2009-05-19_REVISION - M1981021 (32)

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Section 4 Potential for Rock to be Toxic-Forming would be expected to increase during December and January. However, excess ET occurring in • latter months is estimated to be adequate to remove this moisture through evapotranspiration, preventing percolation through the pile. As discussed previously, the primary ET mechanism contributing to this water balance assessment is evaporation. Unsaturated Flow Modeling: Unsaturated flow modeling was used to estimate percolation through the rock piles in a more rigorous manner. UNSAT-H (Fayer 2000) is a Fortran-based model that is used to simulate the one-dimensional flow of water, vapor, and heat in. soils. The code addresses the processes of precipitation, evaporation, plant transpiration, storage, and deep drainage. The procedure for this estimation included: ¦ Estimate physical and hydrological characteristics of development rock using site specific sampling and laboratory analyses. ¦ Compile climate data from locations as close as possible to the site. ¦ Estimate percolation through development rock areas using UNSAT-H. 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 Richards 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). UNSAT-H also is capable of simulating heat flow and plant transpiration, although these options were not applied in this estimation. 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 presented as Attachment 6. 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. • cm 4-12 T164966-Denism MineekTask Order 3 - DMO Sampling and AnaWs PlanlTask 3.12 - Soil, Ora, Rock Data Assessment Report\Sunday Rock Re pomlinal reponfiReporaFinal Sunday Mines Group Soil One Rock Data Assesament Report_061509.doc