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-v2- The effects of the waste piles on flow in the North Fozk of the Gunnison can be evaluated using the mass balance - mixing ~rndel shown above. This requires estimating the total dissolved solids concentration of the leachate from the pile and estimating the rate of discharge to stream - alluvial aquifer system. The total dissolved solids concentration can be overestimated by using a concentration based on analyses of spoil waters in other parts of the state, 3000 mg/1. The chemistry of the leachate would probably be a sodium-bicarbonate type water with relatively low type metal concentrations. The rate of discharge can be estimated from the water balance provided in the Orchard Valley permit application. This water balance estimated subsurface runoff to be 2.8 in/yr. Assuming that most of the water for subsurface flow and deep seepage is the result of snowmelt, it may be that this discharge is limited to a 60 day period, the rate of discharge can be calculated. (13 acres surface area) x (2.8 in) (1/12) = 3 acre feet of water. (3 acre feet) x 43,560 ft3/acre feet e 60 days - 24 hrs - 60 min/hr e 60 sec/min = 0.03 cfs aay Using an estimated concentration of 3000 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) (3500 mg/I) + (0.03. cfs) (3000 mg/1) 432 + 0.25 + 0.03 Cd = 97.2 mg/1 This results in a 2.2 mg/1 increase over baseline conditions, which is insig- nificant. In addition, it is probably an overestimation, since seepage in the piles is the result of snowmelt and would probably occur when stream flows are much higher than average. 3. Depletion of Streamflow 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 this area are ephemeral and would not be expected to be significantly affected by mining. Two mining operations, Mt. Gunnison and Somerset, would undermine perennial drainages tributary to the North Fork of the Gunnison. Subsidence could reduce Streamflow by directly diverting surface flow and alluvial ground water into the bedrock ground water system. This affect is undersirabZe both because of the hydrologic effects and the mine operation problems it could produce. The Mt. Gunnison application has proposed a subsidence protection plan for the Dry Fork of Minnesota Creek to protect Streamflow. The adequacy of this protection plan has not been assessed as a part of this permit review. The Division has stipulated that site specific monitoring data concerning subsidence and its hydrologic effects be taken into account in the design of any protection plan.