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Revisions to Modeling Hydraulic Controls, West Pit Devon Hornvedt <br /> San Luis Project Newmont USA Limited <br /> The Alluvial Extraction Well simulation consists of five extraction wells. The total extraction rate <br /> is 95 gpm. The potentiometric surface within the West Pit stabilizes at an elevation of <br /> approximately 8,596 ft amsl (Figure 10) after approximately three years of extraction. <br /> Groundwater flowpaths indicate complete capture of groundwater exiting the west end of the <br /> window (Figure 10). At rates lower than 95 gpm, groundwater is not fully captured as it exits the <br /> West Pit. <br /> 4.3 Slurry Wall with No Pumping <br /> The slurry wall scenarios simulate placement of a slurry wall across either the alluvial window, or <br /> the alluvial window and portions of the west side of the West Pit with no additional pumping <br /> occurring within the pit. The simulation of a slurry wall across only the alluvial window is referred <br /> to as the "partial slurry wall," whereas simulation of a slurry wall across both the alluvial window <br /> and the area along the former northwest pit wall is referred to as the "full slurry wall." The slurry <br /> wall is keyed into bedrock for each simulation. The Horizontal Flow Barrier package of <br /> MODFLOW is used to simulate the slurry wall. The hydraulic conductivity of the slurry wall is <br /> simulated at 0.001 ft/d (3.5 E-07 cm/sec). The wall is simulated as 3-feet thick. The simulations <br /> were run for a period of 20 years (7,300 days). <br /> The slurry wall without pumping simulations indicate that groundwater is discharged from the <br /> West Pit under both the partial or full slurry wall conditions (Figures 11 and 12). The partial and <br /> full slurry wall simulations both indicate that alluvial water is generally deflected around the wall <br /> and does not enter the West Pit. However, the potentiometric surface within the West Pit rises <br /> above ground surface after approximately seven years and continues to rise throughout the 20-year <br /> simulation period. This rise is in response to inflow along the Pit walls through the Precambrian <br /> and Santa Fe Formation units, even though the rate is relatively low. Hydrographs of well BF-5 <br /> (which is not pumping in these simulations) shows the steady increase in water levels within the <br /> West Pit(Figure 13). This rise in water level elevation within the West Pit would result in surface <br /> discharge and eventual topping of the slurry wall. <br /> There is almost no difference in the potentiometric surface between the partial and full slurry wall <br /> simulations(Figures 11 and 12). This is because the very low permeability simulated for the green <br /> clay(0.0036 ft/d)that extends along the northwest side of the West Pit effectively acts as a natural <br /> hydraulic barrier to groundwater flow (Figure 2). Whether or not the green clay is a continuous <br /> unit, as simulated in the modeling, is unknown. <br /> 4.4 Slurry Wall With Pumping <br /> This scenario is similar to the previous slurry wall simulations but also includes limited pumping <br /> of backfill well BF-5 to reduce the potentiometric surface within the West Pit. The extraction rate <br /> of well BF-5 was adjusted to optimize the capture of groundwater within the West Pit. Only the <br /> optimized simulations are presented. <br /> May 2023 7 Engineering Analytics,Inc. <br />