2022-07-12_PERMIT FILE - C1981019 (23)

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Exh. 7-23G-1 Revision Date: 4/28/22 Revision No.: TR-154 EXHIBIT 7, ITEM 23, PART G Perimeter Area Sediment Control During Mining Introduction Colowyo will have ground disturbance along the perimeter of the Collom Pit that cannot effectively be routed to one of the existing sediment ponds. Colowyo will implement a perimeter sediment control plan for these areas. Generally, this will be ground disturbance on the west and east sides of the pit, and directly adjacent to the crest of the pit. This ground disturbance will be where topsoil is removed from the crest of the pit to a nominal distance back from the crest of the pit to allow a light vehicle road or buffer area for blasting activities from native areas. Typically, this area will be less than 200 feet of disturbance from the crest of the pit to native areas. Please refer to Map 23B for the location of the perimeter sediment control for the Collom Pit. This demonstration will, in accordance with Rule 4.05.2(3)(a), provide an exemption from the requirements of 4.05.2. Therefore, surface water runoff from this disturbed around will be treated through a grass filter. For the model demonstration, SEDCADTM 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). Runoff curve numbers assigned to the undisturbed and/or native lands in various stages of mining and reclamation have been selected in accordance with Table 1 in the Introductory Text for Exhibit 7 in Volume 2D or as further described below. The grass filter physical parameters were taken as the defaults in the SEDCAD drop down tables for a good stand of well established rye grass, except that the stem spacing was conservatively increased from 0.67 inches to 2.0 inches, and the roughness coefficient reduced from 0.0121 to 0.008, as described Reference 1, page 568, for a "fair stand" of grass. Both of these revisions produce a more severe loading on the grass filter by allowing the flow velocity to be higher than in a "good stand". Slope of the grass filters was set at 25% to 50%, which is representative of the land slopes in the areas shown on Map 23B. Two generic SEDCADTM models have been ran to determine the feasibility of using grass filters to capture sediment from these perimeter areas. Specifically, a grass filter length of 20 feet (length is in the direction of flow) and width equal to the similar disturbance and larger disturbances than is anticipated were evaluated for performance in a 10 year, 24 hour storm. Note that the length and width dimensions used in SEDCADTM evaluation are reversed from what seems intuitive. SEDCADTM refers to the "length" of the filter as the length in the direction of flow, and the width as the direction perpendicular to that. For a 20 foot wide strip 1,000 feet long parallel to the contours, with the 20 foot dimension in the direction of flow, the "length" is 20 feet, not 1,000 feet. A disturbance was evaluated for 1,000 feet of disturbance, 100 feet wide. This area is 2.3 acres, assumed to be at curve number of 85. It produces 1.68 cfs of runoff released onto the 1,000 foot wide by 20 foot long grass filter immediately downgradient on the slope. Here the trap efficiency is 94.7% , and it releases water with settleable solids concentration of 0.00 ml/l, well below the 50 ml/l standard.