2025-04-11_REVISION - M1988112 - Laserfiche WebLink

Home Browse Search

Mining and Reclamation Permit Amend.-M-1988-112 Groundwater Management Plan Battle Mountain Resources,Inc. in 2019 and confirmed the decision to proceed with geotechnical and engineering studies for the design and installation of the proposed slurry wall. A final version of the model was run with the simulation of the slurry wall consistent with the final design alignment. Those results are provided in the following section. 7.0 GROUNDWATER MANAGEMENT WITH A SLURRY WALL BMRI plans to install a slurry wall around portions of the West Pit to reduce the inflow of groundwater from the adjacent alluvial aquifer, which decreases the volume of water requiring treatment in the West Pit. The proposed installation of the slurry wall meets the objectives of the GWMP, and does not affect the function of the current pump and treat remedial action. 7.1 Slurry Wall Design Parameters The slurry wall will be located along the southern portion of the West Pit, as shown on Figure G-1. The slurry wall will extend from 140 feet west of monitoring well M-20 to 190 feet north of monitoring well M-30, spanning a total length of approximately 1,5500 feet. The slurry wall will be 3 feet wide and will extend from bedrock (either Precambrian gneiss-granite complex or Santa Fe Formation) up to ground surface. Depth to bedrock varies from 45 to 60 ft below ground surface along the wall alignment. The slurry wall will be installed using a"one-pass trenching" method. As the trencher moves forward, it brings in-situ soils to the surface and mixes in bentonite then places the soil- bentonite mixture back underground. This is a continuous process as the equipment moves along the slurry wall alignment. The soil-bentonite mixture will be between 3%- 5% bentonite by unit weight of soil of 110 pounds/ft3. The difference in bentonite percentage is due to unknown compositions of sands in the in-situ soils. The soil- bentonite mixture will have a hydraulic conductivity of less than 1 x 10"7 centimeters/second. The slurry wall materials are compatible with the range of groundwater quality anticipated prior to, during and following installation of the wall. 7.2 Slurry Wall Operational Parameters for Groundwater Management The slurry wall is a passive hydraulic barrier to groundwater flow. Once installed, no additional maintenance or operations are required. The slurry wall will substantially reduce the volume of groundwater entering the West Pit from the alluvial aquifer. However, groundwater flow will still enter the West Pit from the surrounding and underlying Pre-Cambrian aquifer and, to a much lesser extent, from the Santa Fe Fm that is present along the southwest side of the Pit. Continued groundwater extraction will be required to maintain water levels within the West Pit at or below the prescribed levels in DP-045675. Numerical groundwater modeling predicts that the extraction rate to achieve the prescribed water level of 8582 ft amsl, CSPS NGVD29 (equivalent to approximately 8586 ft amsl using NAVD88) will be between 20 to 25 gpm [10.51 to 13.14 million gallons per year, or 32.3 to 40.32 acre-feet/year (AF/yr)]. Of that total, infiltration of April 2025 11 Engineering Analytics,Inc.