2019-06-28_REVISION - M1977004 (9)

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Homestake Mining Company May 10,2019 Regular(112d)Operation Reclamation Permit Application Package 4.3.2. Best Management Practices to Define and Achieve Lowest Practical Level Consistent with the regulatory status described above, the current objective of the water quality- based reclamation activities is to define the LPL on Indian Creek through the investigation and implementation of BMPs for uranium load reduction, while at the same time pursuing modifications to the regulatory standards on Indian Creek and Marshall Creek. What constitutes a "practical' BMP for uranium load reduction is subjective and as-yet undefined; however, it is assumed that options requiring year-round, in-perpetuity, electricity-powered active water treatment would be deemed low-practicality. Overall, there are numerous complexities at the site that need to be considered, including the partial backfilling and/or draining of the North Pit, overall site stability, the revegetation processes, site access during winter months, the lack of power at the site, and the fact that uranium concentrations are heavily driven by precipitation and snow melt, with uranium loading controlled by subsurface water moving through uranium bearing materials at depth before discharging to surface water. These constraints will need to be taken into consideration to determine the acceptable level of disturbance and the activities that will be conducted to meet and define the LPL. Generally, CDRMS has expressed preference to not disturb areas that have been undergoing reclamation activities; accordingly, potentially conflicting objectives will need to be weighed as the LPL is being defined. In order to develop an amended reclamation plan with specific benchmarks that will govern activities leading to closure, HMC, WQCD and CDRMS will need to work together to meet Mine reclamation objectives and determine what the definition of LPL will be and what can actually be achieved at the Mine in terms of uranium reductions. The BMPs that are currently being investigated at the site include source zone uranium load reduction, water infiltration management and segregation, and passive water treatment. It is anticipated that each of these activities would result in some level of disturbance, which would require time for final closure and revegetation. Once the LPL is determined, HMC will work with CDRMS to determine what will need to be accomplished for final site closure. Modifications to the amended reclamation plan based on future water management requirement will be coordinated with CDRMS through reclamation permit technical revision requests. Water quality BMP elements included in the Water Management System OU are briefly summarized in the following subsections and in Section 5.5. 4.3.2.1. Sediment Control Structures Surface water must be controlled to minimize erosion and sediment transport. A variety of runoff control structures are currently in place, such as rock-lined drainage ditches, a sediment pond, and buried pipe conveyances. Some of these will be maintained for the future, others will be replaced as the final reclamation proceeds. It is anticipated that implementation of water quality improvements/load reduction BMPs, revegetation, and drainage improvements may result in EXHIBIT E-RULE 6.4.5(AMENDED RECLAMATION PLAN) Page 58 HOMESTAKE MINING COMPANY