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West Elk Mine <br />stations on Sylvester Gulch and above and below the mine on the North Fork, provided sufficient <br />baseline water quality information to evaluate the changes in North Fork and Sylvester Gulch water <br />quality that could result from the impacts of this refuse stockpile. <br />SG-1 was abandoned and sealed and replaced with well GB-1, which was subsequently replaced by <br />GP-6 in 1997. GP-1 was abandoned and sealed and replaced with GP-7 in 1997. The wells were <br />lost due to construction of the LRP. Water levels are reported in the annual hydrology reports. <br />Previous studies of chazacteristics of refuse from the F Seam and B Seam have predicted no <br />problems with acidity (the refuse is in fact basic) and no problems with respect to toxic ions, <br />although sodium could be encountered in relatively high concentrations. A leachate study of the F <br />Seam and B Seam refuse was conducted and was presented as part of the application for the <br />permanent refuse disposal site (Exhibit 13 and Exhibit 56). <br />Leachate volume is a function of the amount of infiltration into the pile. An estimate of this <br />volume can be produced by applying the mean annual precipitation less than volume lost to <br />runoff and that volume lost to evaporation. Although runoff is significant in an isolated large <br />storm event, the very small precipitation events which comprise the majority of mean monthly <br />precipitation produce little or no runoff. A conservative estimate would be that 95 percent of the <br />mean annual precipitation enters the soil profile through infiltration. However, only a portion of <br />this volume will be ultimately retained through deep percolation, as much of it is lost to <br />evaporation and transpiration. For this refuse disposal site, we have used a mean annual <br />precipitation value of 14 inches. Evaporation has been estimated based on data for monthly <br />precipitation and monthly temperature collected from a monitoring station one mile southwest of <br />Paonia, Colorado and using procedures outlined in Soil Conservation Service Technical Release <br />No. 21, April 1967, revised September 1970. The net annual volume lost to deep percolation <br />was estimated to be 5.7 inches per year or, over the total surface azea of the pile, 4.3 acre-feet per <br />year. <br />Measured values of in-place permeability were obtained for the foundation soil ranging from 2,4 x <br />l0A cm/sec to 3.6 x 10-5 cm/sec. These values represent horizontal permeability values and in the <br />highly stratified soils observed on this site, the expected vertical permeability should be at least one <br />order of magnitude lower. Previous investigators have reported the permeability of the refuse <br />material to be on the order of 1.2 x l0A cm/sec. This permeability contrast will cause most of the <br />leachate water to perch within the refuse and above the native soils. A shallow water mound will <br />build within the pile and cause the water to migrate laterally to small seepage areas at the edge of <br />the pile. Here, it will enter the surface water collection system and be routed to a sedimentation <br />pond. <br />An analysis was performed for the RPE to determine leachate production, and is presented in <br />Exhibit 70. Rockdrains constructed under the base of the RPE will capture water that may <br />migrate down through the pile. The outlet of the drains discharge into the RPE pond, and will be <br />discharged along with surface water runoff that has accumulated in the pond, in compliance with <br />MCC's NPDES permit. In addition, Phases 1 and 2 of the RPE have been lined with a HDPE <br />liner. The soils investigation of the RPE azea indicated that the majority of this azea is gravel, <br />with high permeabilities. Beyond the Phase 2 boundary, the subsoil is a clay-rich colluvium with <br />1.05-291 Revisedhme 1005 PRIG; Rev. March 1006; Rev. May 1006 PRIO <br />