2017-08-16_PERMIT FILE - C2010089A (4)

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Diversions. The ponds are designed to contain runoff from a 10 -year, 24-hour storm event plus additional storage capacity for water pumped from mine pits. Surface Water Ouality The sediment ponds are designed to prevent additional contribution of sediment to stream flow outside the permit area and incorporates detention times sufficient to ensure that all applicable effluent standards will be met. The ponds are currently planned to be removed and reclaimed following the completion of mining. Plans for sampling overburden and topsoil to identify potentially deleterious materials have been developed and are outlined in Section 2.05.4(2)(d), Overburden and Topsoil Handling. As previously mentioned, investigations have determined that no acid or toxic materials will be replaced in the upper four feet of the regraded spoil (Section 2.04.6, Geology Description). Surface runoff from reclaimed areas will not come into contact with materials that would contribute to elevated levels of acid or toxic constituents in the runoff. Topsoil handling procedures (Section 2.05.4(2)(d), Overburden and Topsoil Handling and revegetation methods (Section 2.05.4(2)(e), Revegetation) have been developed to prevent erosion and minimize the additional contribution of sediment to runoff. Plans presented in both sections include the establishment of topsoil piles, temporary seeding of topsoil piles, disking, mulching, deep ripping and prompt reseeding of reclaimed areas. Culverts presented in the Surface Water Control Plans have been designed to include appropriate headwater and tailwater depths and outfall protection (riprap) to minimize erosion. Surface Water Ouantity The sediment ponds, collection ditches and culverts have been planned and located to control all drainage into, through and out of NHN mining area, and to ensure that changes in surface water quantity are minimized. The post -mining topography is planned to approximate the pre -mining contours (see Section 2.05.4(2)(c), Backfilling and Grading). Replaced topsoil material will exhibit infiltration rates similar to pre -mining soils. Reclamation techniques used in the top soiled areas (disking, ripping, mulching, etc.) in combination with reestablished watershed topography will minimize excessive rates and volumes of runoff. Stream Buffer Zones A section on stream buffer zones in the NHN mining area can be found in Section 4.05.18, Stream Buffer Zones. Collection ditches which routes runoff from disturbed areas to the sediment ponds will be constructed immediately outside the stream buffer zone along Glasier Draw, Meehan Draw, and Nygren Draw. The operations plan for the collection ditch construction calls for no disturbance to the main channel of the draws, no excavation within the 100 foot buffer zone and that the collection ditches are capable of conveying the runoff from a Section 2.05.6(3)(b)(v) Page 3 December 2016 (RN -01)