Authors: Evans, E; Castendyk, D; Verburg, R; Nutini, J; Matznick, A; Cavalieri, L

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Evans, E, Castendyk, D, Verburg, R, Nutini, J, Matznick, A & Cavalieri, L 2023, 'Closure modeling of the Eagle Ni-Cu mine, Michigan: Part 2. Limnology and water quality of the Humboldt Tailings Disposal Facility, a pit lake used for sub-aqueous tailings disposal', in B Abbasi, J Parshley, A Fourie & M Tibbett (eds), Mine Closure 2023: Proceedings of the 16th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth,

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Water quality models are an important tool to predict and manage risks and determine the best operational strategies and long-term solutions for mine waste and water management. Eagle Mine LLC (Eagle), a subsidiary of Lundin Mining, uses a hydrodynamic model to predict stratification and water quality in the Humboldt Tailings Disposal Facility (HTDF) during operations and closure. Ore from the Eagle underground Ni-Cu mine is processed at the Humboldt Mill in Champion, Michigan, and a sulfide-rich tailings slurry is sub-aqueously disposed in the HTDF, a meromictic pit lake which formed in the 1950s. A water treatment plant (WTP) treats effluent prior to discharge through a reverse osmosis system that returns a brine to the bottom of the HTDF. After operations conclude in approximately 2027, the WTP will continue to operate until water quality meets discharge criteria. In 2022, Eagle generated a 34 year, 2D hydrodynamic and water quality model of the HTDF in CE-QUAL-W2. The model included a novel code that adjusted bathymetry over time as a function of tailings deposition and simulated profiles of water temperature and concentrations of several constituents of interest from 2019 to 2053.The modeling was conducted in five parts. Part 1 (Calibration) predicted conditions from 2019 to 2020 and adjusted input variables to match predicted and observed conditions. Part 2 (Validation) assessed the accuracy of the prediction using observed data from 2021 to 2022. Strong agreement between predicted and observed physical and chemical conditions provided confidence in the model and enabled its use in water and tailings management decisions. Part 3 (Operations) extended the model to the end of operations and quantified the impact of specific tailings and water management decisions on water quality, including tailings production rates, tailings deposition plans, minimum depths of tailings injection, start dates for new water treatment equipment, and composition of various inflows. Part 4 (Remediation) predicted the WTP would need to run for approximately 3.5 years (mid-2027 to 2030) after operations to meet closure criteria and this finding was used to estimate closure costs. Part 5 (Post-Closure) predicted conditions during the first two decades of passive discharge (2030 to 2053). The HTDF was expected to remain vertically stratified until approximately 2040 and discharge water quality was predicted to meet required limits. The model is routinely updated as new tailings production and water management plans are generated. Models such as the one described in this paper are reliable and valuable tools for supporting mine waste and mine water management decisions throughout the life of mine.

Keywords: closure modeling, pit lakes, CE-QUAL-W2, subaqueous tailings deposition

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