2020-08-20_REVISION - C1980007 (36)

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a. Ground Water Effects During mining at the West Elk Mine, ground water seeps into the underground workings from rock exposed in the workings. Inflow into the workings was estimated to be a total of 166 acre-feet in 2004. Excess accumulations of this water are pumped out of the workings into Sylvester Gulch through a permitted discharge outfall. After mining is completed, pumping will cease, the portals will be sealed, and the underground workings will flood with the water seeping into the workings. The operator estimates it will take between 200 to 800 years for the workings to fully flood. The water that seeps into the workings will saturate the gob in the down-dip end of the workings and minerals will be dissolved from the gob, creating a gob leachate. This leachate can be expected to be alkaline and have TDS between 1,000 and 5,000 mg/l(for comparison,North Fork alluvial water probably has TDS greater than 1,500 mg/l, based on monitoring at the Bear No. 1 Mine.) As the down-dip end of the West Elk Mine workings fill with gob leachate, this leachate will exert a hydraulic head on the downdip walls of the workings and the leachate may seep into the cleat porosity and fault porosity that is in the coal seams exposed in the workings. This leachate could flow down-dip, parallel to bedding, through the coal seams and discharge from the coal seams into the alluvium of the North Fork of the Gunnison where the coal seams sub-crop underneath the alluvium. This seepage would form a plume of gob leachate in the alluvium that would extend downgradient from the Sylvester Gulch/North Fork confluence toward Somerset. The likely maximum discharge rate of gob leachate into the alluvium would be on the order of 100 gpm based on the maximum head that could develop in the flooded workings. Such a plume would mix with and be diluted by ground water in the North Fork alluvium and would be attenuated where the alluvium significantly widens upgradient from Somerset. North Fork alluvial ground water is monitored upgradient from Somerset at the Bear No. 3 Mine. Low permeability of bedrock units in and near the West Elk Mine (Rollins Sandstone, B through F seams, and the Barren Member) will prevent any significant seepage of gob leachate into these units. MCC has submitted two plans for sealing the portals upon closure of the mine. One plan calls for perpetual gravity discharge of mine water to the surface through a four-inch PVC pipe that will be installed in a block wall at the portal. An alternate plan(to be constructed if water is toxic) is to construct water-tight seals within the mine that will withstand the expected hydraulic pressures. Samples taken in the adjacent Oliver Mine indicate mine waters will not be toxic. 43