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<br />V. Surface Water hydrology -Rules 2.04.5, 2.04.7, 2.05.3(4), 2.05.6(3) and <br />Information on surface water hydrology is found in Volumes 3 and 4, Section <br />2.5 and Volumes 11 and 12, Section 4.6 of the Edna permit document. <br />The Edna Mine is located within the Trout Creek and Oak Creek drainages of the <br />Yampa River Basin of northwest Colorado. The office area and rail loadout are <br />adjacent to Oak Creek and separated from the area of mining in the Trout Creek <br />basin to the west by a ridge which forms the watershed divide. Both streams <br />flow in a northeasterly direction to their confluence with the Yampa River. <br />Trout Creek forms the western boundary of the permit area. The ephemeral <br />streams which drain the permit area flow west or northwest into Trout Creek. <br />Sediment ponds are located at the base of these tributaries above their <br />confluence with Trout Creek. The disturbed area is 4190 acres, or 8 percent <br />of the 54 square mile Trout Creek drainage basin. The average annual runoff <br />in Trout Creek is 37,500 acre-feet. The mine area is estimated to yield <br />approximately 730 acre-feet of runoff annually, or about 2 percent of the <br />total annual stream flow of Trout Creek above the mine. Annual mean minimum <br />discharge is estimated at 10 cubic feet per second (cfs). <br />Only a small area of mine-related disturbance drains into Oak Creek. Although <br />no mining occurs within this watershed, the loadout, conveyor, access road, <br />and office are situated within the Oak Creek Basin. Of the total 140 acres of <br />basin area within the permit boundary, only about 24 acres are disturbed by <br />the mine. This area is a small fraction of the total drainage area of 27.4 <br />square miles upstream of the mine on Oak Creek. Average annual yield is <br />estimated at 7,900 acre-feet. Runoff from the permit area is estimated at 25 <br />ac -f t, or 0.3 percent of the total annual stream flow of Oak Creek above the <br />mine. The estimated mean minimum flow is 5 cfs. <br />Seasonal runoff hydrographs for both streams show peak discharge occurring in <br />April, May, and June as a result of snowmelt runoff. Low flows in both <br />streams begin in the late summer and extend through the winter months until <br />the onset of spring runoff. <br />A large portion of the permit area has been previously mined, replacing <br />consolidated overburden with spoil materials. Deep percolation of <br />precipitation through the spoils has created a water table aquifer which abuts <br />against the now buried highwall or box cut near the foot of the previously <br />mined slope. Numerous springs have formed as a result. These springs emanate <br />from the relatively steep slope which exists above the alluvium on the east <br />side of Trout Creek. <br />The discharge of water from the spoils aquifer into Trout Creek is the most <br />significant surface water quality concern at the Edna Mine. From data <br />provided by the operator, the concentration of total dissolved solids (TDS) in <br />the spoil spring discharges has been observed to average 3030 mg/1. The <br />discharge of spoils springs during the low streamflow months of June through <br />August is a potential concern as the relative amount of dilution water <br />available in Trout Creek is minimal, and this water is used for irrigation <br />-13- <br />