2013-01-29_PERMIT FILE - C1981022A (3)

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Swim ~ 2.05.6 Localized dewatering effects have the potential to temporarily reduce alluvial/colluvial ground water contributions to surface flow in Elk Creek This possibihty is not a significant concern, however, because any potential effects would be limited to the inunediate area of the Elk Creek surface facilities and would «cur over a relatively brief period during construction of the rntne development entnes and ventilation shafu. In addition, the alluvia]/colluvial deposits in the inunediate vicinity of the Elk Creek surface facilities are relatively deep and this portion of Elk Creek appears to be a losing rather than gaining stream reach, so ground water contributions to surface flow are either minimal or negative. OMI's planned underground mining operations may result in passive drainage requiring handling of mine inflows from both existing and proposed mine workings. Potential ground water inflow sources include water which has accumulated in old mine workings, the stratigraphic units directly disturbed by mining, and the overlying perched aquifer system due to downward leakage through subsidence fractures. I~istoric mining operations did not encounter significant quantities of water and it is anticipated that mine inflows maybe limited to specific areas. Any significant mine inflows will be collected and routed to temporary underground sumps for retention and recycling to the operational mine water system Detailed plans for handling of mine drainage are provided in Section 2.05.3, Mme Facilities. As mining praeeds, any significant ground water inflows may result in a temporary 1«alized reduction in the potentiometric surface. During mining operations a 1«alized cone of depression will develop, however, the mine workings will fill with water after operations cease and water levek in the mined area will reach equilibrium with the water table in surrounding areas. Specific mine design, operation, and monitoring measures including layout and sizing of mine openings based on subsidence evaluatiotu, the use of longwall mining methods, retention of bamer pillars and limited extrattion in critical areas, and ongoing subsidence monitoring, should effettively limit subsidence and any related ground water impacts. Mme design and operational control • measures, when combined with mining depth should m;n;m;>P fracture propagation at or neaz the ground surface in areas overlying the underground workings effectively limiting drainage of perched aquifers and alteration of surface infiltration characteristics. Where subsidence fractures do intercept perched aquifers, resulting ground water drainage and overall effects on the ground water system are expected to be minirrtal since the areal extent and ass«iated storage capacity of typical perched aquifers in this area are limited. Mme workings will not extend under the alluvium of the North Fork Mme layout a designed so the lateral extent of subsidence will extend no funkier south than the north edge of old State Id,gltway 133. Old State F~Lgltway 133 is north of the North Fork alluvium therefore, mining should not have an adverse effect on the alluvium of the North Fork of the Gunnison River. There should be no seepage from the Rolliru Sandstone up into the Sanborn Creek NLne B-Seam floor, due to the intervening 8.5 feet of relatively impermeable shale bed, the limited amount of grotrnd water in the Rollins unit, and its low permeability and transmissiviry. The fact that the Beaz Mmes and the old Somerset Mute did not experience seepage up though the floor of the B-Seam further support this conclusion. Similarly, the proposed Elk Creek D-Seam Mine is even higher in the stratigraphic sequence, further m;~;m;z;ng the potential for seepage from below. As described in Section 2.05.6, Subsidence Survey, Subsidence Control, and Subsidence Monitoring Plan, surface subsidence effects are expected to be minimal involving relatively uniform and limited reductions in surface elevations and development of minor surface tension fractures. Relatively small tension fractures will heal relatively quickly with the natural processes of material consolidation and sedimentation resulting in fracture filling and elirrtiriation of the fractures as prnential infiltration channels within a short time period. Underground mining operations will expose coal, floor, and roof materials to oxidation and increased leaching as ntirtirtg is completed and working areas are abandoned. Natural caving will ako expose substantial quantities of broken coal and roof material to oxidation and leaching as groundwater floods these areas. It is anticipated pH will increase slightly and the ground water chemistry will PR04 2.05-85 Revised August 2000