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• Introduction <br />Peabody Energy Twentymile Coal Company (TCC) is constructing a new intake shaft installation (18L <br />shaft) at TCC's Foidel Creek Mine, located in Routt County, Colorado. The purpose of this analysis is to <br />model the hydrology and sedimentology of the new shaft pad area and to design sediment control <br />structures that will treat runoff from that area, both during construction and after construction of the <br />intake shaft pad is complete. <br />The 18L intake shaft installation includes a shaft pad, cuttings pits, a transformer pad, a topsoil stockpile <br />and an access road (Exhibit 1). The majority of the surface disturbance is associated with the shaft pad <br />and the cuttings pits. This report addresses surface runoff from those areas. The access road and the <br />small area that includes the transformer pad and the topsoil stockpile are addressed in other separate <br />reports. <br />The hydrology and sedimentology analyses have been conducted for two different scenarios: during <br />construction, and after construction is completed. From a sedimentology standpoint, the worst-case <br />scenario occurs during construction, when runoff from disturbed areas is most likely to include a <br />significant sediment load. The total area of disturbance for the installation will be 4.0 acres, occurring <br />during construction. After construction is complete only 2.4 acres of disturbance will contribute runoff. <br />To treat runoff during construction we propose using a combination of Best Management Practices <br />(BMPs) and two sediment ponds constructed in series (Exhibit 1). Site runoff from the intake shaft pad <br />would flow overland and through a silt fence before flowing into the first sediment pond. The first <br />• sediment pond, Pond 1, is the larger of the two ponds, and is designed with a combination of principal <br />and emergency spillways. Pond 1 will discharge into Pond 2 for additional runoff treatment. Discharge <br />from Pond 2 is expected to meet water quality standards. <br />Once the cuttings pits are constructed, rainfall over that area will be completely contained within the pits, <br />and a significantly smaller disturbed area will discharge into Pond 1. Due to the reduction in disturbed <br />area runoff and to other BMPs applied throughout the area, Pond 2 will no longer be necessary for <br />additional runoff treatment and can be reclaimed (Exhibit 2). BMPs to be utilized after construction is <br />complete will include surfacing the pad with coarse gravel that will contribute little sediment, and <br />hydromulching the shaft pad outslopes. <br />Pond Design <br />Exhibit 3 provides design details for both ponds. The ponds are designed to work in series during <br />construction of the intake shaft pad, to provide effective treatment of discharge from the tributary areas <br />disturbed by construction. Pond 2 will provide for secondary treatment of discharges from Pond 1. Once <br />construction of the shaft pad is complete, the disturbed area contributing runoff will be reduced, and Pond <br />1 alone will provide sufficient capacity and adequate treatment of runoff. Pond 2 will then be reclaimed. <br />Pond 1 is sized to completely contain the runoff from the 10-year, 24 hour storm event without <br />discharging, and the principal spillway can pass the entire 10-year, 24-hour storm event. However, for <br />the purpose of sizing Pond 2, Pond 1 was conservatively modeled with a permanent pool at the principal <br />spillway elevation. <br />Twentytmile Coal Company 18L Shaft Pad 1 Water & Earth Technologies, Inc. <br />Drainage and Sediment Control System Design March 2, 2009