2017-09-07_PERMIT FILE - C1981019A (4)

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RULE 4 PERFORMANCE STANDARDS diversion ditches and sedimentation ponds, will be located, constructed and maintained to avoid erosion and increased contribution of sediment load to runoff. Facilities to control sediment are typically installed in areas above and/or below the planned sites of disturbance. "Upstream" facilities, such as clean water diversion ditches upslope from the mining activities, serve to divert runoff away from the disturbed areas. Temporary diversion ditches below the disturbed area of the pit will help collect runoff from disturbed areas and route it into the sedimentation ponds. During active mining, the mining areas will aid in retaining sediment within the disturbed areas by catching water in pits, small depressions and dozer basins, etc. Once reclaimed, the basins will drain as they did prior to mining activities (i.e., historic drainage patterns will be re-established). All temporary diversions will be removed and reclaimed when no longer needed for sediment control in accordance with Rule 4.05.2(2) Channel lining rock riprap and energy dissipaters will be used when necessary. As stated above, all temporary diversion structures will be seeded and revegetated after removal. Colowyo does not anticipate that there will be any significant excess material resulting from the construction of diversion ditches. None of the proposed diversions will drain into underground mines. 4.05.6 Sedimentation Ponds The location, design parameters, and detailed sedimentation calculations of all planned sedimentation ponds are presented in Erosion and Sedimentation Control Plan (Exhibit 7, Item 23). The design plans and specifications for the sedimentation ponds are described in this section (Part Q. All sedimentation ponds will be located as close as practical to the areas to be disturbed. Steep terrain in the upper basins precludes location of the ponds at the Collom Pit disturbance boundaries during the critical early phase of operations, necessitating down -valley locations downstream of the Collom Pit and temporary spoil pile footprint. Other methods of sediment control will be located on the reclaimed areas; these methods include the use of contour furrowing, contour drainage ditches, chisel plowing, and revegetation. Colowyo has specificially provided information regarding Rule 4.05.9(7)(a -e) with respect to the construction of sediment ponds on the design drawings. Please see Exhibit 7-23C, Table 1 and Exh. 7-23 Figures C5 through C9. Exhibit 7, Item 23, Part C contains calculations used to determine runoff volumes and flow rates for the theoretical 10 -year, 25 -year, and 100 -year, 24-hour precipitation events, as well as annual sediment volumes. The precipitation data were obtained from the NOAA Atlas 2, Volume 3 for Colorado; soil types were obtained from the Soil Conservation Service, and are shown on the soils survey map (Map 5D). The ongoing mining activities within each watershed of the permit area will create constantly changing hydrologic conditions. The design models are generally based on a static, theoretical scenario, utilizing SEDCAD 4. Please refer to Exhibit 7, Item 23 for a delineation of the areas used for these modeling purposes, the presentation of the assumed worse case scenario, as well as the maps associated with the SEDCAD runs. The scenario used for the sedimentation ponds corresponds to an active, disturbed operation. In terms of groundwater, Colowyo's pits have remained essentially dry. Discharges from the ponds will remain in compliance with Colowyo's CDPS Discharge Permit. The use of flocculants in sedimentation ponds may also be used in accordance with the provisions of the CDPS Permit. Collom — Rule 4, Page 5 Revision Date: 3/3/16 Revision No.: TR -108