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-40- <br />water with relatively low total metal concentrations and neutral pH. The rate <br />of discharge can be estimated from the water balance provided in the Orchard <br />Valley permit application. This water balance estimated subsurface flow to be <br />2.8 in/yr. Assuming that most of the water for subsurface flow and deep <br />seepage is the result of snowmelt, it may be that this discharge is limited to <br />a 60-day period. The rate of discharge can be calculated. <br />(13 acres surface area) x (2.8 in.) x (1/12) = 3 acre-feet water. <br />(3 acre-feet) x (43,560 ft3/acre-feet) - (60 days) - (24 hrs) - (60 <br />mine/hr) - (60 sec/mine) = 0.03 cfs day <br />Using an estimated concentration of 5,000 mg/1 and an estimated discharge of <br />0.03 cfs, the affects of the waste rock piles and mine water drainage upon the <br />North Fork of the Gunnison under average flow conditions can be estimated. <br />j432 cfs)(95 mg/1) + (0.25 cfs)(4,000 mg/1) + (0.03 cfs)(5,000 mg/1) <br />- 432 + 0.25 + 0.03 <br />Cd = 97.6 mg/1 <br />This results in a 2.6 mg/1 increase over baseline conditions, which is <br />insignificant. In addition, it is probably an over estimation, since seepage <br />in the piles is the result of snowmelt and would probably occur when stream <br />flows are much higher than average. <br />Natural faults and fractures, as well as fractures caused by subsidence, could <br />reduce streamflow by directly diverting surface flaw and alluvial ground water <br />into the bedrock ground water system. This effect is undesirable both because <br />of the hydrologic effects and the mine operation problems it could induce. <br />Experience with underground mines throughout the state, as well as in the <br />North Fork area, indicates that mine inflows are not normally significant when <br />ephemeral streams are undermined. Mast of the streams to be undermined in the <br />region are ephemeral and would not be expected to be significantly affected by <br />mining. Flow in these ephemeral streams are concentrated in periods of <br />snowmelt and high intensity precipitation events. The stream gradients are <br />steep and their channels contain little alluvium. For all of the above <br />reasons, very little surface water is retained long enough in the stream <br />valleys to infiltrate into the ground water systems. Therefore, the amount of <br />water observed reaching the mine workings is insignificant. <br />Perennial streams, however, may experience significant depletions of flow if <br />mine workings are extended adjacent to and/or below the level of the stream <br />valley (e.g., the bedrock alluvial contact). The lower stream gradient and <br />the thicker alluvial deposits in perennial stream valleys favor flow retention <br />and recharge to the ground water systems. Due to the increased flow retention <br />and ground water recharge in perennial stream valleys, there is a greater <br />potential for stream depletions by mining adjacent to or under these stream <br />valleys. The mine could generate depletions by increasing mine inflows in the <br />area. These mine inflows could be induced either through the mine workings <br />encountering existing faults or fractures or strata recharged by the stream, <br />or through the mine producing subsidence fractures which extend to the stream <br />valley bottom or to rock strata recharged by the stream. <br />