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impoundments above the affected areas. Related changes in local <br /> piezometric surfaces could result in the modification of springs and <br /> seeps. Waters, having entered the sub-surface strata, could migrate <br /> towards the underground workings, eventually permeating the strata and <br /> gradually migrating downdip to the axis of the Piceance Basin. <br /> None of the three mines will affect the use of ground water from bedrock <br /> aquifers either separately or cumulatively, since there are no bedrock <br /> water supply wells completed in the region hydrologically adjacent to the <br /> mines. Also, the potential for impacting future use of bedrock ground <br /> water is low due to the depth of drilling required and to the problems of <br /> accessing steep slopes on private and U. S. Forest Service land. <br /> The quality and quantity of the alluvial aquifers of Fourmile Creek, <br /> Thompson Creek and Coal Creek will not be cumulatively impacted during the <br /> first 5-year permit period of the three mines. There is a possibility of <br /> quality and quantity impacts on Coal Creek in the distant future when, and <br /> if, the north Thompson Creek Mines progress into the Coal Creek Drainage. <br /> There is a minor potential for the Coal Basin Mine loadout and the North <br /> Thompson Creek Mine loadout to cumulatively impact the quality and <br /> quantity of ground water in the Roaring Fork alluvium, since both loadouts <br /> are located on this alluvial body. However, both of these loadouts have <br /> limited surface disturbance, small amounts of disturbed drainage to <br /> handle, and much of the disturbed drainage water is going into total <br /> containment lined ponds. Therefore, that minor amount of polluted <br /> drainage which may escape from the drainage control system will negligibly <br /> impact the quality of the vast amounts of Roaring Fork alluvial water. <br /> The greatest cumulative ground water impact will be the indirect depletion <br /> of surface water flow through the ground water system to the mine <br /> workings. Since the impacts are to surface water, they are quantitatively <br /> discussed in the Technical Analysis (TA) portion of the decision <br /> document. The mechanism of steam depletion has been discussed in the <br /> general description of ground water in the TA. <br /> One of the potential cumulative impacts that has been effectively <br /> mitigated is unnatural sediment loads generated by the operations. The <br /> sediment control systems in use at all mines and the strict effluent <br /> limitations imposed on all discharges will keep sediment loads near <br /> natural levels. It also should be noted that all three mines considered <br /> in this assessment are underground mines with minimal surface <br /> disturbance. Total surface disturbance is only 0.06% of the 1451 square <br /> mile Roaring Fork River Basin. Because of these facts, cumulative <br /> sediment loads are not expected to be significant. <br /> The cumulative effect of impounding water in sediment ponds at all three <br /> mines will also be insignificant. Because of the small relative surface <br /> disturbance at each mine, pond storage volumes are likewise small. <br /> Sedimentation ponds at all mines are dewatered within 72 hours to minimize <br /> any water loss to the hydrologic system. <br /> -13- <br />