2010-05-07_GENERAL DOCUMENTS - C2009087

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¦ 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 inflow from surface mine pits that are updip from the underground workings. Section 2.05.6(3)(b(iii) of the permit application contains an analytic simulation of the drawdown for three points in time after commencement of underground mining: 2 years, 5 years (end of first permit term), and 35 years (30 years after first permit term, with no mining beyond the current non-retreat room-and-pillar proposal). Figure 2.05.6-1`4.2 in the permit application shows predicted drawdown contours for the Wadge coal potentiometric surface. The contours indicate drawdown at the location of the seam's outcrop on the northeast flank of the Fish Creek Anticline will exceed 50 feet during the initial five-year permit term, and then will recover to a drawdown of less than 5 feet after mining ceases. The Wadge coal seam at this location is within the Trout Creek Sandstone- Twenty7nile Sandstone outcrop belt that has been reported as contributing to Grassy Creek flows (page 38 of U. S. Geological Survey publication titled Evaluation of the upper part of the Mesaverde Group, Northwestern Colorado, Water-Resources Investigations Report 90-4020, by S.C! Robson and Michael Stewart). Seepage out of the outcrop belt was reported as contributing approximately 20% of Grassy Creek's instream flow when sampled in July and September in 1986. The Wadge coal seam comprises roughly 5% of the aggregate thickness of likely permeable units in the outcrop belt, assuming the seam is 10 feet thick and the Trout Creek and Twentymile Sandstones are in aggregate 200 feet thick. If the coal seam's contributions to Grassy Creek flows are eliminated by PSCM's drawdown (which PSCM's analysis indicates is possible), and assuaging the Wadge coal seam contributes 5% of the outcrop belt's total contribution to Grassy Creek flows, then PSCM's drawdown could reduce Grassy Creek's flows by 1% (a 5% reduction of 20% of Grassy Creek's flow). A 1% reduction in flow in Grassy Creek will not be enough to impair the use of Grassy Creek water. There currently are no users of surface water on Grassy Creek. ¦ Underground mine water that accumulates in the downdip end of underground mine workings will seep into the unmined rock in the walls, roof, and floor of the workings. The quality of this leachate is expected to be similar to native ground water in the Wadge coal seam and the roof and floor rock because large amounts of gob (waste rock) are not expected to be produced during; non-retreat room-and-pillar mining. SCCC predicts that a plume of the mine water will advance outward from the workings at a maximum velocity of 11 feet per year, a slower velocity than predicted for the nearby Foidel Creek Mine in the previously mentioned U.S. Geological Survey report of Robson and Stewart. SCCC predicts that seepage of underground mine water from the workings updip into Grassy Creek alluvium after postmining recovery of the Wadge seam potentiometriic surface will be less than 1 gallon per minute and will be too small to have a significant effect on alluvial ground water quality. Peabody Sage Creek Mine 25 May 7, 2010