2019-06-28_REVISION - M1977004 (9)

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Homestake Mining Company May 10,2019 Regular(112d)Operation Reclamation Permit Application Package 4.3.2.3. Passive Water Treatment Passive/semi-passive water treatment focuses on removing uranium from impacted groundwater and surface water downgradient of the uranium release point in ways that do not require year- round, electrically-powered, "active" water treatment. In this context, a "semi-passive" approach would be defined as having non-zero, yet minimal and non-continuous operational requirements following construction; an example would include a gravity-fed biochemical reactor system requiring periodic reactive reagent/media replenishment. Current BMP strategies being pursued for semi-passive water treatment include ETCs employing solid-phase media. Media types investigated have included zero-valent iron, solid-phase (fish bone based) apatite, and biochemical reactor technology using solid organic materials (wood chip, alfalfa hay, and manure/compost mixtures). This strategy has the advantage of high- flexibility with aboveground treatment, but with the disadvantage that such systems may require operation in perpetuity in the absence of source-zone load reduction. These options still present challenges related to year-round operation. To date, screening-level pilot testing of media mixtures has been conducted in 2016 and 2017, including 55-gallon drum-scale flow-through tests installed along Tie Camp drainage. In 2017, a small in-ground pilot test of zero-valent iron was implemented, and in 2018, a pilot test-scale biochemical reactor was installed to capture and treat seepage from the Chester Fault springs on the south wall of the North Pit. These evaluations are currently ongoing and have been summarized in the most recent LPL Update Memorandum submitted to the WQCD and WQCC in July 2018 (Arcadis U.S., Inc. (Arcadis), 2018). 4.3.2.4. Source Zone Uranium Load Reduction via Phosphate Injection Primary uranium load sources at the Mine include residual disturbed uranium bearing material within and adjacent to the underground mine workings, low-grade ore in the rock dumps (dispersed throughout overburden waste rock and/or consolidated within ore stockpiles), and other potential discrete sources located within the rock dumps (e.g., residual heap-leached ore and other waste products). Given the depth of these sources below ground surface (several tens to hundreds of feet beneath overburden material or within bedrock), physical removal of these load sources is currently deemed to have low practicability. Source zone uranium load reduction has therefore to-date focused on insitu passivation and aqueous concentration reduction via the injection of aqueous phosphate reagents. This strategy involves the precipitation of dissolved U(VI) in low-solubility calcium-uranyl-phosphate phases (such as autunite and uranium- substituted apatite), with the precipitate phase acting to passivate/armor uranium phases against further dissolution. This strategy has the primary advantage of eventually achieving source-zone remediation such that remedy implementation can be reduced or eliminated over time. The challenge of this approach involves potential limitations in feasible effectiveness (controlled by the practical ability of achieving reagent distribution and contact with source materials). For example, load reduction efforts on the IRD to date have focused on phosphate injections to capture uranium flowing in from upgradient sources, as opposed to the direct-source reduction EXHIBIT E-RULE 6.4.5(AMENDED RECLAMATION PLAN) Page 60 HOMESTAKE MINING COMPANY