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-32- In light of these observations a range of subsidence impact scenarios are possible. Subsidence related cracking could increase surface and overburden infiltration rates, diverting water from streams and impoundments above the affected areas into underlying aquifers and/or the mine workings. Related changes in local piezometric surfaces could result in the modification of springs and seeps. Waters, having entered the sub-surface strata, could migrate towards the underground workings, eventually permeating the strata and gradually migrating down-dip to the axis of the Piceance Basin. Latteral movements of ground water may be disrupted due to differential settling of overlying rock strata. None of the three mines studied in this CHIS will affect the use of ground water from bedrock aquifers elther separately or cumulatively, since there are no bedrock water supply wells completed in the region which are hydrologically ad3acent to the mines. Also, the potential for impacting future use of bedrock ground water is low due to the depth of drilling required and to the problems of accessing steep slopes on private and U.S. Forest Service land. The quality and quantity of the alluvial aquifers of Fourmile Creek, Thompson Creek and Coal Creek will not be cumulatively impacted during the first 5-year permit period of the three mines. There is a possibility of quality and quantity impacts on Coal Creek in the distant future when, and if, the North Thompson Creek Mines progress into the Coal Creek Drainage. There is a minor potential for the Coal Basin Mine loadout and the North Thompson Creek Mine loadout to cumulatively impact the quality and quantity of ground water in the Roaring Fork alluvium, since both loadouts are located on this alluvial body. However, both of these loadouts have limited surface disturbances, small amounts of disturbed drainage to handle, and much of the disturbed drainage water is going into total containment lined ponds. Therefore, that minor amount of polluted drainage which may escape from the drainage control system will only negligible impact the quality of the vast amounts of Roaring Fork alluvial water. The greatest cumulative ground water impact will be the indirect depletion of surface water flow through the ground water system to the mine workings. Since the impacts are to surface water, they are quantitatively discussed below in the Surface Water section of this Cumulative Impacts Study. The mechanism of stream depletion has been discussed in the general description of ground water. Cumulative Impacts - Surface Water One of the potential cumulative impacts that has been effectively mitigated is unnatural sediment loads generated by the operations. The sediment control systems in use at all mines and the effluent limitations imposed on all discharges will keep sediment loads near natural levels. It also should be noted that all three mines considered in this assessment are underground mines with minimal surface disturbance. Total surface disturbance is only 0.06°~ of the 1451 square mile Roaring Fork River Basin. Because of these facts, cumulative sediment loads are not expected to be significant.