2013-01-29_PERMIT FILE - C1981022A (3)

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Sesirn - 2.05.6 Mute water supply withdrawak from the North Fork of the Gunnison River will reduce the total flow • volume in the river by an amount corresponding to the variable withdrawal rate. OMI's planned water supply withdrawals are covered by existing active senior water righu and do not represent a new reduction in overall flow. Anticipated maximum consumptive use of withdrawals from the North Fork will be limited to less than 100 acre-feet annually (0.14 cubic feet per second). Both the average annual and maximum anticipated water supply withdrawal rate represent a very small percentage of total flow volume for the North Fork (470,000 acre feet annually or an average flow of 650 cfs). OMI recognizes the concem relative to flow depletion, particularly for seasonal low flow periods, and has incorporated specific operational measures in the proposed mining and reclamation plans to miriuruze overall water supply requirements and provide some flexibility relative to required withdrawal rates. Primary mitigation measures for limiting water supply withdrawals include plans for collection and recycling of mine drainage to meet most operational mine water requirements, and the use of a large capacity mine water storage tanks and underground mine water sumps to buffer mine water withdrawal demand and allow some flexibility in required withdrawal rates, especially during any critical periods. In addition mine drainage, which does not result in any reduction in surface flows due to the lack of a direct hydraulic connection, will probably result in some variable discharge of excess mine water to the surface system, effectively augmenting surface flows on a yEar-round basis. These measures should be effective in limiting potential impacu on flow rates in the Nonh Fork, allowing maintenance of adequate year-round flow to suppon both downstream water use requirements and any existing fisheries and aquatic habitat values. Exposure of suificial materiak by mining-related surface disturbance and contact of disturbed area runoff with these materials, infiltration and drainage from coal storage and mine development waste storage areas, and any potential surface discharge of excess mine drainage flows may result in changes • in runoff or discharge water chemistry. Surface water in the mine area u generally a calcium bicarbonate type although chemistry may vary to include sodium bicarbonate and sodium sulfate types. The most probable potential change in runoff water chemistry would be a shift from a strong calcium bicazbonate type toward a sodium sulfate type due to the weathering and leaching of exposed surface materials. OMI's proposed surface drainage and sediment control measures, specifically limitations on total surface disturbance and collection of disturbed area runoff as close as reasonably feasible to the disturbance source area, will be effective in limiting runoff exposure to surficial mateiials and consequent leaching. The fact that the materials which will be exposed, including overburden, soils, coal, and mine development waste materials, have been detemiined as not potentially acid- or toxic- forming u animportant factor which will ako limit the potential for anysignificant changes in surface water chemistry. The Somerset Mrtte has been abandoned and sealed, so the discharge of mine water has been terminated. The mine openings ac the Somerset Mine were in place prior to the adoption of the regulation concerning the location of such openings. However, it h not expected that a gravity discharge from these openings will occur. Mute water levek and quality are measured in wells H 10 and B-6. In the event that discharge should occur, water samples from these wells indicates that the quality is suitable for discharge to the surface under existing CDPS permit effluent limits with the possible exception of total recoverable iron. It is assumed that the Somerset Mtne will eventually fill to a level where ground water inflow and discharge are in dynamic equilibrium, with water inflow to the mine equaling downgndient seepage from the mine and any upgradient seepage from the B Coal Seam at its outcrop/subcrop. The only • potential concem then becomes potential increases in the total dissolved solids (TDS) concentration for the downgradient discharge or upgradient seepage. The flow into the mine was estimated to be 235 gallons per minute according to the Mtne Inflow Study dated September 21, 1990. The average TDS concentration of the water monitored in H-10 and B-6, through 1989 averaged 2J31 mg/L PR04 2.05-91 Revised A~ ,ct 2000