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Alluvial Aquifers The potential does exist for the regional alluvial aquifers to be cumulatively impacted. Many of the mines are located either adjacent to or beneath these aquifers. Both water quality and water levels could be affected. These are discussed individually below. The Edna mining application predicts that a portion of its mine inflow will come from the alluvial aquifer of Trout Creek. Water levels in the aquifer could lower as much as a foot or two in the vicinity of the mine. Dewatering at the Foidel Creek Mine may lower alluvial aquifers, but this will be compensated by the increase in flow in Foidel Creek from mine inflow discharges. The cumulative effect of water level lowering in the regional alluvial aquifers should be insignificant: None of these mines are located close enough to each other such that their individual water level declines would overlap. In addition, the ratio of the amount of existing water in the aquifers to the expected amounts of mine inflows coming from the aquifers is so large that it is unlikely that any impact, beyond the immediate vicinity of the mine, would be realized: Therefore, it is not expected that mining in the Trout Creek Basin will significantly affect water levels in the regional alluvial aquifers in a cumulative fashion. Many of the mines in the basin have the potential to affect water quality in the regional alluvial aquifers. A report by Kaman Tempo (Bishop, et al.; 1982) predicts that mining may cumulatively affect the water quality in the Yampa River, Trout Creek, and Fish Creek. Alluvium in Foidel and Middle Creek will also be impacted. The degradation of the surface water quality is a result of both mine-pond water discharges and spoils-water discharges: Water in the spoils at several of the large surface mines will discharge to the alluvial aquifers and to streams which recharge alluvial aquifers: The alluvial water quality is influenced by the surface water quality during recharge in the spring, and following irrigation by waters from mining disturbed drainages during the summer. Surface water impacts of increased total dissolved solids will impact the alluvial aquifer systems in the Twentymile Park basin, but will not be significant enough to cause material damage. The mines will have their greatest impact on the surface water systems during the late summer when base flows of the streams will be at their lowest and any mine discharge and spoils discharge will have minimal dilution. At this time the alluvial systems will discharge to the streams. Because the alluvium is discharging at low flow periods the high TDS of the creeks will not recharge the aquifers. Generally the alluvial aquifers will be recharged during high flow periods when the TDS of streams are lower. Thus the impacts to the alluvial aquifers by higher TDS flow in streams will be buffered by ground water movement toward the streams, preventing stream flows from entering the alluvial system. Surface Water Cumulative surface water impacts within the basin were analyzed using two different techniques. Initially, a surface water salt-loading model, developed by the USGS in 1983 was loaded with data submitted by the operators from 1983-1985. This model utilizes power curve regression analyses of the relationship between discharge and total dissolved solids or electrical conductivity to predict salt-loading throughout the drainage using a simple accounting system (Parker and Norris, 1985). -55-