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<br />-40- <br />mines will diminish ground water flow rates within the aquifer during and <br />shortly after operation, whereas underground mines will primarily change <br />the ground water flow direction within the aquifers towards the mine <br />workings. Both of these effects will deplete yround water discharge from <br />the Twentymile Park Basin. This depletion will continue after mining has <br />ceased until the spoils or the mine workings have filled with water to a <br />pre-mining or an equilibrium level. <br />The significance of depletions of ground water discharges from the <br />affected aquifers is dependent upon ground water use downgradient of the <br />mines, either directly, as water well supplies, or indirectly by <br />sustaining base flows in streams and thus contributing to surface water <br />uses. <br />Water well supplies from the effected bedrock aquifers will not be <br />cumulatively impacted by the current mining activities and the proposed <br />Foidel Creek underground mine, since no wells are completed in these <br />aquifers within the Twentymile Park Basin. Also, no water wells are <br />completed in these aquifers downgradient of the Twentymile Park Basin <br />alony the Hayden Syncline. <br />The contributions of ground water to surface water uses is more complex. <br />However, the current impacts on ground and surface water systems due to <br />mining in the Middle Coal Group is projected to be minor because of: <br />1) the distance of the presently active mines and the proposed Foidel <br />Creek Mine from the discharge area, 2) the limited contribution of the <br />aquifers to the systems, and 3) the drawdown cone (water level decline) <br />in the aquifer associated with the Foidel Creek Mine is not expected to <br />reach the aquifer's stream/alluvial discharge areas. <br />Following mining, spoil waters from the surface mines and mine waters <br />from the underground mines will eventually recharge aquifers in the <br />Middle Coal Group. These waters will degrade the ground water qualities <br />of these aquifers with primarily increases in total dissolved solids <br />(TDS) and sulfate concentrations. Eventually, this degraded ground water <br />wil] discharge either to Sand Wash Basin through the Hayden Syncline, or <br />to the streams of the Trout Creek-Yampa River drainage basins through <br />subcrops and outcrops. (This long-term effect has been included in the <br />mining scenarios for the surface water cumulative hydrologic assessment.) <br />The operation of the Middle Creek Mine (presently inactive) would have an <br />impact upon the quantities of waters in the Middle Coal Group aquifers <br />and upon the surface stream/alluvial aquifer system. Because this mine <br />is located in a discharge area, it could significantly extend the <br />existing drawdown cone in the Middle Coal Group aquifer beneath the <br />existing discharge areas adjacent to stream/alluvial aquifer systems. <br />Once the drawdown cone has extended beneath the existing discharge areas <br />adjacent to streams, water will be withdrawn from the stream/alluvial <br />aquifer systems. This, in turn, would diminish surface water flows. <br />Withdrawals of water from the stream/alluvial aquifer systems would <br />continue until the water levels in the underground mine and the aquifer <br />reach the level of the aquifer's present stream discharge points. (The <br />extent and significance of this effect will be more fully examined during <br />the permit review of the permit application for the Middle Creek Mine.) <br />