2017-09-07_PERMIT FILE - C1981019 (7)

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Exhibit 7, Item 25B Collom Topsoil Depth Test Small Area Exemption Demonstrations In 2017, Colowyo will be conducting a topsoil depth testing program within the disturbance boundary of the Collom operation. This depth testing program will allow Colowyo to construct a topsoil depth model to ensure that accurate volumes can be calculated for stripping and topsoil pile configurations. At the time topsoil depth testing is scheduled to occur ground disturbing activities for the box cut will not have occurred and the down gradient primary sediment control structures will either not be constructed or will be in process of being constructed. Therefore, in accordance with Rule 4.05.2(3)(a), this demonstration meets the criteria for an exemption from the requirements of 4.05.2; and surface water runoff from the ground disturbance associated with the topsoil test pits will be treated through small area exemptions. One hydrology model is presented within the exhibit which includes a model for one test pit disturbance area. Since there will be multiple locations of testing occurring the worst case disturbance area for a test pit has been modeled. All other test pits will have a smaller contributing watershed or will be smaller in size (e.g. thinner topsoil) thus requiring smaller disturbance areas. For the model demonstration, SEDCAD's grass filter routine has been used to demonstrate that runoff from the largest disturbance area will meet the effluent standards of 0.5 ml/1 in accordance with Rule 4.05.2(7) (US EPA, 40 C.F.R. §434.52). Sedimentology parameters and curve numbers used are defined in Exhibit 7 Methodologies and Assumptions for Sedimentation Pond Designs Evaluations. Drill Pad Demonstration The worst case disturbance area for a test pit was determined by disturbance area, location, topography, slope, and overall contributing watershed above the test pit. Figure 27-4-1 in Exhibit 27, Item 24 — Volume 20 provides the location of the contributing watershed modeled. The test pit modeled has 0.0008 acres of disturbance, 4.3 acres of contributing watershed, and a ground slope for the contributing watershed of 5%. A curve number of 85 was utilized to reflect the disturbed condition of the test pit, and a curve number of 63 was used to reflect the undisturbed condition of the contributing watershed. This demonstration utilized the Manning's roughness coefficient of 0.0054, and a stem spacing of 0.86 inches. These inputs are intended to reflect that the native grass cover density adjacent to the test pits. The 0.03 cfs flow was then run through a native grass filter approximately 6 feet in length (test pit width) and 5 feet in width (direction of the flow through the grass filter). As shown in the attached SEDCAD output files, the calculated resulting peak settleable concentration released and the 24 -hr weighted settleable solids concentration is 0.07 ml/1, well below the effluent standards of 0.5 ml/1. Exh. 7, Item 2513, Page 1 Revision Date: 8/14/17 Revision No.: MR -177