2025-04-11_REVISION - M1988112 (2)

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Revisions to Modeling Hydraulic Controls, West Pit Devon Hornvedt San Luis Project Newmont USA Limited • The models were simulated using the US Geological Survey (USGS) code MODFLOW2005 (Harbaugh, 2005). The original simulations were run using MODLOW SURFACT which is a proprietary code (HydroGeoLogic Inc., 1996). MODFLOW2005 is a public-domain software code. This transition allows for easier review and transfer of the model simulations to regulators and/or independent third-party reviewers, as needed. • A second layer was added to the base of the model to better represent potential vertical groundwater movement into and out of the underlying Precambrian bedrock and Santa Fe Formation from the overlying alluvium and West Pit backfill. The original modeling simulated the base of the alluvium and backfill as a no flow boundary. • Simulated hydraulic conductivity of the alluvium in the vicinity of the alluvial window was adjusted based on analysis of the test data collected during the hydrologic investigation (EA, 2022). • The hydraulic conductivity value assigned to the underlying layer that represents the combined Santa Fe Formation and Precambrian basement hydrologic units was based on analysis of the hydrologic testing of the Santa Fe Formation. • Monthly water level data collected from the newly installed alluvial monitor wells were used to develop additional calibration targets that were included in the model. • The thickness of the slurry wall was increased from one foot to three feet in the relevant simulations. 3.0 MODEL CALIBRATION As was done in the original modeling, water level measurements and operational flow rates that characterize two unique sets of conditions were used to calibrate the updated model. One condition reflects the return of the groundwater flow system to pseudo-steady state non-pumping conditions following the termination of mine dewatering. No pumping occurred at the West Pit from early 1997 until mid-1999. The second condition reflects the pumping of the West Pit backfill in order to reverse the hydraulic gradient between the West Pit and Rito Seco. Continuous pumping of the West Pit from the backfill wells began in 2000 and has continued to the present. The model is calibrated to both sets of conditions. Water level data collected from site monitoring wells in the first quarter of 1999 represent the non- pumping condition. The average water level elevations measured from site monitoring wells from 2012 through 2017 are considered representative of the potentiometric surface during pumping from West Pit backfill well BF-5 at approximately 200 gpm. Pumping was also occurring during that time from alluvial wells M-32 and M-33, at a combined rate of approximately 15 gpm. The first quarter 1999 and the 2012-2017 average water level data sets were used as calibration targets for the two unique sets of conditions(steady state,non-pumping vs.transient,pumping).The water level data were adjusted to the measure point elevation determined from the NAD83 CSPS survey. Average water level data from the alluvial wells installed in the 2020 hydrologic testing program May 2023 4 Engineering Analytics,Inc.