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Calculation Brief Title: Evaluation of the Sensitivity of Percolation Estimates through Mine Rock Stockpiles on the Simulated Precipitation Rate 1.0 Purpose/Objective UNSAT-H was used to estimate the percolation of water through development rock piles at the Van 4 Mine, Uravan Mining District, Colorado. UNSAT-H 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 (Fayer 2000). • In a review of the initial modeling results presented in Environmental Geochemistry Investigation of Soil and Rock Materials, Van 4 Mine (CDM 2009), the Colorado Division of Reclamation, Mining and Safety (DBMS 2009) provided the following comment: Unsaturated (UNSAT-H) model and percolation estimation. Denison has provided information showing the limited mobility and reduced threat from various minerals in the mined and unmined rock material, based on the geologic characteristics as well as the and climate at the mine. The wettest months are August through October, which suggests that precipitation during those months is delivered via thunderstorms - potentially high intensity events. It is known that plumes can form beneath waste rock facilities in and climates in response to seasonal patterns of such high intensity events. Please address the following: Does the model account for high-intensity, short- duration precipitation events such as thunderstorms? If not, what effect would these have on percolation estimates? Additional UNSAT-H simulations were completed to address this DRMS comment by evaluating the sensitivity of the UNSAT-H model to various rates of precipitation. The simulated rate the simulated called HPR, which is ulat d hourly at of precipitation. This calculation sheet 0 am C:Welson wodJDenison\UNSAT_MHPR eensicivity\Ven 4 HPR Sensitivitycalc bW.dac