Yoshino, Miori 1 ; Ge, Shemin 2
1 University of Colorado
2 University of Colorado
Limited groundwater studies exist for aquifers in the Denver Basin north of the Greeley Arch. With continued proposals for uranium mining in this part of the basin, it is necessary to acquire a detailed understanding of groundwater flow in order to estimate impacts, if any, of in-situ mining processes. This study focuses on discerning groundwater flow at the proposed Centennial Project site and its surrounding areas in west-central Weld County, Colorado, by using a steady state, regional scale groundwater flow model and a local scale model in the proposed mining zone. Generalized stratigraphy of the Pierre Shale, the Fox Hills Formation, and Laramie Formation was simulated. Data from public well records of the Colorado Department of Water Resources were used for hydraulic head contour mapping and calibration. In the study area, recharge to groundwater is low, typical for non-irrigated or natural grasslands. The depth of the groundwater table varies between 3 m and 9 m below the surface. MODFLOW, a finite-difference numerical model, was used to simulate the background steady state groundwater conditions. Model calibration achieved a correlation coefficient of 0.891 between model hydraulic head and observed water level data. Groundwater in the study area flows mainly to the south and slightly east, although there are local exceptions to this general flow direction. Slug and bail tests conducted in low sandstones of the Laramie Formation yielded hydraulic conductivity between 9.2 x 10-7 m/s to 1.8 x 10-6 m/s. A multi-well aquifer pumping test in the Fox Hills Formation yielded hydraulic conductivity of 7.2 x 10-6 m/s. The Pierre Shale was assumed in the model to be equal to or less than 1.0 x 10-8 m/s. Results suggest the potential impact of uranium mining in the Fox Hills Formation on groundwater in areas overlying, underlying and downgradient of the proposed mining zone within the Laramie-Fox Hills aquifer is limited to the proposed mining zone boundaries during in-situ recovery.