2018-11-21_REVISION - M2004044 (21)

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elevations rise during the wet season. The reduced extent of drawdown during the wet season is likely due increased recharge satisfying the water balance criteria quicker and the active ditches creating wet boundary conditions. Figures 7 and 8 show the maximum simulated change in alluvial groundwater levels for scenario 5,the most active mining stage, during the wet and dry seasons,respectively. The contours show the effects of dewatering the Challenger Pit and Baseline Mine. Near those mines, simulated water levels drop 18 to 22 feet. The zone of notable drawdown(more than 2 feet) extends about 8000-8400 feet north-south and 3500-4000 feet east-west during the wet and dry seasons respectively. Figures 9 and 10 show the maximum simulated change in alluvial groundwater levels for scenario 6,the west cell mining and post-mining stage,during the wet and dry seasons,respectively. The key result shown is the water level drawdown at the greenhouses,where the Tucson South east and west Cells, Challenger Pit,and Rogers Pit meet. The close spacing of the sites reduces the-area through which groundwater can flow. Because pumping the greenhouse wells takes more water out of the ground than can easily flow through the constricted alluvium,the water levels near the wells are shown to drop 10 to 16 feet,likely reducing their capacity and potentially rendering them dry during significant usage periods. The figures show water mounding up to 4 feet between the Brighton Ditch and west cell slurry wall. Figures 11 and 12 show the maximum simulated change in alluvial groundwater levels for scenario 7, the isolated east cell mining stage,during the wet and dry seasons,respectively. Because the Challenger Pit and Baseline Mine do not exist in this scenario,there is no drawdown from dewatering. The figures do show water mounding up to 4 feet west of the east cell and in the gap between the Rogers Pit and east cell slurry walls. Figures 13 and 14 show the maximum simulated change in alluvial groundwater levels for scenario 8, the west cell mining and post-mining stage,during the wet and dry seasons,respectively. Because the Challenger Pit and Baseline Mine do not exist in this scenario,there is no drawdown from dewatering. The figures do show water mounding between the Brighton Ditch and west cell and in the gap between the Rogers Pit and east cell slurry walls. The figures also show a simulated_ drop in water levels of around 2 feet by the greenhouses during the dry season. Figure 15 shows simulated groundwater elevations after mining all cells at Tucson South and nearby sites. Outside of the mined areas,they are similar to the pre-mining elevation contours. 4.0 CONCLUSIONS AND RECOMMENDATIONS A finite difference groundwater model was constructed for the proposed Tucson South Mine. The model indicates the groundwater near the mine will be lowered or locally slightly raised during mining in response to dewatering and slurry wall installation. The most widespread drawdown was found to occur during the dewatering of the Challenger Pit,Baseline Mine,and southern portion of the Rogers Pit. The dewatering of the Challenger Pit could potentially draw some water out of the Brighton Ditch (depending on the hydraulic connection between the Brighton Ditch and groundwater table in that area) unless local mitigation steps are taken.The largest local drawdown occurred near the greenhouses at the northeast corner of the Tucson South west cell,after the installation of all slurry walls and slope Iiners. _ 10- August 2004 V3919 0191TS GW McdcATS RgmrATucson South Rpt DraB.doc