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Comment Response Page 2 42. The applicant provided SEDCAD4 modeling results dated August 4, 2006 for four ponds proposed for the South Taylor mining proposal. These materials are part of Exhibit 7, Item 20 of the PR-02 application materials. Introductory text is included that describes the methodologies and assumptions for sedimentation pond design and evaluations. In Section I.5 of the introductory section, the text states, "The watershed or sub-watershed delineations reflect the worst-case scenario. Conditions will be milder if reclamation phasing is implemented ". From looking at the maps showing the pond location and worst-case watershed conditions, it appears reclamation phasing was considered in the modeling for each pond. Please explain this potential discrepancy. Response: The sentence in question has been deleted from- Section 1.5 of the Introductory text of Exhibit 7-20. 43. The applicant provided SEDCAD4 modeling results dated August 4, 2006 for four ponds proposed for the South Taylor mining proposal. These materials are part of Exhibit 7, Item 20 of the PR-02 application materials. Introductory text is included that describes the methodologies and assumptions for sedimentation pond design and evaluations. In Section 1.4, Hydrograph Response it is stated that "medium and fast hydrograph responses were identified as representative for mine areas" yet fast and slow hydrograph responses were used exclusively in the SEDCAD design runs. Please explain. Response: This comment is a duplicate to comment 41 44. In Section 1.5 of the introductory section, the text states "The watershed or sub-watershed delineations reflect the worst-case scenario. Conditions will be milder if reclamation phasing is implemented". From looking at the maps showing the pond location and worst-case watershed conditions, it appears reclamation phasing was considered in the modeling for each pond. Please explain this potential discrepancy. Response: This comment is a duplicate to comment 42 45. Section 1.8 Curve Number provides general discussion and Table 1, DMG Approved SCS Curve Numbers. The Division has a long history with Colowyo regarding pond design and SEDCAD input assumptions. Colowyo has also exceeded effluent limitations on occasion and received a Notice of Violation related to sediment deposition in an off-site hay Seld below the Prospect drainage. While the Division may agree [hat site-wide curve numbers have been approved in the past, the Division believes these curve numbers were never discussed and agreed upon as being appropriate for future mine expansions, like South Taylor, Lower Wilson, or the much discussed Collom Project. The Division has no way ofpre-approving modeling input parameters for future mining aeeas a[ any mine site for the obvious reason that at any time we have no knowledge of future mining proposals. Specifically, using a curve number of 60 for areas of 1-2 years reclamation with a fair hydrologic condition and a B soil is in question. Regardless of any approved modeling, Colowyo is held to water quality effluent limits through their Colorado Discharge Monitoring permit and is liable to maintain sediment capacity in ponds in accordance with the coal roles and with prndent consideration of the ponds ability to perform. Given the steep slopes present a[ South Taylor, the south-facing nature of the reclaimed area, and the likelihood of grazing pressure from wildlife, the Division is concerned that the current modeling efforts minimize storm runoff and result in under-sized pond designs. Furthermore, positioning sediment ponds thousands of feet below the mining disturbance has been problematic in our experience. Severe erosion has occurred in the charmels passing through undisturbed areas due to large volume runoff events from the dismrbed and reclaimed area. In addition, the Division believes the pond cleanout under the current proposal will be necessary but may be made much more difficult and expensive due to the pond locations. Trucking sediment back to the disturbed area via Highway 13 seems inefficient and there is the potential for off-site impacts to the native channels below the areas controlled by the sediment ponds.