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Coal Company calculated runoff and erosion/ sediment yield rates from disturbed <br />areas when designing all sediment ponds. (Tab 13). Sediment leaving the site will <br />be controlled to meet effluent limits placed upon sediment pond discharges in Seneca <br />Coal Company's Colorado Discharge Permit System permit. Baseline flows in <br />Hubberson Gulch exhibit TSS levels from 5 mg /l to 606 mg /1 with an average of <br />100 mg /1. The total suspended solids increase to Hubberson Gulch that can be <br />attributed to mining is expected to be minimal. Once vegetation is successfully <br />reestablished, the erosion rates are expected to return to the pre- mining level of <br />approximately 140 tons /mi' /yr. <br />Sediment ponds will act to control flood flows from the disturbed area. The ponds <br />will regulate flood discharges, minimizing the impact of the floods. The ponds are <br />not expected to significantly reduce the quantity of runoff water available in Dry <br />Creek. The mine area makes up only a small portion of the Dry Creek watershed <br />(0.9 %). <br />The major impact of mining will be the development and subsequent discharge to <br />the surface system of spoils aquifer waters. As each successive pit is reclaimed in <br />the Wadge and Wolf Creek/Sage Creek mining areas, a spoils aquifer will begin to <br />develop. The water necessary to recharge these aquifers will be derived from <br />precipitation, overland flow, and discharge from the Williams Fork formation <br />overburden and the coal seams being mined. <br />The backfilled pits in and of themselves will display unconfined conditions, however <br />the pits will be surrounded on the sides and be underlain by relatively confining <br />units. The underlying material will be even more confining than the sides. <br />As water enters the reclaimed areas it will tend to migrate through the spoils toward <br />the lowest point in the pit. While some of this water may be lost to the walls, the <br />major portion of it will stay in the pit, as the permeability of this material is greater <br />than that of either the walls or the floor. It is projected that the water quality of <br />spoil aquifers at the Seneca II -W Mine site will approximate that at the nearby <br />Seneca II Mine. The Seneca II Mine spoil aquifers exhibit elevated levels of total <br />dissolved solids, magnesium, manganese, molybdenum, and sulfate. Complete <br />water quality analyses of two spoils aquifer wells are published in the Seneca II <br />Mine 1984 Annual Hydrology Report, Appendix B. <br />As the aquifer is developing, the water will be of slightly poorer quality than the <br />above assumptions, but as the more easily leached constituents are lost, the quality <br />should approximate that at Seneca II Mine. Analysis of spoils water from the <br />Seneca II Mine indicates that this magnesium - calcium sulfate water has elevated <br />levels of sulfate and magnesium. TDS values vary between 2,200 and 4,000 mg /l. <br />Seneca II -W Findings Document 40 C- 1982 -057 <br />Permit Revision No. 6 January 6, 2012 <br />