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-32- The projections for the Yampa River near Hayden (Table 3) show effects similar to those identified in the other drainages. However, the Yampa River is the major drainage in the area and is less affected by mining operations than the other drainages considered. Affected water quality in the Yampa River would remain suitable for both flood irrigation and domestic use. The mining at Grassy Gap is expected to raise the natural levels of dissolved solids in Grassy Creek, both in the short-term and long-term scenarios. Due to the small acreage affected by the mine, coupled with dilution, these levels should not rise to a point where usability of the surface water for irrigation will be affected. Iron and manganese levels are also expected to increase in Grassy Creek due to leaching of waters through the spoil piles. This increase will taper off as the readily dissolvable metals are leached from the reclaimed areas. When considered with other mining activity in the area, the Grassy Gap mine will contribute to the cumulative increase in dissolved solids in the Yampa River. Due to dilution in this major river, the projected increase is small and the usability of the water in the Yampa should not be affected. This cumulative hydrologic assessment indicates that the Grassy Gap mining operation has been designed to prevent material damage to the hydrologic balance of surface water outside the permit area. B. Ground Water The Grassy Gap mine and a portion of the Seneca II mine lie on the Hayden Syncline structural and ground water basins. Both of these mines are surface operations and do not mine through any of the major sandstone aquifers of the ground water basin. Grassy Creek, downstream of the Grassy Gap mine, may provide partial recharge of the Trout Creek Sandstone. The increased levels of dissolved solids, resulting from mining at Grassy Gap may impact the quality of water in the Trout Creek Sandstone. The recharge area downstream of the mine is small and represents a minor portion of the total recharge area of the aquifer. The impacts on quality of the water in this aquifer should be insignificant. There is little possibility for long-term impact to water quality of regional aquifers from mining at Peabody Coal Company's mine, Seneca II W. There is the possibility that a vertical leakage by capillary action may occur through small fractures resulting from depressurization of the strata from the base of the Wolf Creek Coal to the Trout Creek Sandstone. A monitoring well in this aquifer at the mine site has measured five feet of hydraulic head loss. It is not known whether this is due to depressurization or if it is a result of other factors such as increased use of the aquifer. At any rate, the long-term effect of capillary movement would be mitigated once the overburden is replaced and pressure on the underlying strata is restored. The small fractures will seal themselves over time.