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D. Cumulative Hydrologic Assessment -Summary and Findin s A cumulative hydrologic assessment of all anticipated mining on the hydrologic balance in the general area of the Hayden Gulch Mine has been made by the Division. This assessment has considered the short-term and the long-term effects of mining to the surface water and ground water hydrologic systems. The ground water assessment indicates that mining may affect several aquifers in the general area. The regional aquifers, Trout Creek sandstone, Tow Creek sandstone, and Twentymile sandstone would not be significantly affected. The upper tJilliams Fork Coal aquifers may be qualitatively affected by the Hayden Gulch Ptine. This effect is not significant because there is no documented utilization of this aquifer downdip of the operation, the aquifer is not regionally significant and, any loss of ground water contribution to surface streams is expected to be minor. Following mining and reclamation, water of degraded quality may discharge to surface systems. This effect was included in the surface water analysis. F4ining may affect the quantity and quality of flow in alluvial aquifers. Changes in water levels in the alluvium are not expected to be significant, and the expected water quality degradation would not limit pre-mining water uses. The surface water basin is most sensitive to coal mining impacts in the southeastern portion of the basin where 5,300 acres of surface mines contribute spoils flows to Foidel, Middle and Fish Creeks, small (average annual flows less than 10 cfs.) tributaries of Trout Creek. In lower Middle Creek, stream flow and total dissolved solids concentrations would increase primarily as the result of backfilled spoils discharges and secondarily as a consequence of discharge from the underground mine workings. Water from Middle Creek is used for irrigation. However, water is diverted upstream of the reach that would be affected by mining. The proposed Foidel Creek underground mine would have no adverse effect upon water use in this area. Water from Fish, Creek is also used for flood irrigation. The analysis indicates that the total dissolved solids concentrations would not exceed the use criteria (2000 mg/1) during the low flow months (August through September) of dry years. The long-term effects of mining, primarily discharge of affected ground water, are directly added to surface flows. In reality, ground water discharge is to the stream/alluvial aquifer system. Flow in the alluvial aquifer would help to buffer the effects of mining. Both surface and alluvial ground water quality would be degraded, but surface flows would not be affected to the extent predicted by the analysis. The water quality on Fish Creek would still be within acceptable limits to support existing and potential water uses. Lower Trout Creek and the adjacent Yampa River are used for both flood irrigation and domestic water. The projected effects of mining would not limit these uses. The cumulative hydrologic assessment indicates that the proposed mining operation has been designed to prevent material damage to the hydrologic balance outside the permit area. -26- }