2025-04-11_REVISION - M1988112 (2)

Home Browse Search

Revisions to Modeling Hydraulic Controls, West Pit Devon Homvedt San Luis Project Newmont USA Limited were also used as calibration targets for the pumping condition (well BF-5 continues to pump at approximately 200 gpm). The calibration target values for the steady state and transient simulations are provided in Tables 2 and 3,respectively. The variables that were used to recalibrate the model to the non-pumping and pumping conditions included hydraulic conductivity, heads and conductance of General Head Boundaries GHBs on the perimeter of the model domain, the conductance of the river cells representing the Rito Seco, and recharge via infiltration of precipitation. Additional zones of hydraulic conductivity were added to the model for the alluvium south of the West Pit to incorporate some of the findings of the recent hydrologic investigations (Figure 2). Extraction rates for the pumping scenario were assumed constant at 200 gpm for backfill well BF-5 and 15 gpm split equally between alluvial extraction wells M-32 and M-33. A second layer was added to the model to represent the underlying Santa Fe Formation(and deeper portion of the Precambrian basement). A uniform thickness of 100 feet was assigned to this layer. Figure 3 shows a west to east cross-section of the model layers through the center of the model. The potentiometric surfaces resulting from the recalibrated model simulation of the non-pumping and pumping conditions are shown on Figures 4 and 5,respectively. The calibration residuals(the difference between observed and simulated values) are also shown on the figures. Calibration of the revised model resulted in an improvement in the overall calibration statistics. Table 4 presents a comparison of the calibration statistics from the original base model to the current model. Table 5 compares the hydraulic conductivity zone values from the original base model to the current model. One key difference between the original and revised model is that a much lower value of hydraulic conductivity of the backfill was estimated (500 ft2/d compared to 1,600 ft2/d). This change in calibrated hydraulic conductivity was likely the result of the additional transmissive capacity of the deeper layer that was added to the model. A plot of the observed versus simulated heads for the non-pumping simulation is provided in Figure 6. A plot of the observed versus simulated heads for the transient simulations is provided in Figure 7. The plots illustrate the generally good fit between observed and computed heads throughout the model under both the steady state and transient simulated conditions. The particle tracking code MODPATH (Pollack, 1994) was used to assess groundwater flow direction for the calibration simulation. The flowpaths resulting from the calibration simulation show groundwater moving into the West Pit across the alluvial window and eventually being captured at backfill extraction well BF-5 (Figure 8). Overall,the revisions to the model resulted in similar potentiometric surface and groundwater flow patterns as the original model for the calibration simulation. 4.0 HYDRAULIC CONTROL SIMULATIONS The hydraulic control measures that were evaluated using the revised model include: May 2023 5 Engineering Analytics,Inc.