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Revisions to Modeling Hydraulic Controls, West Pit Devon Hornvedt <br /> San Luis Project Newmont USA Limited <br /> The partial and full slurry wall simulations that include extraction from well BF-5 at 20 gpm <br /> indicate that no groundwater is discharged from the West Pit (Figures 14 and 15). The <br /> potentiometric surface within the West Pit stabilizes at an elevation of approximately 8,584 ft amsl <br /> for both simulations as shown in the hydrographs of well BF-5 (Figure 13). This suggests that <br /> inflow into the West Pit also stabilizes at approximately 20 gpm, under the simulated conditions. <br /> The results of the partial or full slurry wall simulations with pumping from well BF-5 meet the <br /> objectives of hydraulic control of groundwater within the West Pit. There is no groundwater <br /> outflow from the pit and the volume of water that would require treatment is reduced from 200 <br /> gpm under current conditions to approximately 20 gpm. <br /> 5.0 SUMMARY <br /> Numerical models used to evaluate hydraulic controls for reducing groundwater inflow into the <br /> West Pit were revised and updated. Key revisions included the addition of a deeper layer in the <br /> model and incorporating the findings of recent hydrologic investigations. Updates included re- <br /> projection of the model domain and site features to NAD 83 CSPS coordinates,and a switch to the <br /> USGS MODFLOW2005 code. <br /> The model was recalibrated to the relatively steady state non-pumping conditions that existed from <br /> 1997 through 1999, and to the transient, pumping conditions (200 gpm from well BF-5) that <br /> currently exist. Recalibration of the model resulted in improved calibration statistics. The <br /> potentiometric surface and groundwater flowpaths for the calibration simulation were very similar <br /> to the original model results. <br /> The no-action scenario was run to provide a baseline for comparison to various hydraulic control <br /> simulations. This simulation indicates that, if no remedial action is taken, alluvial groundwater <br /> entering into the West Pit through the southeast end of the alluvial window will exit the pit through <br /> the west end of the window and eventually discharge to the Rito Seco. <br /> Results of the alluvial extraction well simulation indicate complete capture of groundwater that <br /> would otherwise exit the West Pit through the alluvial window. The simulation includes five <br /> extraction wells positioned along the southwest side of the West Pit. However, complete <br /> groundwater capture requires a total extraction rate of approximately 95 gpm.Although this is less <br /> than the current 200 gpm pumping that is occurring at well BF-5, this scenario would still require <br /> treatment of a substantial amount of wastewater prior to discharge. <br /> The slurry wall without pumping simulations indicate that groundwater is discharged from the <br /> West Pit under both the partial and full slurry wall condition. The slurry wall is simulated as three- <br /> feet thick with a hydraulic conductivity of 10-7 cm/sec.Both simulations indicate that alluvial water <br /> is generally deflected around the wall and does not enter the West Pit.However,the potentiometric <br /> surface within the West Pit rises above ground surface after approximately seven years in response <br /> to inflow along the pit walls through the Precambrian and Santa Fe Formation units. This rise in <br /> May 2023 8 Engineering Analytics,Inc. <br />