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-41- (13 acres surface area) x (2.8 in.) x (1/12) = 3 acre-feet water. (3 acre-feet) x (43,560 ft3/acre-feet) = (60 days) (24 hrs) (60 min/hr) '~ (60 sec/min) = 0.03 cfs day Using an estimated concentration of 3,000 mg/1 and an estimated discharge of 0.03 cfs, the effects of the waste rock piles and mine water drainage upon the North Fork of the Gunnison under average flow conditions can be estimated. Cd (432 cfs)(95 mg/1) + (0.25 cfs)(3,500 mg/1) + (0.03 cfs)(3,000 mg/1) 432 + 0.02 + 0.03 Cd = 97.2 mg/1 This results in a 2.2 mg/1 increase over baseline conditions, which is insignificant. In addition, it is probably an over estimation, since seepage in the piles is the result of snowmelt and would probably occur when stream flows are much higher than average. Natural faults and fractures, as well as fractures caused by subsidence, could reduce streamflow by directly diverting surface flow and alluvial ground water into the bedrock ground water system. This effect is undersirable both because of the hydrologic effects and the mine operation problems it could induce. Experience with underground mines throughout the state, as well as in the North Fork area, indicates that mine inflows are not normally significant when ephemeral streams are undermined. Most of the streams to be undermined in the region are ephemeral and would not be expected to be significantly affected by mining. Flow in these ephemeral streams are concentrated in periods of snowmelt and high intensity precipitation events. The stream gradients are steep and their channels contain little alluvium. For all of the above reasons, very little surface water is retained long enough in the stream valleys to infiltrate into the ground water systems. Therefore, the amount of water observed reaching the mine workings is insignificant. Perennial streams, however, may experience significant depletions of flow if mine workings are extended adjacent to and/or below the level of the stream valley (e.g., the bedrock alluvial contact). The lower stream gradient and the thicker alluvial deposits in perennial stream valleys favor flow retention and recharge to the ground water systems. Due to the increased flow retention and ground water recharge in perennial stream valleys, there is a greater potential for stream depletions by mining adjacent to or under these stream valleys. The mine could generate depletions by increasing mine inflows in the area. These mine inflows could be induced either through the mine workings encountering existing faults or fractures or strata recharged by the stream, or through the mine producing subsidence fractures which extend to the stream valley bottom or to rock strata recharged by the stream. Two mining operations, Mt. Gunnison and Somerset, will undermine perennial drainages, and one mining operation, Blue Ribbon will be mining adjacent to a perennial drainage tributary to the North Fork of the Gunnison.