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 from beneath the pit are considered to be relatively minor. Additional hydrologic investigations were conducted to better characterize aquifer properties and the groundwater flow regime in the vicinity of the alluvial window,and to validate the conceptual model. The investigations included extensive pumping tests of the alluvial aquifer(EA,2022)and subsequent testing of the underlying Santa Fe Formation (EA, 2023). The results of the investigations confirmed the original conceptualization that the bulk of groundwater flowing into the West Pit enters through the alluvial window from the alluvial aquifer. Results of the hydrologic testing and refinement of the conceptual model have been incorporated into the numerical models to better represent site conditions. This technical memorandum summarizes the revisions to the models and the results of the hydraulic control simulations. 2.0 MODEL REVISIONS The results of the hydrologic investigations confirmed the conceptualization that most of the groundwater flowing into the West Pit is sourced from the alluvial aquifer and enters through the alluvial window (EA, 2022). The hydrologic testing also indicated apparent hydraulic communication between the alluvial aquifer and the Santa Fe Formation in the vicinity of the alluvial window. However, additional investigation demonstrated that the Santa Fe Formation has extremely low permeability(consistent with historic test results)and is unlikely to be a significant source of inflow to the West Pit (EA, 2023). Even though the contribution of groundwater inflow to the West Pit from the deeper portions of the Santa Fe Formation and Precambrian basement is anticipated to be negligible, these hydrogeologic units were incorporated into the models for further evaluation. As part of the hydrologic investigations, the horizontal location and vertical elevation of all monitor and recovery wells were resurveyed using the North American Datum (NAD) 1983 Colorado State Plane South (CSPS) projection, to provide a single, consistent coordinate system for the San Luis Mine project. The model was updated to include the recent survey data. The following revisions were applied to the numerical modeling of the West Pit. • The model domain was re-projected using NAD83 CSPS coordinates (in feet). Figure 1 presents the revised coordinates of the model domain. • All monitor and production wells were also re-projected to NAD83 CSPS for horizontal locations (easting and northing). Well locations are shown on Figure 1. • Measure point elevations for each monitor well were re-projected based on the revised vertical NAD83 coordinates and the water level measurements used as calibration targets were adjusted accordingly. The revised coordinates and measure point elevations are provided in Table 1. May 2023 3 Engineering Analytics,Inc.