2009-12-02_GENERAL DOCUMENTS - C1981010 (2)

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• 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 predicted subsurface saturation elevation (section 4.3.4.6 of permit application). Almost all of the waste ash material will be coal combustion ash and sludge from the Craig power plant. The main elemental components of the combustion wastes are barium, calcium, iron, magnesium, potassium, silicon, and titanium. Adequate ground water monitoring is in place to detect any ground water problems early-on so mitigative measures can be implemented. Drawdown of water levels in aquifers - The original permit application predicted that the excavation of pits at the Trapper Mine could temporarily draw down water levels in local aquifers by a few feet at a distance of one mile from the permit boundary (page 4-226 of permit application). The predictions proved inaccurate as no draw-downs were detected in wells at those distances. Observed draw-downs have been temporary and are limited to the immediate vicinity of the • pits. The operator expects such limited draw-downs to continue with future Trapper Mine 26 September 21, 2009