2025-04-11_REVISION - M1988112 - Laserfiche WebLink

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Mining and Reclamation Permit Amend.-M-1988-112 Groundwater Management Plan Battle Mountain Resources,Inc. flowpaths and capture zones calculated from the groundwater flow simulations. Multiple numerical models were constructed to address uncertainty in the site hydrogeology (i.e., the source of water inflow to the West Pit). The hydraulic control measures modeled included the following: (1) an interceptor trench, (2) an alluvial extraction well array, (3) a slurry wall with no pumping, and (4) a slurry wall with limited pumping. The no-action scenario was also simulated to provide a baseline for comparison. The models were initially calibrated to the current conditions (200 gpm extraction from backfill recovery well BF-5) and to historic post-mining, pre- GWMP conditions (without extraction). Results of the initial modeling effort indicated that the scenario of a slurry wall with limited pit pumping could control groundwater discharge from the West Pit. The required pumping rates would be substantially less than the 200 gpm that is currently being extracted and treated. The reporting of that modeling effort is provided in Appendix C. The initial numerical models were reviewed by a reputable third-party consultant and then collaboratively modified, recalibrated and rerun to evaluate hydraulic control options (Lytle Water Services 2020, included as Appendix D). The revised models were simulated using the U.S. Geological Survey (USGS) code MODFLOW2005 (Harbaugh 2005). MODFLOW2005 is a widely accepted, well-vetted, public-domain software code capable of simulating a wide array of hydrologic conditions. As previously discussed, additional field investigation was conducted to better characterize aquifer properties in the "alluvial window" and to validate the results of the groundwater modeling. The groundwater models were updated to incorporate results of the 2022 and 2023 hydrologic investigations and to include a deeper layer to evaluate potential vertical groundwater movement between hydrologic units (Engineering Analytics, Inc. 2023, included as Appendix E). The model domain and associated calibration targets were updated to reflect current survey data using the more recent horizontal and vertical Colorado State Plane South (CSPS) projections using North American Datum 1983 (NAD83) and North American Vertical Datum 1988 (NAVD88), respectively. The projection of elevation using NAVD88 is approximately 4 feet higher than elevations projected using NGVD29. Table G-3 provides a comparison of the elevation difference at each existing monitoring well at the site. The revised model was calibrated to the same conditions as the original model, representing relatively static, non-pumping conditions that existed following the termination of mine dewatering, and the current pumping of the pit backfill that reverses the natural hydraulic gradient between the West Pit and Rito Seco. The overall calibration of the revised model was improved compared to the original model. Four options were evaluated and included the no-action option, alluvial extraction wells, slurry wall without pumping, and slurry wall with pumping. Based on simulations using the revised and updated model, the most effective means of hydraulically controlling groundwater movement from the West Pit to the Rito Seco and reducing the volume of water that will require treatment is the installation of a slurry wall coupled with limited pumping. These results were consistent with the results of the initial modeling conducted April 2025 10 Engineering Analytics,Inc.