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SURFACE MINING ~'1 /~ Sediment pond interception and evaporation These are losses from streamflow due to evaporation from impounded water. The number of sediment ponds used in surface mining operations is general]y mucn larger than for underground operations, reflecting the much larger area of surface disturbance. Acco,^dingly, the potential far evaporation loss is mucn greater for surface mines than for underground mines. Tne mode of operation of sediment ponds should be considered when estimating evaporation. In Wyoming, for instance, the storage permit requirement for + evacuation of ponds results in minimum water surface areas for evaporation. Regardless of storage permit requirements, howeve.^, most mining operations will evacuate ponds under regulatory dewatering requirements, taus minimizing evaporation surfaces. Operations like that at Black Mesa/Kayenta mine, Arizona, however, design heir ponds as zero discharge facilities with no provision for downstream dewatering and generally with substantial excess capacity, thereby ensuring the retention of most storm runoff. Combining these considerations with the large number of ponds (over 140) and an ave^age annual evaporation of over 50 inches per year will result in a substantial annual evaporation lass at the mine. USGS Professional Paper 272-D, "Evaporation from the 17 Western States," by t4eye.^s, 1962, gives average annual lake evaporation which ranges from about 2.0 feet for eastern North Dakota to over 6.0 feet fo.^ soutne.^n Texas. An alternate worst-case approach to sediment-pond losses is to assume that _ all rote..^cepted runoff from pond drainage basins is lost fran streamflow. Such estimates are based on average annual unit ^unoff fo.^ the mine area and 1gn0.^e dewate.^ing Outflows. The ddvdntdg2 Of this p.^ocedu,^e 15 that estimates of average annual unit runoff are available which can be combined with known pond basin areas to arrive at a streamflow depletion estimate. The direct evaporation estimate depends on an estimate of total pond water surface area, which may be difficult to achieve. It is recortvnended that runoff rote^cepted by sedimentation ponds be considered a depletion from streamflow and estimated, as indicated above, either on the basis of evaporation loss from ave.^age pond surface areas or unit runoff roterception, the option being based on best available information. Internally-drained basins Closed basins are generally created from final cuts which are incorpo,^ated into postmining topography for stock watering and wildlife enhancement. This type of impoundment, unlike sediment ponds, is the only impoundment resulting from mining wnich will permanently deplete streamflow; and it is, the.ref 0.^e, .^eCOmmended Chat all ,^unoff Wlthln the C1052d bdsln be considered streamflow depletion. Estimates of streamflow depletion resulting from closed basins may be made using either procedure discussed above for sediment ponds. However, because regulations dictate that such permanent impoundments maximize water retention for postmining uses, the unit ^unoff approacn, wnicn assumes tnat the entire basin area is noncontributing, may be more appropriate in most Cases. -7-