2008-05-13_REVISION - C1981008 (5)

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• Environmental Issues 1) Sediment Loss from Reclaimed Areas The land requested for this bond release is dryland pasture and irrigated pasture, which was originally dryland and irrigated pasture before mine disturbance. Since a comparison must be made of the sediment loss from the reclaimed area to that of the original mine community, the best manner to accomplish this is to use the vegetation statistics from the reclaimed areas and the same areas prior to mining in 1987. The areas analyzed for sediment loss are located east of 2700 Road and to the north and south of BB Road. The irrigated pasture area totals 241.5 acres and the dryland pasture area is 62 acres. The Universal Soil Loss Equation was developed in 1965 by Wischmeier and Smith using earlier equations by Musgrave in 1947. The equation was modified in 1993 to account for steeper slope areas, change in erosion over time among other improvements. The revised equation (RUBLE) does account for rill and inter-rill erosion. Its complete form is listed below: • A=RxKx(LxS)xCxP where: A =soil loss in tons/year per acre R =rainfall factor (from standard chart of area) K =soil erodibility factor (function of site specific soil) LS =length slope factor (from contour map & chart) C =management factor dependent upon vegetation & mulch P =erosion control practice factor Rainfall Factor "R" The rainfall factor is the product of rainfall energy times the maximum 30-minute intensity for a given rainstorm. It is considered as the erosive power of the rain for that particular area. A chart of "R" values for Colorado developed by the Transportation Research Board in 1980 shows that the "R" value for the entire Nucla area is 26. Since the reclaimed land slope is identical to the pre-mine slopes but the manner in which • settling of the reclaimed surface may cause some ponding on the large flat areas, a lower 13