Numerical simulation of uranium in-situ leaching contaminant transport in a spatially heterogeneous confined aquifer
Asteria-Lea Mwetulundila, Abdon Atangana
Abstract
Abstract Low-grade, sandstone-type uranium deposits in aquifers are exploited by the in-situ leaching process involving the dissolution of uranium by diluted sulphuric acid, oxygen in the ore. This process produces most of the world’s uranium but has never been used in Africa. The Auob aquifer in Namibia contains a uranium deposit that is amenable to in-situ leaching, but it is also the only permanent water resource that supplies a semi-arid area. International experience associates in-situ leaching with aquifer pollution, this has serious implications for Namibia. The aim of this study is to develop a model of the migration of ISL solutions, reaction products against a background of a complex patterning within a confined heterogeneous aquifer through the presence of geological faults, fractures. The model also includes potential ion exchange during leaching, addresses contaminant migration prior to, during, after fault transits. A set of governing equations that can simulate advection, dispersion, reaction, ion exchange in a faulted, heterogeneous aquifer system was developed. The flow–transport formulation was discretized using a Crank–Nicolson scheme from dispersivity, an upwind approach from advection. Conditions for the stability of the numerical scheme were derived. To study the flow, dissolution of uranyl-sulphates over time, a computer simulation was introduced, assuming the hydraulic parameters, initial concentrations. Numerical simulations show that the model captures several field-scale features, such as non-symmetric plume migration due to geological heterogeneity. Stability analyses demonstrate that the scheme is stable for the given discretization, that the plume shapes are influenced by the fault zone affecting the contaminant pathway. Site-specific data from the Auob aquifer should be incorporated to validate model predictions, refine parameter estimates. Once calibrated, the numerical framework can serve as a decision-support tool for mapping contaminant transport, guiding monitoring, remediation strategies in in-situ leaching operations