Groundwater Flow of a Proposed Uranium In-Situ Recovery Mine Site and Surrounding Areas, Weld County, Colorado

摘要

With continued proposals for uranium in-situ recovery in northeastern Colorado, it is necessary to acquire a detailed understanding of background groundwater flow in order to estimate potential mining impacts. Groundwater studies for northern Denver Basin aquifers are limited. This study focuses on discerning in-situ recovery influences on groundwater flow in the Laramie-Fox Hills aquifer by using a steady state, regional scale groundwater flow model and a local scale model in the proposed mining zone. General stratigraphy of the study area includes, in ascending order, the Pierre Shale, the Fox Hills Formation, and the Laramie Formation. Data from public well records were used for hydraulic head mapping and calibrating the groundwater model. Slug and bail tests conducted in the Lower Laramie Formation yielded a hydraulic conductivity range between 9.2 x 10-7 m/s and 1.8 x 10-6 m/s. The Pierre Shale is considered less permeable with an assumed hydraulic conductivity of 1.0 x 10-8 m/s or less. MODFLOW, a finite-difference numerical model, was used to simulate the background steady state groundwater conditions. Groundwater generally flows south and slightly east. Model calibration achieved a correlation coefficient of 0.86 between model hydraulic head and observed water level data. The local scale model used two 7-spot pumping and injection well configurations to examine water level drawdown in response to long term pumping. For a given rate of pumping at 275.22 m3/d and injection at 54.5 m3/d at the pumping and injection wells respectively, drawdown effects were limited to a ~80 meter radius surrounding the well field in the proposed sandstone. MT3D, a three-dimensional solute transport model, was used for simulating solute transport in groundwater around the well field. Modeling results suggest that solute concentrations are generally confined by low-permeability shale layers under the given groundwater flow and pumping and injection scenarios. Lateral transport was limited to the radial extent of drawdown. Impacts of in-situ recovery on groundwater flow in areas overlying, underlying and downgradient of the Upper Fox Hills Formation can be minimized given an adequate understanding of the hydrogeologic conditions surrounding the recovery site.