Groundwater Flow Simulation of Uranium In Situ Leach Mining in North-Central Colorado

A uranium deposit in north-central Colorado is under consideration for in-situ leach mining.  The stratigraphy of the area includes, in ascending order, the Pierre Shale, the Fox Hills sandstone, and the Laramie Formation.  Two roll front mineral deposits are located in the upper Fox Hills sandstone.  This study seeks to understand if groundwater exchange between the upper Fox Hills sandstone and the neighboring strata exists during in-situ leaching. A three-dimensional, finite-difference groundwater flow model is developed over various domains.  Groundwater flows predominantly from west to east through the upper Fox Hills sandstone under a hydraulic gradient of approximately 0.01.  To examine the impact of in-situ leaching, a five-well configuration was simulated with four injection wells symmetrically surrounding one extraction well, all terminating in the upper Fox Hills sandstone.  The extraction rate is approximately four times the injection rate. The vertical conductivity of the upper Fox Hills is set in the range of 10^-5 m/s to 10^-7 m/s.  Isotropic and anisotropic conditions will be evaluated.  Smaller vertical hydraulic conductivity restricts upward flow into the Laramie formation and thus is more conducive to in-situ leaching.  Preliminary results suggest that while most of the injected solution flows toward the extraction well, some of the injected solution may travel in vertical and eastward directions, outside the flow field of the five-well configuration.  In addition, preliminary results imply that the amount of leakage into the overlying Laramie formation depends on hydraulic conductivity values and anisotropy.