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Twentymile Coal Company 6RT Borehole Pad & Access Road Drainage and Sediment Control System Design <br />Introduction <br />Peabody Energy Twentymile Coal Company (TCC) is constructing a thickener borehole installation (6RT <br />Borehole) at TCC's Foidel Creek Mine, located in Routt County, Colorado. The 6RT Borehole <br />installation includes an existing light use road, and a 70 x 70 ft drill pad. Following completion of the <br />6RT Borehole installation, the 70 x 70 ft drill pad will be reclaimed. This report addresses surface runoff <br />from the access road and borehole pad. <br />Small area exemptions may be applicable for facilities that disturb less that 1 acre and are outside the <br />boundaries of conventional runoff control. The 6RT Borehole pad and access road disturbs 0.28 acres, <br />thus meeting the small area criteria. This facility is located in an isolated area within the permit <br />boundary, and has a low potential for runoff. If construction of traditional runoff control structures is <br />required (i.e. sediment ponds), the resulting disturbance would likely be greater than that of the facilities <br />themselves. Alternative sediment controls (ASC's) will effectively reduce sediment transport from the <br />disturbance to acceptable limits. Straw wattles or Curlex sediment logs placed along the contour, down - <br />gradient from the disturbance combined with gravel surfacing of the disturbed area will reduce the <br />sediment concentration of stormwater runoff resulting from the 10 -year, 24 -hour storm to meet NPDES <br />requirements. The site was modeled in SEDCAD to determine the effectiveness of ASC's in controlling <br />erosion and sedimentation, and to ensure that stormwater runoff velocities are not erosive over the area of <br />disturbance. <br />Appropriateness of Small Area Exemption <br />A small area exemption for this site is appropriate for the following reasons: <br />Its location is isolated from other facilities constructed under this permit that require the use of <br />sediment control structures. <br />2. The total area disturbed is small (0.28 acres). <br />3. Alternate sediment control methods ( Curlex sediment logs or straw wattles) are utilized at the site to <br />prevent surface degradation and protect the hydrologic balance. All runoff is expected to exit the area <br />in the form of sheet flow. <br />4. The site location is geologically and hydrologically stable. <br />5. The existing access road and 70 x 70 ft borehole pad will be surfaced with coarse gravel. Sediment <br />logs or straw wattles placed down - gradient, and along the contour, from the disturbance will <br />effectively trap suspended sediment and settleable solids in runoff from the disturbed area. <br />6. The 70 x70 ft borehole pad will be reclaimed following completion of the thickener underflow <br />system. <br />SEDCAD Modeling <br />A SEDCAD analysis was conducted to determine the effectiveness of alternative sediment controls <br />(sediment logs or straw wattles, and gravel surfacing of the disturbed area) to treat stormwater runoff <br />Water & Earth Technologies, Inc. Page 1 of 4 October 23, 2009 <br />