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

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Scrlia: - 2.05.6 combination of grading, use of sedimentation ponds, and progressive reclamation will protect water • quality and mimmi~r the potential for erosion and excess sediment contributions. Becatue this is a semi-arid area and for the reasons previously discussed under water chemistry and water quality impacts, no significant impacts are anticipated due to any limited infiltration and leaching which may occur. OMI's planned mining and related activities have the potential to cause localized and temporary impacts to ground water. These probable ground water hydrologic consequences include: Alterations of ground water flow patterns Drainage of seeps/springs Increases in concentrations of TDS and certain individual chemical constiruents Muting-related probable hydrologic corssequences for ground water resources in the Somerset, Sanborn Creek and Elk Creek pemut and adjacent areas will be limited by the lack of significant ground water recharge in the area, the existence of a thick sequence of low pemteability bedrock layers between the coal seams to be mined and the ground surface, the general lack of significant regional ground water movement due the absence of well defined regional aquifers, and very limited beneficial ground water use in the permit and adjacent areas due to depth and water quality considerations. Probable ground water consequences are disctsssed in the following sectioru. There is no identified regional aquifer above or below the mine workings. The only ground water units of any significance in the OMI permit and adjacent areaz are the petthed ground water occurring in certain coal seams and the associated clastic interburden sequences and in the overlying overburden materials; and the alluvium/colluvium of the Nortkt Fork of the Gunnison River. The Rollins Sandstone is not coruidered an aquifer for the reasons previously discussed, is well below the • sequence to be mined, and is hydrologically isolated From potential mining-related effetts by a thick sequence of relatively impermeable shales, between the Rollins Sandstone and the B-Seam The alluvium/colluvium of the Noah Fork is at an elevation below the D coal seam where it outcrops on the north side of the Noah Fork Valley, with the seam dipping to the nonheazt. While the B-Seam is at a lower elevation than the North Fork in the eastern pan of the mining area, it is hydrologically isolated from the Noah Fork and the associated alluvium/colluvium by a sequence of low-permeability units which will not be affected by mining or subsidence since a broad bamer pillar will be retained between the [nine workings, the North Fork and both the old and the existing State Highway. The strata above and those associated with the mine workings may contain perched aquifers, which could be depleted directly through mining or indirectly through fractums into the mine. However, no water supply wells are known to exist in any local perched aquifers. The potential for future rue of these perched aquifers is severely limited because of the their depth, the steepness of the valley slopes (access corutrairtts), and the limited lateral extent and coruequent storage capacity of the perched units. Even if the peached aquifers were impacted, therefore, it is doubtful that water supply wells or water resources would be impacted now or in the future. Alteratioru of ground water flow patterns will occur az a result of underground mining excavation and consequent mine drainage. These activities will depress the water table in the irrurtediate vicinity of the mine workings, which will induce ground water flow into the underground workings az the hydrologic system adjusts to reestablish stable water levels. Potential ground water inflows are discussed in Section 2.04.7, Hydrologic Description. A shift in ground water flows toward the mine workings will alter existing ground water flow patterns. Natural ground water flows in the immediate vicinity of the underground mine twrlangs will tend to be deflected toward the mine workings under the influence of the cone of depression. This deflection o[ flow will provide some natural mitigation • of flow alteration az flow lines converge around the active mining area to reestablish downgradient flow continuity at some point downdip from the mine workings. PR04 2.05-94 Revised August 2000