2009-02-19_PERMIT FILE - C1982056A (3)

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The DEPOSITS model (Ward et. al 1979) was used to determine the capacity and effectiveness of Sedimentation Pond D to handle inflows from all tributary sources, during a 10-year, 24-hour precipitation event. The output from the DEPOSITS model is given in Exhibit 28, Deposits Model Output. Several assumptions were made in calibrating the DEPOSITS model. First, it was assumed that two perennial sources of water will be tributary to Pond D: drainage from the upgradient spoils through the overflow channel, and discharge from the underground mine through Ditch D-2. The inflow to the pond from these sources was assumed to fill the impoundment to the sediment storage level; thus dead storage was set equal to the permanent pool volume. Second, it was assumed that the runoff from the design event will enter the pond from all tributary areas simultaneously, without regazd to lag time due to diversion. These assumptions make the flood routing and sedimentation analyses more conservative. The inflow hydrograph calculated by the DEPOSITS model is derived by applying the 10-yeaz, 24-hour SCS Type II storm and the SCS unit hydrograph to derive the runoff response. The outflow hydrograph is derived by calculating the cumulative volume in the pond and applying stage-storage relationships to detemrine head on the decant orifice, which is used in the orifice equation to derive the dischazge. The outflow sediment-graph is derived by applying the variable rates of inflow and outflow to determine detention time, and applying representative particle-size distributions to Stokes' Law in an iterative analysis. Leachate studies conducted at Energy Mine No. 1 with mixed overburden spoil, and at the Foidel Creek Mine with underground waste rock, yielded similar results. Monitoring of actual Leachate water at Energy Mine No. 1 yielded results which are comparable to the column leach tests. Ground water chemistry at the mine site will change from a predominantly sodium-sulfate type to acalcium-sulfate type. Total dissolved solids concentration in Leachate waters will be approximately 2,800 mg/1. This concentration is less than the maximum allowed for agricultural uses. Solute transport will carry Leachate into the adjacent overburden aquifer. Dilution of Leachate through intermixing with ground water in the adjacent overburden aquifer will occur, and concentrations of Leachate outside the permit and adjacent aeeas will be below the level of detection. Therefore, the production of Leachate will not affect the postmining land use, and Bound water quality will not be degraded by the proposed disposal operations. The output from the DEPOSITS model shows that the average eliluent concentration from Pond D of Total Suspended Solids (TSS) is 20.7 mg/1. This is in compliance with the average monthly effluent limitation for TSS set in the NPDES pemrit. Based on this analysis and monitoring records for pH, total iron, total manganese, oil and grease, and other pazameters, the affluent limitation set for discharge point 005 (Site 84 in NPDES Permit CO-0027154 are expected to be met. Therefore, coal waste disposal operations in the Area 1 Pit will not compromise the existing sediment control plan and will not degrade surface water quality. As noted in the geology section, under the discussion of the geology of the Eastern Mining District, the overburden strata is the same as that overlying the initial mining azea. Roof and floor analysis was completed on those materials obtained from exploration holes 93001, 93003, 93004, and 93005. The results of the analysis do not indicated any problems with these materials. The data is presented in Exhibits ] OA through 10E. As stated in the preceeding sections, no leaching problems, which would affect the postmining land use, aze anticipated from mining in the Eastern Mining District. As is the case with the current mining area, increases in TDS are associated with the spoil water that is used for underground dust suppression. The present demonstration, however, does not indicate that the elevated TDS concentrations are impacting existing land uses. EMD (Panels 2-Kiehl through 6-Right, PR 97-04) The geology discussion provided for PR 93-03 applies to the aeeas covered by this permit revision. As stated in PR 93-03, and as it carries forward into this permit revision, no leaching problems are anticipated from mining in the EMD. Also, it is planned to return all of the mine water to the underground workings so that no discharge is anticipated into Foidel Creek. Therefore, the mining operation should not impact the postmining land use. TROS-49 2.05-95 Revised -May 2005