The Midnite Mine is an inactive uranium mine on the Spokane Indian Reservation located in Stevens County, Washington, and approximately 40 miles northwest of Spokane. Geologic units exposed in the area consist of metasedimentary rocks of the Precambrian Togo Formation, a series of quartz monzonite intrusions of late Cretaceous age, igneous dikes, and alluvial deposits derived from these rocks. Mining activities have left behind two open pits, several waste rock piles (burying three previously excavated pits), and stockpiles of low-grade uranium ore (protore). Groundwater at the site occurs in the crystalline/metamorphic bedrock and overlying unconsolidated material, including alluvium and waste rock. The hydrologic system presents several technical challenges because of the transient nature of discharge from seeps at the southern extent of the mined area; numerous potential preferred pathways, which include buried pre-mining drainages and buried mining haul roads; and flow through fractured bedrock. A strong correlation (R2=0.992) between the water table and topographic surface indicates that topography is the primary control of groundwater flow within the fractured bedrock system, consistent with that of an unconfined, equivalent porous media flow system. Mathematical modeling was employed to simulate and integrate the surface water, unsaturated zone and groundwater systems. The focus of this presentation is the method used for calibration of the groundwater model. Calibration of the MODFLOW model was accomplished using MODAC, an automated parameter estimation code that is based on Darcy’s Law to estimate extremely heterogeneous permeability distribution. Calibration of the model to observed data reveals that the permeability distribution is more closely related to the topographic setting as opposed to being solely related to the distribution of the lithologic units. These results are consistent with recent water resources investigations in fractured bedrock. The calibrated model has been used to simulate various remedial alternatives for feasibility study.