2020-08-20_REVISION - C1980007 (36)

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Inflows into underground mine workings from faults and fractures located outside stream valleys generally dry up with time or flow intermittently at discrete points along the fault or fracture. Those which continue to flow have flow rates which diminish to a trickle. Such mine inflows from faults and fractures may represent the dewatering of lenticular sandstone units with limited recharge areas, or may represent flows through fracture zones extending to the surface which have narrow recharge zones on steep slopes. The large initial flow rates are much larger than what would be expected from intergranular porosity in low- permeability sandstone (Darcy-type flow), indicating flow probably is from fractures or faults. Discharge of groundwater occurs through numerous seeps and springs identified in the life-of-mine area. The locations of these springs are shown on Map 37 and 37A, Springs and Stock Ponds. C. Surface Water The North Fork of the Gunnison River is the principal drainage in the mine area. Smaller streams in or adjacent to the life-of-mine areas are tributary to the North Fork. The North Fork has an average annual streamflow at Somerset of approximately 487.8 cfs per year (1980-2017). Flows in the North Fork usually peak in May or June at around 2,000 to 3,000 cfs, and generally range between 50 and 200 cfs during the rest of the year. The flow is regulated by the Paonia Reservoir on Muddy Creek five miles upstream of the town of Somerset. The reservoir became operational in 1962. Water yields during that period have ranged from a high of 601,800 acre feet per year in water year 1984, to a low of 82,270 acre feet in water year 1977. Water in the North Fork has a neutral to alkaline pH, and is a sodium-bicarbonate type with total dissolved solids generally less than 100 mg/l. Streams in the northern part of the West Elk Mine permit area are Sylvester Gulch, Lone Pine Gulch, and Gribble Gulch. These three streams flow directly to the North Fork. Lone Pine and Gribble Gulches have ephemeral flows. No flow has been observed in Lone Pine Gulch for several years. Sylvester Gulch is ephemeral in its upper section, but has perennial flow in its lower section due to a spring. Sylvester Gulch has a drainage area of 4.25 square miles. The hydrologic yield of Sylvester Gulch is small for its size compared to other area watersheds due to its lower elevation and gentler slopes. Raven Creek crosses the northeastern corner of the permit area over a distance of approximately 2,000 feet. This perennial stream and its riparian zone are protected from undermining and subsidence by a buffer zone of 660 feet (1/8 mile) on either side of the stream, as stipulated in Lease COC-67011. Mining approved by PR-11 terminated to the south 40