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for simplicity the annual simulations were divided into two stress periods; a "wet" season (April 1 to September 30) and a "dry" season (October 1 to March 31). For these seasonal baseline simulations, recharge rates developed during calibration were distributed between the two periods as described in Section 2.4. River and ditch cells were modified to mimic conditions during wet and dry seasons. Ditches that do not transmit flow during the non-irrigation season were turned off during the dry season portions of the simulations or allowed to only receive flow if the head conditions dictated it. The stage level in the South Platte River and other perennial flow channels were reduced during the dry season. The seasonal simulations were run for a period of six years to establish aquasi-equilibrium condition for each season. Typical maximum seasonal fluctuations in the alluvial valley were simulated to be one to four feet. The quasi-steady water table conditions at the end of the wet and dry periods were used as a basis For comparing the effects due to mining at the same time periods. 3.3 Mining and Post-Mining Simulation Results The model was used to simulate both the water table effects during mining and the long-term changes in water table conditions as a result of the mining activities and associated water storage facilities. The potential effects on neighboring wells and surrounding areas were evaluated by comparing simulated pre-mine wet (high) and dry (low) season levels with simulated during- and post-mine seasonal high and ]ow water levels. A slurry wall around the main cell is planned to be installed at the front end of the mine operation. A small area on the east end of the property will be dewatered for mining and then backfilled with unmarketable fines. Three steps were taken to convert the model into mine operation mode: 1) the Bull Seep within the property was realigned to the east, closer to the Fulton Ditch; 2) the main cell was converted to no-flow cells to simulate the effects of the slurry wall; and 3) to simulate the effects of dewatering, the east cell was represented by constant head cells set at five feet above the average bedrock elevation assigned to each cell along the perimeter of the cell. For post-mining simulations, the simulated slurry wall was left in place, the east cell was assumed to be backfilled with unmarketable fine-grained material having a hydraulic conductivity two orders of magnitude lower than the alluvial aquifer (three feet per day), and the simulations were run for an additional six years to allow the seasonal water table elevations to return to new quasi-steady state levels. Figures 5 and 6 provide contour maps of the simulated changes in water levels during mining operations for the wet and dry seasons, respectively. The magnitude of the drawdown varies with direction from the mine due to the thickness of the gravel aquifer, groundwater flow gradients, and hydrologic features such as rivers and ditches. The most significant impact from the mining is predicted to come from dewatering the east cell. Significant drawdown, defined as more than two feet, was predicted to extend over an area approximately 4,800 feet wide and 7,000 feet long during mining in the wet season. During the dry season the magnitude of drawdown decreased (maximum of 44 feet instead of 47 feet), but the area of drawdown increased to 5,400 feet by 7,000 feet. While the bedrock under most of the mine site is quite flat, there is a relatively deep "paleo channel" that runs under the southern part of the east cell and north section of the main cell. To mine out the bottom of this channel, the water level would have to be lowered almost 50 feet in the immediate area. The model simulated this "worst case" scenario by assuming that the deepest portion of the cell would be open and dewatered continuously. The combination of the dewatering, adjacent lined mines, and - 6 - July 2004 -.U919 018\GW ModcNtcport\Hazeltine Rp\ "/ D.doc