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The surface configuration changes are limited to one site at the Nucla Mine. The West Return Ditch runs through the mine. Its course was altered in the late 1970's to accommodate the mining operation and subsequently re-established in the early 1980's. Re-establishment resulted in the shortening of the channel by less than 11% and an insignificant decrease in the slope of less than 0.001 ft/f t. Hydrologic monitoring has documented that this channel has stabilized. There does not appear to be increased sediment loads associated with this reach and no downcutting has been observed. Therefore, this change has had limited hydrologic consequences to surface water. The changes in runoff characteristics can be apportioned between operational periods (short-term effects) and post-mining periods (long-term effects). During mining the sediment ponds will delay storm runoff through Tuttle and Calamity Draws fora few days. This will have a beneficial impact on the drainage by limiting storm-related downcutting from high flows and improving the opportunities for infiltration of these waters into the aquifers. Peabody also has storage rights for 13 acre-feet/,year at Ponds 001 and 007. In addition, water from pit inflows will be added to the drainage from one pond at each site, in contrast to being discharged from the bedrock aquifers into the draws along their length. This will result in higher flows upstream but will have an insignificant impact on the watershed. At Nucla East average pit inflows are predicted to be approximately 0.06 cfs (27gpm). Reclamation plans for both mines will result in decreased slope gradients and increased slope lengths. At Nucla East the average watershed slope will decrease by 18 percent and the slope length will increase by 27%. Inadequate pre-mining information precludes a comparison at Nucla, but results are expected to be similar. In any case, the reclaimed topography will cause a slowing of surface run-off which may result in greater infiltration within the disturbed portions of the mine site. As the permitted mine areas make up less than two percent of their respective drainages, these slope reductions will have no significant impact on the basin. Disturbed lands can contribute to an increase in sediment loading within their drainages, However, the presence of sediment ponds at the mine sites should mitigate this effect. The sediment ponds at the two sites are designed to handle the 10-year 24-hour event. The six existing ponds have showed only one exceedence of the approved National Pollution Discharge Elimination System (NPDES) Total Suspended Solids (TSS) standard during their existence at the mine site. Increased salinity is another common impact from surface mining. Weathering processes result in the increased dissolution of the broken rock and subsequent increases in TDS. Some increases will occur during the actual mining operation. More increases will occur as a consequence of spoils spring discharge. A discussion of the calculation of these impacts follows. -22-