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• HYDROLOGIC BALANCE -RULE 2.05.6 uestions 1 through 8 INTRODUCTION Leachate resulting from the movement of infiltration waters through the refuse pile is expected to has little impact on underlying ground water or to the North Fork of the Gunnison River. An estimate of leachate quality and analysis of the impact to the North Fork after discharging to the river during low flow conditions is presented herein in response to questions posed by the Colorado Mined Land Reclamation Division (CMLRD) in their letter to West Elk dated December 6, 1985. It is not anticipated that ground water will be significantly affected because leachate will be collected by the rock underdrains as discussed in the report submitted by Rocky Mountain Geotechnical during August, 1985. ASSUMPTIONS AND BASIS OF ANALYSIS Impacts of leachate collected by the sedimentation ponds discharging to the North Fork were analyzed on the basis of the following assumptions which provide a high degree of conservatism. 1) Impacts were calculated assuming that undiluted leachate discharges from the sedimentation ponds to the North Fork during the one in ten year return period, seven day duration low flow. The U. S. Geological Survey Open File Report 84- 137 (1984) states the ten year, seven day low flow as 33.9 cubic feet per second (cfs) for the Somerset gauging station, which is the near the proposed refuse pile. Discharge during this assumed low flow period is not anticipated as the • sedimentation ponds have a storage capacity designed to retain the runoff from a 10 year, 24 hour duration precipitation event. Flow in the North Fork is expected to be at least IO times greater during the time leachate is discharged than assumed here for this analysis. 2) The flow of leachate is calculated by assuming that an annual infiltration of 5.7 inches flow through the maximum area of the pile of 20 acres, generating 10.3 acre-feet per year (8.4 gpm) of leachate (p. 13, RMG report). The full area of the pile will not be developed for approximately 10 years and the initial infiltration will be consumed in filling the unsaturated pore space in the pile. Based on a McWhorter-Nelson analysis of infiltration conducted by RMG, approximately 15 years will be required before the quantity of leachate discharging from the pile is equal to the quantity infiltrating the surface of the pile. The annual infiltration rate of 5.7 inches per year may be as much as twice the actual infiltration. It has been further assumed that the natural springs along the southwestern margin of the pile will generate leachate. The estimated seasonal maximum discharge of the springs is 20 gpm, giving a total assumed leachate discharge volume of 26 gpm. This assumption has been made only to provide an additional factor of safety to the analysis. While it is possible that the spring discharge may pick up a small amount of dissolved solids from the refuse, the rock drains are designed specifically to conduct the spring water beneath the refuse pile to the sedimentation pond without contacting the refuse. Therefore, the actual leachate discharge is expected to be about 10 times smaller than that assumed for this analysis. It is also expected that the spring water collected by the rock drains will dilute the concentration of the leachate rather then add to the leachate volume. ~J ,o