2021-08-17_REVISION - M1977342 (4)

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TR-34 to Permit M-1977-342 Environmental Protection Plan Tailing slurry is deposited across 1-Dam and 3-Dam by multiple spigots in the spring and summer months, transitioning to lead-off deposition during the winter months. Process water is reclaimed from the TSF for reuse in the Mill through a barge pump system, which conveys decant water from the TSF to the EBR. Effectiveness: As discussed in TR-14, the TSF has been designed, engineered, and constructed to contain the flow from a probable maximum storm event without compromising dam stability. TR-14 also addresses operational plans which are now in place to manage post storm event operations including maintaining at least 3,582 acre-feet of storage. Under emergency situations, where the stability of the dam may be threatened, stormwater can be pumped from the TSF to the EBR and diverted to the Williams Fork River via the Upper Bypass line. The TSF has been designed, and is being operated, with effective engineering controls and redundancies, including seep water collection and return, a groundwater interceptor system, and regular internal and external monitoring and inspections. Monitoring: Routine monitoring associated with the TSF includes the following: • Inspections are performed on a daily, monthly, and annual basis by Henderson Tailing personnel and include visual observations of the tailing dam and impoundment, benches, starter dams, 3-Dam step-back, access roads, and diversion ditches for the appearance of structural weakness and other hazardous conditions. • Weekly piezometer monitoring and data assessment. • Completion of a detailed monthly inspection of the TSF and the Seep Water Collection and Return System. • Annual inspection by a qualified registered professional engineer or other qualified professional specialist under the direction of a professional engineer. Maintenance and/or updates to facilities, systems, equipment, or procedures are performed as needed to be protective of the environment and to prevent releases. Additionally, the Henderson Annual Reclamation Permit Report to DRMS includes both a flood storage evaluation report, documenting that a minimum of 3,582 acre-feet of flood capacity is maintained throughout the year, and an annual TSF evaluation report, which summarizes the results of operation, maintenance, and inspection activities for the previous year. Mill-EPF 1.5 — Seep Water Collection and Return System: Seepage water from the base of 1- Dam and 3-Dam is collected and returned to the process water circuit.The TSF contains a series of abutment, foundation and horizontal drains that manage the phreatic level in the tailing embankment and maintain dam stability. Subterranean seepage at 3-Dam is captured using a wick-drain system. The seepage collection system conveys seep water from the base of 1-Dam and 3-Dam to the Ute Park pump station via the seep water collection canals, where it is returned to the TSF and back into the process water circuit. When needed, sediment is excavated from the seep water collection canals and placed within the seepage collection boundary, drained and moved to the dam face. Effectiveness: This EPF is one of several engineering controls and redundancies to prevent releases of acidic waters and has been effectively operated (as amended and improved) since Climax Molybdenum Company August,2021 Henderson Operations Page 22 of 45