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<br />Leachate formation - <br /> Slightly acidic snowmelt and rainwater at the Trapper <br />Mine infiltrates into the spoil that fills the reclaimed pits and fill structure (Horse <br />Gulch Fill). Groundwater may also seep into spoil from adjacent or underlying <br />rocks. As the water percolates through spoil, chemical reactions occur with the <br />spoil. The main reactions are dissolution, pyrite oxidation, and cation exchange. <br />These reactions enrich the infiltrating water with minerals from the spoil, forming <br />a leachate that has high levels of dissolved solids. The leachate is alkaline, <br />magnesium-sulfate type water. The concentration of dissolved solids can be <br />predicted to not exceed 5,000 mg/l. The leachate samples at Trapper have <br />remained in the range of 1,100 to 4,100 mg/l, (page 4-238g of the permit). <br /> <br />Leachate quality predicted by batch leaching tests - <br /> Batch leaching tests <br />performed by Trapper indicated that the metallic constituents in coal spoil at the <br />Trapper Mine which are most likely to form a leachate are manganese, <br />molybdenum, and vanadium (section 4.8.2.5 of permit application). As of 2008, <br />manganese has been detected in samples from three of the four wells at the <br />Trapper Mine that monitor water quality in spoils-filled pits (wells GD-3, GF-5, <br />GF-7, and GF-11) \[Appendix W of permit application\]. Molybdenum and <br />vanadium have not been detected in samples from the four wells. <br /> <br />Leachate migration - <br /> The leachate that infiltrates the spoil in a back-filled pit or <br />fill flows through the spoil to the lowest point in the pit or fill structure. In a fill, <br />the leachate can be expected to discharge to the land surface from the lowest point <br />of the fill. In a pit, the leachate accumulates against the un-mined low-wall of the <br />pit, creating a saturated zone in the spoil against the low-wall of the pit. Leachate <br />from this saturated zone then discharges into any permeable bedrock units in the <br />low-wall whose hydraulic head is less than the head in the saturated zone. This <br />discharge results in a leachate plume extending out into the bedrock. The <br />Division predicts leachate plumes have formed or will form in Johnson, Pyeatt, <br />and Flume gulches (Hydrology Notes map in Technical Revision 90). If the water <br />quality of the leachate is worse than the native ground water quality, the leachate <br />degrades the ground water. If the saturated zone in the spoil in a pit builds up <br />high enough, the leachate overtops the lip of the pit and flows out onto the ground <br />surface as a spoil spring. The only major aquifer that would be in the flow path of <br />a subsurface plume of leachate at the Trapper Mine is the Third White Sandstone. <br />Page 4-226a of the permit application explains that if leachate were to flow <br />beyond the permit area, two bedrock wells (W-624-74 and 80998) could be <br />affected. <br /> <br />Duration of leachate formation and migration - <br />Rainwater, snowmelt, and <br />ground water will perpetually infiltrate the spoil in the pits and fill structures. <br />Leachate will continue to be generated in the spoil until the infiltrating waters are <br />no longer coming in contact with minerals in the spoil that can react with the <br />water. Over time, the amount of this contact between the water and the minerals <br />diminishes as the flowing water chemically removes minerals from the spoil, <br />resulting in a network of channels in the spoil. Eventually the mineral supply in <br /> <br /> <br />31 <br /> <br />Trapper MineJuly 9, 2013 <br /> <br />