2021-08-17_REVISION - M1977342 (4)

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TR-34 to Permit M-1977-342 Environmental Protection Plan the beginning of Mill operations in 1976. As described in TR-30, the seep water road, which provides the initial capture barrier at 1-Dam, was raised to increase the storage capacity to approximately 78 acre-feet. This allows for the 1-Dam seep water collection system to contain a 100-year snowmelt event under life of mine conditions. Monitoring: Routine monitoring associated with the Seep Water Collection and Return System includes the following: • Routine inspections are performed on a daily, monthly, and annual basis by Henderson Tailing personnel and include visual observations of the 3-Dam Seepage Collection System, 1-Dam Seepage Collection System, Underdrain System, Emergency Seep Water Pump System, Decant Barge, and Pipeline. • Completion of a detailed monthly inspection of the TSF and the Seep Water Collection and Return Systems. • Annual leakdown, or static pressure test is performed on the pipeline that conveys seep water from 3-Dam to the Ute Park pump station. Maintenance and/or updates to facilities, systems, equipment, or procedures are performed as needed to be protective of the environment and to prevent releases. The results of the annual leakdown test are submitted with Henderson's Annual Reclamation Permit Report. Mill EPF 1.6 — Groundwater Extraction System: Henderson operates a series of five extraction wells along the north seep water collection canal to prevent potential process water influences on groundwater quality; the wells are labelled MLEX-1 through MLEX-5, from northwest to southeast. Based on previous characterization studies, the alluvial and glacial drift deposits were reported to be the primary water-bearing unit underlying and downgradient of the TSF. Seepage from the 1-Dam tailing facility that is not captured in the seepage collection canals reports to the underlying alluvium. The MLEX extraction wells are installed parallel to the toe of 1-Dam in the alluvial and glacial drift deposits and range in depth from 76 to 95 feet. Flows from wells MLEX-1 through MLEX-4 are combined into a single underground header that discharges to the 1-Dam seepage collection area. The discharge from MLEX-5 is piped separately into the Ute Park pump station inlet channel. All water pumped from the extraction system is pumped back to the tailing pond for reuse in the milling circuit. Effectiveness: This EPF is one of several engineering controls and redundancies to prevent releases of acidic waters.All the MLEX well pumps are submersible VFD electric pumps and can be controlled either to maintain a certain water level in the well or a specific pumping rate, ensuring capture performance under a variety of conditions. Monitoring: Flow rates and volumes are continuously measured and recorded with the use of electronic recording devices. Monitoring of ground and surface water quality downstream of the Groundwater Intercept System is conducted on a routine basis as part of the GWMP (see Sections 8-11 below). Mill EPF 1.7 — Ute Park Pump Station: The Ute Park pump station is an important component in the process water management. Collected seepage is pumped back from the Ute Park pump station to the water pool on the TSF by turbine pumps with a total capacity of 5,000 gpm. A Climax Molybdenum Company August,2021 Henderson Operations Page 23 of 45