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Mountain Coal Company, L.L.C. Exhibit 5l <br />West Elk Mine ~ Lower Refuse Pile <br />the lower refuse pile. Cadmium and mercury are parameters which also have existing concentrations near <br />• the ISS. The ISS for mercury is below the detectable measurementability of most analytical laboratories. <br />Detection limits reported in the data reviewed for this analysis range from 4 to 200 times erecter than the <br />ISS. Therefore. i[ is not possible to accurately estimate the existing backeroundconcentrationof mercury in <br />the North Fork in relation to the ISS. <br />Data for some of the parameters which have 1SSs were not available. These included residual chlorine, <br />hexavalent chromium, cyanide, dissolved magnesium. nitrite, silver, and sulfide. The ISS for residual <br />chlorine was developed to regulate the discharge of treated sewage effluent and is not a parameter of <br />concern when considerinethe impacts of the refuse leachate. Discussions with CDMG indicate cyanide is <br />not expected to be a parameter of concern based on their experience with other sites. Silver is very <br />insoluble and is not expected to leach at significantconcen[rationfmm the refuse. Sulfideand nitrite, which <br />are reduced chemical species, are expected to convert to oxidized ionic forms after mixing with the high <br />dissolved oxyeen concentration of the North Fork water. Hexava{ent chromium and dissolved manganese <br />were assumed [o be at the same concentration as trivalent chromium and total manganese. which added to <br />the high degree of conservatism incorporated in the analysis. <br />5.4.5 Underdrains <br />Two underdrain systems have been located within the pile a[ approximately normal to natural gradients on <br />the ground surface. The location of the underdrains is provided on Map 54 of the MRP. A typical cross- <br />section of the underdrains has also been provided in Figure LW'P-009 in Appendix 1. An estimate of the <br />amount of seepage which will ultimately be collected and removed on a continual basis by the underdrain <br />system has been made using estimates of infiltration. <br />• Capacity of the underdrains was then checked using a safety factor of 2 for the anticipated design flow rate, <br />minimum anticipated gradient, and an assumed permeability for [he material within the rock drain of 10 <br />cm/second (20 feet per minute). <br />The lower underdrain system is anticipated to have a gradient of not less than 3.3 percent and is expected to <br />intercept a Flow of approximately 4 gallons per minute (design flow of 8 gallons per minute). A nominal <br />drain dimension of 4' x 3' will have a capacity which exceeds this requirement. <br />The upper underdrain is anticipated to intercept a Flow of leachate of approximately 3 gallons per minute. <br />However, at this location, the drain will also be required to collect and convey Flows associated with <br />existing seeps and springs at the toe of the steeply sloping area along the west edge of Sylvester Gulch <br />Road. Flows from these springs are seasonally variable and will range from no flow to an estimated <br />maximum flow of'_0 gallons per minute. Therefore, the maximum total Flow is estimated to be 23 gallons <br />per minute (design Flow of 46 _:allons per minute). The upper drain is anticipated to have a minimum <br />gradient of approximately 6.'_~ percent. Using an assumed permeability of 10 cm/second (20 feet per <br />minute), the =S' x 3' drain would have a minimum capacity of 80 gallons per minute. <br />S.S Slope Stability Analysis ofPhases[throughlV <br />5.5.1 Critical Sections <br />A total of five cross-sections have been identified through various portions of the pile and may be found on <br />. Figures LWP-Ol I through LWP-013 and Figure 3.0 in Appendix K. In general. the cross-sections were <br />selected to pass through available soil boring locations m order to provide the clearest picture of foundation <br />18 <br />