2009-05-19_REVISION - M1977285 (37)

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Section 13 Geochemical Data and Analysis The UNSAT-H model was chosen for this evaluation because it simulates both downward movement of water into the development rock piles during infiltration and upward movement of water from the rock piles caused by evaporation. Consideration of upward unsaturated flow is critical for accurate simulation of percolation through mine rock piles in semiarid and and environments (Swanson et al. 2000). Application of rigorous unsaturated flow models based on Richard's equation is becoming a widely used approach to simulate unsaturated flow through mine waste piles (e.g. Ford et al, 2000; Swanson et al., 2000, Fala et al., 2005). Benson et al. (2006) provides basic requirements for unsaturated flow models including: ¦ The model must simulate unsaturated flow in a rigorous manner (i.e. must solve Richard's equation); ¦ The model must include a surface boundary simulating soil-atmosphere interactions (precipitation, infiltration, evaporation, runoff); and ¦ The model must integrate climatic data into the solution. UNSAT-H meets these requirements. Although the EPA Hydrological Evaluation of Landfill Performance model (HELP) has been used in the past for these types of estimates, HELP does not meet the criterion of rigorously simulating upward movement of water through the vadose zone through application of Richard's equation (Benson et al. 2006). Additional details of the calculation methods and procedure applied by UNSAT-H are presented in Fayer (2000) and are not discussed further in this document. A calculation brief, model input files, input data tables, and model output files are included Attachment O. The calculation brief provides a summary of the modeling approach including description of model nodes and boundary conditions, description of climate data sources, and definition of soil characteristics. Exhibit 13-3 summarizes the results of the modeling effort for the West Sunday and Topaz Mines on an annual basis. Additional detail, including daily percolation estimates are included in Attachment O. The estimate was completed for the West Sunday and Topaz Mines and for average annual precipitation and the wettest year on record. • 13-8 T:184988-Denison Mines\Task Order 4 - EPP Sunday Mines Group%FINAL EPPTINAL - Environmental Protection Plan Sunday Mines.doc