2013-07-09_REVISION - C1981010 (7)

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These reactions enrich the infiltrating water with minerals from the spoil, forming a leachate that has high levels of dissolved solids. The leachate is alkaline, magnesium - sulfate type water. The concentration of dissolved solids can be predicted to not exceed 5,000 mg /1. The leachate samples at Trapper have remained in the range of 1,100 to 4,100 mg /1, (page 4 -23 8g of the permit). Leachate quality predicted by batch leaching tests - Batch leaching tests performed by the applicant indicated that the metallic constituents in coal spoil at Trapper which are most likely to form a leachate are manganese, molybdenum, and vanadium (section 4.8.2.5 of permit application). As of 2008, high levels of manganese have been detected in samples from three of the four wells on Trapper that monitor water quality in spoils - filled pits (wells GD -3, GF -5, GF -7, and GF- 11) [Appendix W of permit application]. Molybdenum and vanadium have not been detected in samples from the four wells. Leachate migration - The leachate that infiltrates the spoil in a back - filled pit or fill flows through the spoil to the lowest point in the pit or fill structure. In a fill, the leachate can be expected to discharge to the land surface from the lowest point of the fill. In a pit, the leachate accumulates against the un -mined low -wall of the pit, creating a saturated zone in the spoil against the low -wall of the pit. Leachate from this saturated zone then discharges into any permeable bedrock units in the low -wall whose hydraulic head is less than the head in the saturated zone. This discharge results in a leachate plume extending out into the bedrock. The Division predicts leachate plumes have formed or will form in Johnson, Pyeatt, and Flume gulches (Hydrology Notes map in Technical Revision 90). If the water quality of the leachate is worse than the native ground water quality, the leachate degrades the ground water. If the saturated zone in the spoil in a pit builds up high enough, the leachate overtops the lip of the pit and flows out onto the ground surface as a spoil spring. The only major aquifer that would be in the flow path of a subsurface plume of leachate at Trapper is the Third White Sandstone. Page 4- 226a of the permit application explains that if leachate were to flow beyond the permit area, two bedrock wells (W- 624 -74 and 80998) could be affected. Duration of leachate formation and migration - Rainwater, snowmelt, and ground water will perpetually infiltrate the spoil in the pits and fill structures. Leachate will continue to be generated in the spoil until the infiltrating waters are no longer coming in contact with minerals in the spoil that can react with the water. Over time, the amount of this contact between the water and the minerals diminishes as the flowing water chemically removes minerals from the spoil, resulting in a network of channels in the spoil. Eventually the mineral supply in the spoil will be exhausted, and the leachate quality will approach the pre- mining ground water quality. The amount of time necessary for the mineral supply to become exhausted is predicted to be on the order of a few hundred years. Leachate formation from buried ash - The permit's restrictions on waste placement were designed to limit the amount of subsurface water that comes into contact with buried ash. The restrictions require ash to be buried above the Trapper Mine 25 July 9, 2013