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