2025-04-11_REVISION - M1988112 (2)

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Newmont Corporation July 23, 2020 Page 8 3.8 Full Cutoff Wall with Pumping of BF5 The full cutoff wall remediation scenarios were simulated with well BF5 pumping at a rate of 60 gpm for the BASE, PITWALL, and UPFLOW models. This simulation with additional pumping changes the groundwater flow gradient within the West Pit, which induces a small amount of groundwater flow through the cutoff wall into the West Pit. Groundwater flow paths shown in Figure 11 indicate that a small amount of groundwater may seep through the cutoff wall in the east portion of the West Pit. However, the cutoff wall greatly reduces this flow from a current operations estimate of 135 to 152 gpm, down to an estimated flow through the cutoff wall of approximately 10.5 gpm for the BASE model, 3.9 gpm for the PITWALL model, and 3.4 gpm for the UPFLOW model. The total simulated groundwater flow into the West Pit is 60 gpm in the BASE model, 64 gpm in the PITWALL model and 65 gpm in the UPFLOW model with additional groundwater coming from the North Pit area and Precambrian unit. Nearly all the groundwater simulated in the BASE model is captured by pumping BF5 at 60 gpm. Most of the groundwater simulated in the PITWALL and UPFLOW model is captured by pumping BF5 with 4.1 gpm in the PITWALL model and 4.6 gpm in the UPFLOW model flowing out to the upgradient alluvium. These flows are commensurate with the current operations estimated water balance and should be able to be controlled by the M32 and M33 alluvial wells. Therefore, the total pumping could be reduced to an estimated 75 gpm from the current 205 gpm that has to be captured and treated. This remediation option appears to have potential to reduce the current water handling and treatment requirements. 4.0 PROPOSED QUANTIFICATION OF RITO SECO INFLOWS While it is generally understood that a major source of inflows to the West Pit has historically occurred from the Rito Seco alluvium on the east side of the pit, the flow has not been quantified. HYS and LWS recommend that field studies be initiated to assess the flow through the saturated alluvial sediments that are entering the West Pit. Generally, we envision that this will require the installation of shallow monitoring wells capable of being pumped so aquifer hydraulic characteristics can be assessed. Similar to how surface flow would be calculated in an open channel, the drilling of the boreholes for the monitoring wells will establish the higher permeability of the alluvial/colluvial sediments and be drilled to intercept the basement rock so the flow cross-section can be established. We would recommend the boreholes be drilled perpendicular to the direction of groundwater flow to establish the saturated cross-sectional area. An additional well should be drilled upgradient or downgradient of this cross-section to establish the hydraulic gradient in the sediments. Then, after the wells are completed and fully developed,constant-rate pump tests can be performed to estimate the permeability of the saturated deposits. From these data, groundwater flow can be estimated using the Darcy equation.