2008-09-29_PERMIT FILE - C1981019A (6)

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RULE 2 PERMITS interception, evaporation, surface and bank storage and most other abstractions from precipitation. will be negligible or be compensatory. The area in the process of being mined and the areas being reclaimed will have increases in runoff as bare soils tend to have lower infiltration rates than soils protected by a vegetative cover. On bare soils, the impacting raindrops tend to puddle the soil. The energy of the falling rain breaks down soil aggregate, and small particles are carried into the soil pores. The net result is a lowering of the infiltration rate and an increase in runoff. This increased runoff from areas in the process of being mined and from areas being reclaimed is compensatory. Runoff from the areas being mined flow to the bottom of the pit which has no outlet. It then evaporates, infiltrates or is otherwise lost to the surface drainage system. Increased runoff from areas in the process of being reclaimed flows through constructed channels to a detention pond and eventually enters the natural drainage system. It is estimated that the combined total area that is in these conditions at any one time will be not more than 1000 acres. The net effects of the lost runoff and the increased runoff on the hydrologic balance are considered compensating or negligible and involves a small portion of the total area being mined. It is also considered that infiltration into the subsurface soils will not appreciably change. Deep percolation into the subsurface soil cannot occur until the potential evapotranspiration demand for the vegetative cover has been met and the normal root zone of the plant is saturated. The Colowyo Mine is situated in a semi-arid region where this potential evapotranspiration is almost always greater than the available moisture held by the soil. The net result is water rarely, if ever, is given the opportunity to percolate into the subsurface material. The major abstraction from precipitation, evapotranspiration will be affected by mining because the infiltration rate and moisture-holding capacity of surface soils will change. It is believed the infiltration rate and moisture-holding capacity will temporarily increase due to the stripping, stock-piling, handling and spreading of surface soils. This is a temporary condition; and over an extended period of time, these surface soils will consolidate to approximate premining condition. Nevertheless, the short-term effects of mining will be an increase in the infiltration rate and moisture-holding capacity of the surface soils. This will result in an increase in evapotranspiration and a net decrease in surface runoff. The process of infiltration of water into soils and water movement within surface soils is an exceedingly complex process. In general, the infiltration rate is dependent on soil physical properties, vegetative cover, antecedent soil-water conditions rainfall intensity, and the slope of the infiltrating surface. The EMRIA Report Number 3-1975 indicates the permeability for surface soils before mining were slow to moderately slow, 0.20 to 0.60 inch per hour. Moisture retention or water-holding capacity for these soils was rated good, ranging between four and six inches per effective soil depth. In studying the infiltration rate of reclaimed lands at Colowyo, Striffler and Rhodes (1981) calculated a mean final infiltration capacity of 5.5 cm per hour at the Colowyo Mine site (2.17 inches per hour). The National Engineering Handbook, Section 4, Hydrology, by the Soil Conservation Service outlines several methods for studying the hydrology of water sheds and for solving special hydrologic problems that arise in planning and evaluating small to moderate sized watersheds. One method outlined in this handbook utilizes curve numbers for various soil types and vegetative covers and presents the relationships: Runoff, Q = (P-0.2S)2 P+O.8S Curve Number, CN = 1000 10+5, and Initial abstration, la = 0.25, Rule 2 Permits 2.04.7-27 Revision Date: 6/23/08 Revision No.: MR-91