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topogaphic point on the highwall, and conforms to the requirements for temporary diversions set forth in Rule 4.05.3. Designs for the overflow channel aze given in Table 51, Overflow Channel Specifications. The DEPOSITS model (Wazd et. al 1979} was used to detemvne the capacity and effectiveness of Sedimentation Pond D to handle inflows from all tributary sowces, dwing a 10-yeaz, 24-how precipitation event. The output from the DEPOSITS model is given in Exhibit 28, Deposits Mode] 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 upgadient spoils through the overflow channel, and discharge from the underground mine through Ditch D-2. The inflow to the pond from these sowces 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 hydrogaph calculated by the DEPOSITS model is derived by applying the 10-year, 24-how SCS Type II storm and the SCS unit hydrogaph to derive the runoff response. The outflow hydrograph is derived by calculating the cumulative volume in the pond and applying stage-storage relationships to determine head on the decant orifice, which is used in the orifice equation to derive the dischazge. The outflow sediment-gaph 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 overbwden spoil, and at the Foidel Creek Mine with undergound waste rock, yielded similaz 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 overbwden aquifer. Dilution of leachate through intermixing with ground water in the adjacent overbwden aquifer will occw, and concentrations of leachate outside the permit and adjacent azeas will be below the level of detection Therefore, the production of leachate will not affect the post-mining land use and ground water quality will not be degraded by the proposed disposal operations. The output from the DEPOSITS model shows that the average eftluent concentration from Pond D of Total Suspended Solids (TSS) is 20.7 mgA. This is in compliance with the avemge monthly effluent limitation for TSS set in the NPDES pemtit. Based on this analysis and monitoring records for pH, total iron, total manganese, oil and grease, and other parameters, the effluent limitation set far dischazge point 005 (Site 84 in NPDES Pemrit CO-0027154 are expected to be met. Therefore, coal waste disposal operations in the Area 2 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 overbwden strata is the same as that overlying the initial Honing 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 above sections, no leaching problems, which would affect the postmining land use, are anticipated from mining in the Eastern Mining District. As is the case with the current mining area, increases in TDS aze associated with the spoil water that is used for underground dust suppression. However, the present demonstration does not indicate that the elevated TDS concentrations are impacting existing land uses. EMD (Panels 2 right through 6 Righfl fPR 97-04) -The geology discussion provided for PR 93-03 applies to the azeas covered by this permit revision. As stated in PR 93-03 and as it carries forwazd 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 dischazge is anticipated into Foidel Creek. Therefore, the mining operation should not impact the postmining land use. TR 05-47 2.05-95 Revised -January 2005