2019-06-28_REVISION - M1977004 (9)

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Homestake Mining Company May 10,2019 Regular(112d)Operation Reclamation Permit Application Package 3.3.4.3. North Pit/Underground Mine Workings System Water enters the North Pit Lake from shallow and deep sources along its northern, northwestern, eastern, and southern boundaries. However, the primary source of uranium load to the North Pit Lake is believed to be from the Chester Fault/underground mine workings system. Importantly, little to none of the uranium load is assumed to originate from the walls of the North Pit itself. This understanding is based on the following lines of evidence: • Uranium concentrations in the Chester Fault seeps and springs (CFS-1 and 2) are elevated in uranium relative to North Pit Lake water. These seeps are similar in composition to underground mine workings piezometer water quality. • Water levels and fluorescent dye tracer results(described above)demonstrate a generally northward groundwater movement through the underground mine workings and surrounding Chester Fault with discharge into the North Pit Lake. • In contrast, surface water, seeps and springs located on the north and east walls of the North Pit are very low in dissolved uranium. The Chester Fault extends north from the underground mine workings, with unmined ore deposits beneath and to the north of the North Pit footprint. However, these zones likely contribute less uranium to surface water relative to the shallower and highly disturbed portion of the Chester Fault associated with the underground mine workings. Specifically, residual deep ore zones beneath and to the north of the North Pit are relatively undisturbed and are buried beneath backfill; these zones would contribute a uranium load via flow through natural undisturbed fractures that may effectively be considered background. In contrast, the underground mining to the south of the North Pit resulted in creation of voids for preferential water flow and exposure of ore material due to formation collapse surrounding the mine workings that has likely enhanced uranium release. For this reason, efforts to reduce uranium load entering the North Pit Lake have focused on the underground mine workings as the ultimate source in this area. Uranium load reduction efforts are described in more detail below. 3.3.4.4. Indian Rock Dump As noted above, the IRD toe drain no longer conveys substantial water flow. Regardless, gains in water flow and uranium mass load along Indian Drainage downstream of IRD demonstrate that the dump represents an important source of uranium mass load. However, it is currently unknown how much of this uranium mass load originates from low-grade ore material dispersed through the entire dump, or whether the majority of the load originates from discrete local hot-spots. Importantly, most of material in the dump represents displaced overburden material that is not particularly high in leachable uranium. Characterization efforts conducted in 2018 (including drilling, solids characterization, and piezometer installation) indicate that a buried former mine landfill/heap leach pad area adjacent to the shop building, referred to as the Pinnacle Mine Dump, may represent one of the key uranium loads within the dump (Exhibit F, Map F-03A), with a smaller portion of the load coming in from upgradient of this area, possibly from water recharging the IRD from Indian Drainage as it passes adjacent to the IRD downgradient of the North Pit Lake. EXHIBIT E-RULE 6.4.5(AMENDED RECLAMATION PLAN) Page 37 HOMESTAKE MINING COMPANY