundwater potentially discharges to streams. 3.7 Recharge Distributed recharge is entered directly into the top of the model. Based on the model calibration (Section 4), it is necessary to apply different recharge rates in three separate zones of the model: two areas at elevations above 7500 ft, and one below 7500 ft. These zones are illustrated in Figure 3.12. The higher elevation areas (above 7500 ft) receive more recharge than the lower elevation areas since precipitation generally increases with elevation. The two higher elevation recharge areas are divided based on the outcrop areas of units below the KM to the Trout Creek Sandstone and outcrop area of the bedrock units above the KM. 2572-R3 Colowyo Coal Company, L.P. Water Management Consultants