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-47- <br />discharge can be estimated from the water balance provided in the <br />Orchard Valley permit application. This water balance estimated <br />subsurface flow to be 2.8 in/yr. Assuming that most of the water for <br />subsurface flow and deep seepage is the result of snowmelt, it may be <br />that this discharge is limited to a 60-day period. The rate of <br />discharge can be calculated. <br />(115.2 acres surface area) x (2.8 in.) x (1/12) = 26.9 acre-feet water. <br />(26.9 acre-feet) x (43,560 ft3/acre-feet) - (60 days) - (24 hrs) - <br />(60 min/hr) - (60 sec/min) = 0.23 cfs day <br />Using an estimated concentration of 5,000 mg/1 and an estimated <br />discharge of 0.23 cfs, the affects of the waste rock piles and mine <br />water drainage upon the North Fork of the Gunnison under 10 year, 7 day <br />low flow conditions can be estimated. <br />t33_9cfs)(95 mg/1) + (0.25 cfs)(4,000 mq/1) + (0.23 cfs)(5,000 mg/1) <br />33.9 + 0.25 + 0.23 <br />Cd = 156 mg/1 <br />This results in a 61 mg/1 increase over baseline conditions, which <br />will not pose any impairment to water users in the valley. In <br />addition, it is probably an over estimation, since seepage in the <br />piles is the result of snowmelt and would probably occur when <br />stream flows are much higher than average. <br />Natural faults and fractures, as well as fractures caused by <br />subsidence, could reduce streamflow by directly diverting surface <br />flow and alluvial ground water into the bedrock ground water <br />system. This effect is undesirable both because of the hydrologic <br />effects and the mine operation problems it could induce. <br />Experience with underground mines throughout the state, as well as <br />in the North Fork area, indicates that mine inflows are not <br />normally significant when ephemeral streams are undermined. Most <br />of the streams to be undermined in the region are ephemeral and <br />would not be expected to be significantly affected by mining. Flow <br />in these ephemeral streams are concentrated in periods of snowmelt <br />and high intensity precipitation events. The stream gradients are <br />steep and their channels contain little alluvium. For all of the <br />above reasons, very little surface water is retained long enough in <br />the stream valleys to infiltrate into the ground water systems. <br />Therefore, the amount of water observed reaching the mine workings <br />is insignificant. <br />Perennial streams, however, may experience significant depletions <br />of flow if mine workings are extended adjacent to and/or below the <br />level of the stream valley (e.g., the bedrock alluvial contact). <br />The lower stream gradient and the thicker alluvial deposits in <br />perennial stream valleys favor flow retention and recharge to the <br />