2020-08-20_REVISION - C1980007 (36)

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Subsidence fractures that develop over mine workings have the potential to dewater natural springs and wetlands in the West Elk permit area. The permit application explains that the risk of such depletion is significantly reduced by the overburden thickness of greater than 280 ft. and the resistance to fracturing of interbedded fine-grained units that would deform ductiley(bending), rather than brittley (fracturing). Monitoring of springs since before the mine was developed has not detected impacts from mining. This monitoring will continue. The operator predicts overall ground water/surface water balance will not be significantly affected if mine inflows are much greater than predicted because mine inflows will ultimately be discharged back to the North Fork of the Gunnison. i. Refuse Material Disposal Impacts on Groundwater - Lower Refuse Disposal Area—Coal mine waste will be disposed in the Lower Refuse Disposal and Refuse Pile Expansion sites. The lower refuse pile is permitted for permanent disposal of 1.09 million cubic yards of mine development waste and sediment pond cleanout material. The refuse pile expansion is designed to hold 1.38 million tons of material over a life of 9.4 years. The Refuse Pile Expansion is discussed in Permit volume IOB. An Upper Refuse Disposal Area has also been approved for construction, and has not been built. MCC has verbally indicated this pile will not be needed. Stipulation#7 requires MCC to complete a geotechnical investigation prior to constructing Upper Refuse Pile. To date, refuse piles have not impacted the groundwater zone near Sylvester Gulch. Because of the low permeability of the colluvial soils and the lack of a groundwater table near the surface in the colluvium, no impacts on groundwater are anticipated. Water monitoring near the site of the soil stockpiles and waste piles, other water quality monitoring stations on Sylvester Gulch, and sites above and below the mine on the North Fork of the Gunnison, provide data on impacts of this refuse stockpile. The wells in the colluvium in the vicinity of the refuse piles have been mostly dry since 2000 or yielded insufficient water to provide an analysis. A leachate study of the F-seam refuse was conducted and included in the application for the permanent refuse disposal site in Exhibit 51 of the permit application. In addition, the effects of B-seam refuse on leachate were also evaluated and can be found in Exhibit 56. The permeability contrast found in the pile will cause most of the leachate to perch within the refuse pile and above the native soils. A shallow water mound will probably 44