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Prior to reservoir construction, sediment control materials (straw wattles or silt fence) will be placed downgradient
<br />of the construction area, and an estimated 12,800 cy of topsoil (average depth 12 -inches) will be recovered and
<br />placed in a stockpile approximately 90 feet wide and 600 feet long adjacent to the existing topsoil stockpile for the
<br />6MN Ventilation Shaft Pad. The stockpiled topsoil will be stabilized by seeding with the standard temporary
<br />stabilization seed mixture within 6 months following completion of placement, and drainage from the pile will be
<br />controlled through placement and maintenance of erosion control material (straw wattles or silt fence) around the
<br />stockpile perimeter until effective vegetative cover is established. The reservoir embankment will be an earthen
<br />dam constructed of suitable fill material excavated from the reservoir basin (approximately 15,800 cy) and placed
<br />and compacted consistent with the engineering design specifications and construction recommendations provided in
<br />Exhibit 490, 6MN Mine Water Storage Reservoir — Design Drawings
<br />The resulting embankment structure will be approximately 14 feet high at its highest point, with the dam crest at an
<br />elevation of 6,717 feet. Actual constructed height will take into account anticipated embankment settlement, which
<br />is estimated at a maximum of 5 percent (constructed top elevation 6,717.7 feet). The surface area of the maximum
<br />pool (operational mine water storage + pump buffer storage + design stormwater storage)at an elevation of 6,714.4
<br />feet will be approximately 5.1 acres, resulting in a maximum design storage volume of approximately 34.3 acre-feet
<br />of water. Even though the 6MN Storage Reservoir has been designed and will be operated as a closed reservoir, a
<br />single open -channel emergency spillway, with an invert of 6,714.9 feet, has been incorporated into the design in
<br />compliance with applicable regulatory requirements for impoundments. The design of the reservoir also
<br />incorporates a compacted soil liner (approximately 4,800 cy with an estimated permeability of 2 x 10-7 cm/in) to
<br />minimize infiltration and water loss. Due to the capacity and embankment height for the reservoir, reservoir design
<br />and construction must also meet the requirements of the Mine Safety and Health Administration (MSHA, 30CFR
<br />77.216). Reservoir design and construction specifications meet all applicable design and regulatory requirements.
<br />Separate applications for approval were submitted to this agency, with a copy of final approval provided to the
<br />CDRMS.
<br />TCC's intent is to consistently maintain approximately 9.55 million gallons, or 29.3 acre-feet, of water in the 6MN
<br />Reservoir, to supply the mine water system. As designed, the Reservoir has sufficient additional capacity to
<br />provide buffer storage for another 1.0 million gallons, or 3.1 acre-feet, of emergency mine water storage to address
<br />any maintenance or emergency shut -downs of the pumping system, and 1.9 acre-feet (equivalent to the runoff from
<br />a 100 -year, 24-hour storm event) of runoff from the limited contributing watershed. At maximum water storage
<br />level, taking into account the incremental storage capacities outlined (34.3 acre-feet), reservoir freeboard, at the
<br />maximum pool level, will be approximately 0.5 feet to the invert of the open -channel spillway, and 2.6 feet to the
<br />top of the embankment. Hydrologic design information for the 6MN Storage Reservoir is provided in Exhibit 8CC,
<br />6MN Mine Water Storage Reservoir - Drainage Designs. The discharge pumping system for the 6MN Storage
<br />Reservoir has been designed with the capacity to dewater the design storm -water storage volume in less than 12
<br />hours. Any discharge of stored mine or stormwater from the 6MN Storage Reservoir, will be directly to the mine
<br />water system, with no discharge through the emergency spillway to natural drainage. While it will not be utilized
<br />during normal operations or under design storm conditions, the open -channel spillway, and the lower portion of the
<br />stormwater diversion channel are designed to safely pass the peak discharge (approximately 125.5 cfs), and the
<br />total runoff, respectively from the design storm event.
<br />On the edge of the Reservoir, near the embankment, a pump platform will be constructed, with a suspended 50 HP
<br />high-capacity submersible pump, pumping mine water from the Reservoir at a rate of up to 1,000 gpm to the
<br />Million -Gallon Tank at 6MN (MR06-210), which is set-up to gravity -feed to the mine water system through the
<br />existing mine water utility borehole. A pad and short (130 feet) light -use road segment will provide access for
<br />inspection and maintenance. The approximately 1,400 foot pipeline from the 6MN Reservoir to the Million -Gallon
<br />Tank and utility borehole will also be 10 -inch HDPE pipe buried to a depth of approximately six feet within the
<br />previously disturbed 6MN road right-of-way. Pipeline construction disturbance will be reclaimed concurrently
<br />with pipeline installation, with backfilling, soil replacement, and reseeding using the standard rangeland seed
<br />mixture.
<br />In order to assure adequate and consistent water quality for the feed to the mine water system, the 6MN Mine Water
<br />Storage Reservoir installation will include some basic water treatment measures, potentially including one or more
<br />aerators, floating baffles within the pond, and a simple filtration system in the discharge pipeline between the
<br />MRI 1-259 2.05-45.14 11/17/11
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