2018-05-18_PERMIT FILE - C1981019A (8)

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Exhibit 7, Item 25A Collom In Pit Drilling Small Area Exemption Demonstrations In 2018, Colowyo will be conducting an in -pit drilling program within or directly adjacent to the Collom box cut to obtain data to further define the geologic coal structure. At the time drilling is scheduled to occur, ground disturbing activities for the box cut will not have occurred yet, and the primary sediment control structures for the west side sediment control plan will be in process of being constructed. The majority of the drill pads will be within the sediment control system for the Section 25 Pond, but eight pads are within the watershed of the Sidehill and Section 26 Ponds and ditches which will be under construction. 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 drill pads and roads will be treated through a small area exemption. This small area exemption will be specific to drill pads and association access roads listed below. These drill pad locations are shown on Figure Exh. 27-3-1 in Volume 20. C-18-01 C-18-02 C-18-03 C-18-04 C-18-06- C-18-07 C-18-08 C-16-09 Two hydrology models are presented within the exhibit. The first is a hydrology model for the drill pads themselves, and the second is for the drill pad roads. Since there will be multiple pads sizes based on topography and roads lengths constructed to access drill pads, for conservatism, the worst case drill pad and road were determined and modeled. All other drill pads and roads will be much smaller in size and length thus requiring much smaller grass filters. For both 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/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 drill pad was determined by pad size, location, topography, grade, and overall contributing watershed above the drill pad. This area is depicted on Figure Exh. 7-14-1. The drill pad has 0.5 acres of disturbance, 3.8 acres of contributing watershed, and a ground slope for the contributing watershed of 11.6%. 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 0.86 inches. These inputs are intended to reflect that the native grass cover density adjacent to the drill pads. The 1.17 cfs flow was then run through a native grass filter approximately 150 feet in length (drill pad width) and 30 feet in width (direction of the flow through the grass filter). Exh. 7, Item 25A, Page 1 Revision Date: 5/4/18 Revision No.: MR -185