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Mountain Coal Company, L.L.C. Exhibit 51 <br />West EIk Mine Lower Refuse Pile <br />estimated based on data for monthly precipitation and monthly temperature collected from a monitoring <br />station one mile southwest of Paonia, Colorado and using procedures outlined in the Soil Conservation <br />Service Technical ReleaseNo. 21, April 1967, revised September 1970. <br />Leachate will be collected in rock underdrains and is not expected to impact ground water underlying the <br />North Fork of the Gunnison River. <br />Impacts of leachate collected by the sedimentation ponds discharging to the North Fork were analyzed on <br />the basis of the following conservativeassumptions. <br />Impacts were calculated assuming that undiluted leachate discharges from the sedimentation ponds <br />to the North Fork during the one in ten year return period, seven day duration low flow. The USGS <br />open file report 84-137(1984) states that the ten year, seven day low Flow is 33.9 cubic feet per <br />second for the Somerset, Colorado gauging station. Discharge during the assumed loty flow period <br />is not anticipated as the sedimentation ponds have a storage capacity designed to retain the runoff <br />from a 10-year, 24-hour event. Flow in the North Fork is expected to be at least 10 times greater <br />during the time leachate is discharged than assumed for this analysis. <br />• The flow of leachate is calculated by assuming an annual infiltration of ~.7 inches through the <br />maximum area of the pile of 20 acres, generating 10.3 acre-feet per year (6.4 gpm) of leachate. The <br />full area of infiltration will be consumed in filling the unsaturated pore space in the pile. Based on <br />a McWhorter-Nelson analysis of infiltration conducted by Rocky Mountain Geotechnical, <br />approximately 15 years will be required before the quantity of leachate discharging from the pile is <br />equal to the quantity infiltratingthe surface of [he pile. The annual infiltration rate of 5.7 inches per <br />year used for this analysis may be as much as twice the actual infiltration. It has also been assumed <br />that the natural springs along the southwestern margin of the pile will generate leachate. The <br />estimated seasonal maximum discharge is 20 gpm, giving a total assumed leachate discharge <br />volume of 26 gpm. This assumption has been made to provide an additional safety factor. While it <br />is possible that the spring discharge may pick up a small amount of dissolved solids from the <br />refuse, the rock drains are designed specifically to conduct the spring water beneath the refuse pile <br />to the sedimentation pond without contacting the refuse. Therefore, the actual leachate discharge is <br />expected to be about 10 times smaller than assumed for this analysis. ]t is also expected that the <br />spring water collected by the rock drains will dilute the concentration of the leachate rather than <br />add to the leachate volume. <br />• The leachate discharged to the North Fork is assumed to completely mix with the flow in the river <br />accordingto the following equation: <br />(QnPI)(Cal) _ (`lnl)(Cnf) + (QI)l Clm) <br />where, <br />Q,~ =The flow rate of the North Fork (33.9 efs) <br />Q, =The flow rate of the leachate (26 gpm) <br />Q~r,i =The Flow rate of the North Fork after leachate added (I 5,240 gpm) <br />C,, =The concentration in [he North Fork which meets the in-stream standards (ISS) set by the <br />Colorado Department of Natural Resources (I 984). <br />C,r =The quantity of water in the North Fork upstream of [he refuse pile during low flow <br />conditions. <br />C,,, =The maximum allowable concentration in the leachate that will meet the [SS after mixing <br />with the Flow in the North Fork. <br />15 <br />