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adversely affected, the impacts of the sediment ponds on downstream users will be minimal. • Effects of Sediment Pond Discharges, Diversions and Culverts at Road Crossings on Surface Water Quality. The effects of sediment pond discharges on stream water quality will be negligible. All sediment ponds are designed to contain the 10-year, 24-hour runoff volume plus sediment. Based on several years of NPDES monitoring at the Nucla Mine, pond discharges should be limited to the 001, 006, and probably the proposed 007 pond. A modification to the existing NPDES permit for the Nucla Mine will be applied for which will incorporate the proposed 007 pond. To date, no discharges have been observed at the outfalls of Ponds 002, 003, 004 and 005. Tables 17-24, 17-25, and 17-26 present mean annual values of select chemical parameters representative of both the irrigation (mid-Apri1 through mid-October) and non-irrigation seasons at NPDES Sites 001, 006 and Site SW-N3, respectively. During both seasons, TDS concentrations have remained fairly consistent over time at both NPDES 001 and 006. Also, levels of total iron, dissolved iron and total manganese have never exceeded the effluent criteria outlined in the existing NPDES Permit (No. CO-0000213, see Appendix 7-6, Tab 7). • During the life of mining operations at both the Nucla Mine and Nucla East mining area, diversions and culverts will be employed to control surface runoff within and around the disturbed areas. Five temporary diversions will be constructed in both areas to prevent additional surface water runoff from entering disturbed areas and to convey runoff to sediment ponds (see Tab 13, Facilities). All diversions are designed to handle the runoff from a 10-year, 24-hour runoff event. Peak discharge rates for each diversion were determined using SEDIMOT II. Calculatec channel velocities were found to be well under design-limit velocities. Culverts have also been designed to pass the 10-year, 24-hour, and in some cases, the 25-year, 24-hour runoff event using SEDIMOT 11. Culvert headwater design incorporates adequate freeboard of a minimum of one foot. Culvert outlets on applicable sediment ponds will be riprapped to reduce the potential for erosion below the outfalls. In addition, periodic inspections will be conducted at the other culvert sites to insure that any accelerated erosion at the culvert outfalls does not go unchecked. • The effects of sediment pond diversions and culverts on surface water quality will be 17-78 Revised 04/11/88