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Comment Response Page 3 Based on Feld evaluations conducted by the Division, it is the Division's belief that the curve numbers used by Colowyo are too low. Please provide site-specific evidence or data to support the curve numbers currently used in the South Taylor SEDCAD hydrologic modeling. This evidence or data could be in the form of vegetation sampling of 1-2 year old reclaimed parcels or other information as appropriate. The Division strongly suggests incorporating additional sediment control features in the pond designs in addition to using a more conservative approach with curve numbers and representation of the hydrologic conditions within the disturbed watershed. Response: The Colowyo Mine has a long history of using these curve numbers and their use has been previously agreed to by the State. These are the same values used for all structures within the current permit area. See Volume 2D, page 3 of the Introductory text of the current permit for documentation of this. The particular curve number values are the results of many years of consultation by hydrologists and have been in use at least since the early 1990's. Further, there is good evidence that these values lead to workable designs and effective sediment control facilities. While the South Taylor amendment covers an adjacent area, there is no reason from a hydrologic standpoint that the current curve numbers should change. Elevation, slope, aspect, soil types and vegetative cover are all similar. It is our understanding that the reviewer's comment has been somewhat mitigated by an inspection of typical examples of the 1-2 year reclamation. No changes to the curve numbers have been made as a result of this review comment. 46. The Division encourages Colowyo to consider using best management practices (BMP) to help control sediment at the interface between disturbed and undisturbed drainage and to minimize the need for possible yearly pond cleaning. An example of BMP would be incorporating stock ponds or presettling basins into the designs. If Colowyo chooses to use this approach, the Stockponds or presettling basins should be incorporated into the SEDCAD designs. Additionally, these features need to be placed on the appropriate maps. Prior to submitting final designs, the Division would like Colowyo to submit a preliminary design for the Division's consideration. Response: A variety of additional control features have been implemented in the revised design calculations. 47. Under section 1.9 of the introductory section, text reads "base flows were not used in any pond design calculations. Discharge of collected water from the open pit is not anticipated. Any required discharge through pumping will not be permitted during a storm event equal to or greater than the 10 year, 24-hour event storm event". This seems to imply discharge might occur during storm events of less than the 10-year event when sediment effluent limits are in effect. Please explain and or modify this text to clearly represent the any plans to discharge pit water through sediment ponds. Response: Colowyo is responsible for meeting discharge requirements under the NPDES program and wit! meet the effluent limitations as required and as described in the permit. 48. The Division would -ike Colowyo to provide additional support for the control factors used in the SEDCAD model for topsoiled and seeded condition and the 1-2 year growth condition. These values seem remarkably low and/or overly optimistic for these conditions. Support could be actual sampling results or observations made from areas depicting these conditions.