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Exhibit 7, Item 25A <br /> Collom Pit Drilling <br /> Sediment Control Exemption Demonstrations <br /> Colowyo will be conducting a drilling program within or directly adjacent to the Collom Pit to <br /> obtain data to further define the geologic coal structure. The majority of the drill pads are within <br /> the primary sediment control system for the Section 25 and 26 Ponds, but three pads and <br /> associated roads will be outside of the primary sediment control structures. 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 these drill pads and roads will be <br /> treated through a grass filters, and sedimentation ponds or other treatment facilities will not be <br /> necessary. The drill pads this grass filter demonstration is applicable for include C-41-20, C-59- <br /> 20, and C-60-20, and are shown on Figure Exh. 27-3-1 in Volume 20. <br /> For the model demonstration, SEDCAD's grass filter routine has been used to demonstrate that <br /> runoff from this area will meet the effluent standards of 0.5 ml/l in accordance with Rule <br /> 4.05.2(7) (US EPA, 40 C.F.R. §434.52). Sedimentology parameters and curve numbers used are <br /> defined in Exhibit 7 Methodologies and Assumptions for Sedimentation Pond Designs <br /> Evaluations. <br /> Drill Pad Demonstration <br /> The hydrology model presented within the exhibit evaluates a grass filter for the worst case <br /> scenario for one drill pad location(C-41). Since there are three drill pads the largest contributing <br /> watershed to one single pad was chosen. All other drill pads watersheds are much smaller in size <br /> thus requiring much smaller grass filters. The roads to access the drill pads were not evaluated <br /> as Colowyo has already demonstrated larger roads footprints can be treated with grass filters. <br /> Please see Volume 18C Exh. 7-25D. Therefore, this demonstration presents the worst-case <br /> scenario for the C-41 drill pad location only. This pad and contributing watershed are presented <br /> on Figure Exh. 7-25A-1. <br /> The drill pad has 0.5 acres of disturbance, 9.7 acres of contributing watershed, and a ground <br /> slope for the contributing watershed of 9%. A curve number of 85 was utilized to reflect the <br /> disturbed condition of the drill pad and a curve number of 47 was used to reflect the undisturbed <br /> condition of the contributing watershed. <br /> This demonstration utilized the Manning's roughness coefficient of 0.0054, and a stem spacing <br /> of 0.67 inches. These inputs are intended to reflect that the native grass cover density adjacent to <br /> the drill pads. The 0.36 cfs flow was then run through a native grass filter approximately 150 <br /> feet in length (drill pad 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 0.2 ml/l, which is <br /> below the effluent standards of 0.5 ml/l. <br /> Page Exh.7-25A-1 Revision Date: 4/27/20 <br /> Revision No.: MR-216 <br />