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Mine Dewatering Pipeline to the 6MN Reservoir, and a pumped outlet line from the Reservoir to the 6MN Million - <br />Gallon Tank. The 6MN Storage Reservoir will be used to provide back -up storage for TCC's 10 -Right Mine <br />Dewatering Well (also previously identified as the EMD Dewatering Facility), allowing the well to pump <br />continuously at full capacity, providing buffer storage for pump maintenance or failure, and limiting the distance <br />over which the mine water must be transferred. <br />The 6MN Storage Reservoir is designed as a closed reservoir, with no discharge, and provision for adequate <br />operating free -board to allow for storage of runoff from a small upslope drainage area. The 6MN Mine Water <br />Storage Reservoir will be constructed south of the current 6MN Shaft pad and northwest of the NW Mains <br />Ventilation installation. The reservoir embankment will be an earthen dam constructed of suitable fill material <br />excavated from the reservoir basin and placed and compacted consistent with engineering design specifications and <br />construction recommendations. The engineering designs are based on site geotechnical investigations, which <br />included drilling of six, 4 -inch hollow -stem auger test holes; examination and testing of soil samples; placement of <br />1.5 -inch slotted PVC pipe in two of the auger holes to measure water levels (piezometers); and excavation of seven <br />shallow (6 -11 feet in depth) test pits. The location of the 6MN Mine Water Storage Reservoir geotechnical <br />investigations is shown on Map 24 (sheet 2 of 3), Surface Facilities and Freshwater Systems, and by the detail <br />drawings in Exhibit 490, 6MN Mine Water Storage Reservoir — Design Drawings. <br />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 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 />TR06 -57 2.05 -45.6g 07/28/09 <br />