2021-08-17_REVISION - M1977342 (4)

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TR-34 to Permit M-1977-342 Environmental Protection Plan Upon final closure,the Mill WTP is expected to be closed and all related chemicals would be used up before WTP decommissioning. 6.1.2. Acid-Forming Materials and Acid Mine Drainage Handling Mine - The Henderson ore body contains sulfides that, in the presence of oxygen, water and bacteria, have the potential to become an acid-forming material as defined in Rule 1, Section 1.1 (1). In addition, water which accumulates in the mine contacts this sulfide bearing material and, in turn, has the potential to become acidified, thus, meeting the definition of "Acid Mine Drainage" in Rule 1, Section 1.1 (2). Impacted underground water at the Mine is diverted,conveyed to,and treated at the URAD WTP. The structures and facilities at the Mine site are constructed on a fill base of barren development rock extracted during construction and development of the underground mine and its associated access and ventilation shafts (see Figure 1). Samples of this material were characterized, and the results were presented to DRMS in TR-04 related documents. The results of those analyses indicate that the material is non-acid forming. Further, water quality monitoring data collected, including monitoring requirements of the Henderson Groundwater Management Plan (GWMP), comparing metal concentrations above and below the Henderson Mine provide demonstration that, after over 40 years, the fill materials at the Mine is not causing adverse impacts to downstream water quality. Henderson maintains adequate controls in place to detect acid forming conditions and/or elevated concentrations of key parameters through routine surface and groundwater monitoring (refer to the GWMP, approved as TR-16). Mill - The tailing delivery line (TDL) gravity transports slurry, a mixture of tailing material and process water consisting of approximately 40 percent solids, to the TSF (see Figure 4). Tailing material deposited into the TSF has the potential to be acid-forming. Adequate controls and protection facilities are in place related to these systems and are discussed in detail in Section 7.2 below. 6.1.2.1. Temporary Cessation Mine - During temporary cessation, mine water will continue to be conveyed to the URAD WTP for treatment. Mill - At the Mill site, process waters, including seep water from the TSF, are impounded and recycled. Should Mill operations temporarily cease, tailing seep water would continue to be pumped back to the process circuit and the East Branch Reservoir (EBR). Prior to construction of the Mill WTP there would be no discharge from the TSF during periods of temporary cessation except for potential emergency situations as allowed pursuant to the CDPS permit. After construction of the Mill WTP, impacted water would be treated and discharged under a CDPS permit. Handling of the acid-forming tailing material during temporary cessation would be the same as under active operations.Seep water would be pumped back to the process circuit,the water pool would be maintained, and wind-blown particulates from the TSF would be controlled through utilization of dust suppressants. Climax Molybdenum Company August,2021 Henderson Operations Page 15 of 45