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

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RULE 2 PERMITS Where S is the maximum potential difference between P, Precipitation, and Q, Runoff, in inches at the time of a storm's beginning. Estimating composite Curve Numbers of 67 and 63 for the premining and postmining conditions respectively results in maximum potential differences of 4.93 inches for the premining condition and 5.87 inches for the postmining or reclaimed condition. Utilizing these relationships results in an estimated mean annual runoff or 0.35" and 0.24" for the premining and reclaimed postmining conditions, respectively. These relationships are tabulated below. Item Pre-Mining Post-Mining Curve Number CN 67 63 Runoff, Q, in inches 0.35 0.24 Potential difference, S, in inches 4.93 5.87 Initial abstraction, la, in inches 0.99 1.17 During the life of the mine, it is anticipated that a total of 2298 acres will be eventually disturbed in Streeter Gulch and other areas tributary to Goodspring Creek. Another 1512 acres will be disturbed in areas which are included in the Taylor Creek-Wilson Creek drainage basins. Mean annual runoff from these reclaimed areas are estimated to be as follows: Drainage Basin Premining Postmining Difference Good Spring Creek 79 ac. ft. 55 ac. ft. -24 ac. ft. Taylor Creek 40 ac. ft. 28 ac. ft. -12 ac. ft The hydrologic consequences of mining on the various drainage basins downstream of the Colowyo Mine site are summarized below: Drainage & Premining Postmining Difference Post/Pre Location AF AF AF % Good Spring Creek at Axial 1,479 1,455 -24 98.38 Taylor Creek at Mouth 110 98 -12 89.09 Wilson Creek Near Axial 1,372 1,360 -12 99.13 Milk Creek at Mouth 20,600 20,564 -36 99.83 Yampa River at Maybell 1,120,000 1,119,964 -36 100.00 Runoff in the permit area, which is diverted or detained, will continue to reach the historic drainages. A portion of the water rights held by Colowyo on Goodspring Creek may be consumed in accordance with the augmentation plan discussed in Exhibit 7. This possible consumption will possibly affect the agricultural practices on a minor portion of the Goodspring Creek Valley Floor. However, since the Colowyo Coal Company owns the property to be withdrawn from irrigation, and these lands are not an integral part of any ranching or farming operation, the effect will be minimal. Runoff detained in the detention ponds will be released from time to time in accordance with the NPDES permit. These ponds will only interrupt flow resulting from precipitation falling directly on the mine and associated areas. As Rule 2 Permits 2.04.7-28 Revision Date: 6/23/08 Revision No.: MR-91