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-44- Cumulative Hydrologic Impact Assessment - Surface Water (Cont'd) Seepage from Waste Rock/Coal Refuse Piles The North Fork region contains several historic, existing, or proposed waste rock/ coal refuse piles. The Mt. Gunnison Mine facilities includes a small waste rock pile ( I acre) with a larger pile of approximately 10 acres anticipated in the future. The Orchard Valley Mine has an existing waste rock pile (5 acres), and Western Slope Carbon has recently reclaimed a small waste rock pile ( 9 acres). The Somerset life-of-mine area includes two historic waste rock/coal waste disposal areas. The Hubbard Creek site was used from 1970 until 1977. Tyre East Yard site ( 9 acres) was used as a cvaste dump prior to 1969, but is currently part of the surface facilities area. All future waste rock at the Somerset Mine will be disposed of underground, or at a waste disposal site to be established at the area of the "C" seam mine portals (1.2 acres) after the final sealing of these portals. Most of these waste rock/coal refuse piles are located in areas away from the North Fork stream - alluvial aquifer system, and seepage could reach the stream - alluvial aquifer system only after considerable dilution. Two sites, however, the Somerset East Yard, and western Slope Carbon's Crystal Meadow site are adjacent to streams, where seepage from the piles might enter the stream - alluvial aquifer system. The effects of the waste piles on flow in the North Fork of the Gunnison can be evaluated using the mass balance - mixing model shown above. This requires estimating the total dissolved solids concentration of the Ieachate from the pile and estimating the rate of discharge to stream - alluvial aquifer systems. The total dissolved solids concentration can be estimated by using a concentration based on analyses of spoil waters in other parts of the state, 3,000 mg/1. The chemistry of the leachate would probably be a sodium-bicarbonate type water with relatively low total 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 water. (3 acre-feet) x 43,560 ft3/acre-feet ? 60 days r 24 hrs 3 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 overestimation, since seepage in the piles is the result of snowmelt and would probably occur when stream flows are much higher than average.