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