2018-01-31_REVISION - C2009087 (30)

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underground mine water is not consumed in the workings, then the water will be pumped from the portal area sumps to Pond 002. Table 2.05.6(3) of the permit application contains a mass -balance mixing calculation of the pumped underground mine water and the water in Pond 002. The underground mine water is expected to have a much lower TDS content (approximately 1,050 mg/1) compared to the TDS content of Pond 002 water (approximately 4,000 mg/1) because the pillars will not be removed during the proposed mining, resulting in minimal creation of waste rock (gob) in the workings. The mixing calculation indicates that, compared to the current water quality of Pond 002, the pumped mine water will improve the quality of water in the pond and its discharges to Little Grassy and ultimately Grassy Creek. Discharges of spoil leachate from the backfilled Seneca II Mine pits are expected to continue indefinitely, with the dissolved solids content of the spoil leachate declining over a long period of time (possibly centuries) as the chief source of dissolved solids in the spoil (pyrite) is exhausted. After underground mining is complete, the workings of the PSCM are predicted to fill with water up to a maximum elevation equal to the portal elevation over a time period of less than 22 years. In addition to salt loading of streams, PSCM activities are predicted to result in the following impacts: Spoil leachate from the reclaimed surface mine pits will continue to load alluvial ground water with salts in the Little Grassy, Grassy, and Cow Creek drainages. As the alluvial/colluvial water quality closely mirrors the quality of the stream, a degradation (i.e. increase in TDS) is expected to be observable in alluvial monitoring wells down -gradient of the mine site. The Division has previously found that in portions of the Little Grassy Creek drainage that are in contact with previously disturbed lands, TDS increase over baseline conditions is observable (Seneca II Mine findings). The potentiometric surface of the Trout Creek Sandstone will continue to be depressed by the mine operator's pumping of water from this unit in their supply well located in the mine facilities area. The pumping rate will be subject to SCCC's water rights associated with this well. PSCM predicts no impacts to potential aquifers above the Trout Creek Sandstone from pumping the well, based on the presence of thick intervening shale beds that act as aquicludes. Removing coal in the underground workings will cause ground water inflow into the mine void, resulting in drawdown of the potentiometric surface in the Wadge coal seam around the perimeter of the workings. The drawdown will extend into beds immediately above and below the coal seam. Maximum inflow of ground water into the workings is predicted to be 75 gallons per minute, including less than 2 gallons per minute of predicted spoil leachate 17