2020-05-18_PERMIT FILE - C1981019A (5)

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