2018-10-12_REVISION - C1982056 (2)

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System Design Basis 2.1 System Type For treatment of the Foidel Creek mine water, the type of passive treatment system selected was a free water surface constructed wetland. Land is available at the project location that encompasses a slope in the terrain sufficiently steep enough to ensure that water will flow through the system by gravity. Future use will include polishing of stormwater prior to discharging. Design layout, details and drawings of this system are included in Appendix C and are also covered in the System Description section of this TM. 2.2 Flow and Water Quality Design Criteria TC provided flow and water quality data which serve as the design basis for the proposed wetland treatment system. Table 2 summarizes the water quality characteristics and inflow/outflow criteria used to determine an appropriate loading rate and size for this system. The average flow data was determined based on the Mine Dewatering Schedule provided by TC as summarized in Conceptual Study in Appendix A. The East Mine District period will have the highest average flow of 357 gpm and is used as the design inflow rate for wetland sizing. Expected mine discharge water quality data was also provided by TC and conducted by ACZ Laboratories (Appendix B). For conservativism in analysis, the highest concentration values for iron, sulfate and TDS were selected as the influent design concentration. Table 2-1: Basis of Design, Foidel Creek Passive Treatment System Water Quality Data Water Quality Iron (mg/L) Sulfate (mg/L) TDS (mg/L) Influent Design Parameters 1.8 2,510 5,310 Discharge Goal < 1.0 < 2,000 < 1,200 Flow and Temperature Average Design Rate (gpm) 357 Average Inflow Temperature (degrees F) 46 2.3 Performance Modeling and Results During the conceptual study phase, treatment performance was modeled to determine size and configuration using a first -order wetland treatment model based on published first -order removal rates (Kadlec and Wallace, 2009). Details of the performance modeling can be referenced in the conceptual study in Appendix A. The system was modeled as a series of four wetlands that discharge into the 6MN pond. A minimum wetland treatment area, or Design Water Surface Elevation (DWSE) area, for each cell was determined to meet the water quality criterion for iron and sulfate. To ensure that iron in the outflow will be less than the design target, the first four wetland basins were sized to exceed the minimum treatment area requirement. In addition to this requirement, the existing 61VIN Pond was included in the model to perform as the final polishing cell for additional treatment prior to final discharge into Fish Creek. The same PEABODY_FINAL_DESIGN,TM-20181010 COPYRIGHT 2012 BY CH2M HILL INC. • COMPANY CONFIDENTIAL