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NMD Power Borehole -Drainage and sediment control for this site involves gravel surfacing of the pad and use of
<br />the surrounding vegetation as a downslope vegetative filter. The site is approximately 1 acre and will be surrounded on
<br />• three sided by topsoil stockpiles, which will limit surface ninon to the pad. The SAE demonstration for this site is
<br />presented in Exhibit 8.6.
<br />NW Mains Ventilation Shaft and Thickener Underflow Proiect (TRO1-391- Drainage and sediment control for
<br />the NW Mains Ventilation Shaft site is provided by upgradient diversion and access road ditches, perimeter topsoil
<br />berms, a rock containment berm/filter along the entire southeast side of the pad, pad topsoil stockpile collection
<br />ditches, gravel surfacing, downgradient containment berm, and the combination of sump and rock check-dam at the
<br />low point of the berm, and the natural vegetative filter existing between the shaft site and Fish Creek. Under
<br />elevated flow conditions, discharges may pass through the sump and rock filter berm and flow into adjacent low-
<br />lying areas. Under these conditions, effective control and treatment will still occur, since the sump and berm will
<br />act as sediment traps, the flow path will be extended, and the low-lying areas and vegetative filter will serve to
<br />control flows and remove settleable solids. As a relatively small area (approximately '1.8 acres), and given the
<br />effectiveness of the drainage and sediment control measures, the additional disturbance that would be involved in
<br />constructing a sediment pond is not warranted and off-site sediment contributions can effectively be controlled by
<br />these measures under a Small Area Exemption, consistent with applicable provisions of Rule 4.05.2(3)(b). In mid-
<br />2005, TCC experienced an unanticipated discharge of production mine water due to the failure of both primary and
<br />back-up secondary pressure-relief valves on the underground system. While the flows associated with this
<br />discharge significantly exceeded anticipated runoff flows for the design storm, the drainage control system of the
<br />sump, rock check dam, and downgradient vegetative filter functioned together to effectively control and treat
<br />discharge flows. The effectiveness of the existing control and treatment measures was demonstrated by the fact
<br />that no flow ever reached Fish Creek, and sampling results for the ponded discharge water in the low-lying areas
<br />associated with the vegetated filter indicated that the water would meet applicable effluent discharge standards.
<br />Under normal conditions, there will be no discharge or mixing of mine water with surface drainage from the site.
<br />• One of the cased holes, completed in the pad area, is used as a pressure relief/accumulator for the production mine
<br />water system. System pressure relief and back-up were designed to prevent any surface discharge of production
<br />mine water, however, a catastrophic failure of both primary and back-up pressure relief valves resulted in a
<br />discharge in mid-2005. A manual ball valve was installed at that time to seal the casing and prevent any future
<br />discharge. If suitable system pressure relief controls can be installed to assure that no future discharge will occur,
<br />the manual valve may be removed or replaced with afail-safe valve that would allow for free movement of air in
<br />the pressure relief/accumulator pipe but prevent fluid discharge, at some point in the future. So long as no
<br />discharge is anticipated or occurs, the provisions of Rule 4.05.2 are not applicable.
<br />Given the limited site area, a "D" soil type (Binco silty clay loam), the drainage and sediment control measures, and a
<br />distance of approximately 500 feet between the shaft site and Fish Creek, calculated peak flow and runoff from the site
<br />for the 10-year, 24-hour storm event would be approximately 1.65 cfs and 0.13 acre-feet, respectively. Gravel and
<br />vegetative filter efficiency was modeled using the SEDCAD4 check dam and grass filter utility. The SEDCAD4
<br />demonstrations are presented in Exhibit 8S. The estimated settleable solids concentration under peak discharge
<br />conditions would be approximately 0.42 mUl. This settleable solids level is consistent with natural sediment levels for
<br />runoff from undisturbed lands, given similar conditions, and would be significantly reduced as the runoff passes
<br />through the vegetated area between the shaft site and Fish Creek.
<br />EMD Ventilation Shaft -The SAE for the EMD Ventilation Shaft covers approximately 2.66 acres of disturbance and
<br />is located in the NWl/4SE1/4 Sec. 14, TSN, R86W, 6th P.M. on Routt loam (a C soil). SAE demonstrations have
<br />characterized three areas; the east side of the east topsoil pile; the south and west sides of the southwest topsoil pile; and
<br />other topsoil slopes and the benches and cut and fill slopes of the facility itself. Sediment control will be achieved
<br />through the installation of an upland clean water diversion ditch, seeding and mulching the topsoil piles and cut and fill
<br />slopes with 2 tons/acre straw, and graveling the pads and roads with gravel adequate to achieve at least 80 percent
<br />• surface coverage. Vegetative filters will provide adequate sediment detention to achieve effluent limitations following
<br />a 10-year, 24-hour storm event. Peak flows from the design storm are 0.05 cfs, 0.27 cfs, and 1.96 cfs for the east side of
<br />the east topsoil pile, the south and west sides of the southwest topsoil pile, other topsoil slopes, and the benches and cut
<br />and fill slopes of the facility, respectively.
<br />TR08-65 2.05-83 08/20/08
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