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The significance of depletions of ground water discharges from the affected aquifers is dependent upon ground water use down gradient of the mines, either directly, as water well supplies, or indirectly by sustaining base flows in streams and thus contributing to surface water uses. Water well supplies from the affected overburden aquifers will not be cumulatively impacted by the current mining activities and the Foidel Creek underground mine. The contributions of ground water to surface water uses is more complex. However, the current impacts on ground and surface water systems due to mining in the Middle Coal Group is projected to be minor because of; 1) the limited permeability of the overburden aquifer; 2) the low permeabilities of formations at depth; 3) the discontinuity of the lenticular sandstones in the' overburden aquifer; 4) the shallow gradients and distances between recharge and discharge areas; and 5) the thick sequence of shales between major aquifers. Following mining, spoil waters from the surface mines and mine waters from the underground mines will eventually recharge aquifers in the Middle Coal Group. These waters will degrade the ground water qualities of these aquifers primarily with increases in total dissolved solids (TDS) and sulfate concentrations. Eventually, this degraded ground water will discharge either to Sand Wash Basin through the Hayden Syncline, or to the streams of the Trout Creek-Yampa River drainage basins through subcrops and outcrops. (This long-term effect has been included in the mining scenarios for the surface water cumulative hydrologic assessment.) Iles Formation Aquifers: Trout Creek Sandstone and Tow Creek Sandstone The Trout Creek sandstone may be impacted by the Foidel Creek underground mine. The coal seam to be mined is located stratigraphically 150 feet above the Trout Creek sandstone: The permit application for the Foidel Creek mine has identified 9 faults where inflows from this aquifer may occur. To date, however, three of the faults have been mined through and have not encountered increased flows, Should inflows be encountered while crossing the other faults, local draw downs in the Trout Creek Sandstone can be expected. The aquifer will not be dewatered and draw downs are not expected to adversely affect ground water wells. The Little Middle Creek tract of Eckman Park may influence Trout Creek water quality near the recharge area as a consequence of lithic unloading and blasting. However, there is sufficient aquifer storage and distance between the mine discharge point to minimize significant regional impacts. The Trout Creek sandstone is used by the mines in the area for domestic and industrial uses, and will produce small draw downs adjacent to the wells. These uses of water will not significantly impact the discharge rate of Trout Creek sandstone to the surface water system. The mines currently are not using ground water from the Tow Creek sandstone and, therefore, will have no impact on the quantity of water in this aquifer. -54-