Fissure channeling caused anomalous uranium concentration in groundwater downstream of a uranium mine tailings impoundment site

Widespread uranium mining resulted in large amounts of radioactive wastes. While impoundment has been commonly used to store U-laden wastes, information has been lacking on the secondary contamination potential. We conducted a field-scale study at a prototype uranium mine tailings impoundment site in China to understand the abnormally high concentrations (0.90-36.7 μg/L) of uranium recently observed in the downstream groundwater. Based on on-site hydrogeological measurements and modeling analysis, fracture channels were identified in the aquifer. To consider the fissure channeling flow in the solute transport modeling, a modified discrete-continuum coupled modeling approach was established, which considers the flow dynamics within both the matrix domain and the preferential flow through the main fractures. The model was not only able to simulate the observed uranium concentrations at various locations, but also to reveal the U release rate from the impoundment. Moreover, the contaminant transport modeling supported that the abnormal uranium concentrations were attributed to the channeling flow in the aquifer fractures, which was confirmed through geological analysis. In addition to valuable field data, this work offered an effective modeling approach for assessing contaminant transport through fissure channeling and the associated environmental impact of waste disposal facilities in complex hydrogeological settings.