Finite element 2-D transport model of groundwater restoration for in situ solution mining of uranium

Abstract

Developing technologies such as in situ solution mining of uranium represent a new, more complex solute transport problem in site restoration than traditional transport problems such as contaminant migration. The method consists of injecting through wells a lixiviant into the host aquifer containing the uranium. The uranium is preferentially dissolved and the uranium-bearing groundwater is recovered through pumping wells. The environmental advantages of solution mining over conventional mining techniques are several; however, it has the disadvantage of potentially contaminating the groundwater system. A computer model of groundwater restoration for the in situ solution mining of uranium is developed and documented. The model is based on the Galerkin-finite element method using triangular elements and linear shape functions. The computer model calculates the dual changes in concentration of two reacting solutes subject to binary cation exchange in flowing groundwater. This cation exchange process is important in the groundwater restoration of solution mining. Both the concentration in solution and the concentration adsorbed on the solid aquifer material are calculated for both solutes at specified places and times due to the process of convective transport, hydrodynamic dispersion, mixing from fluid sources and cation exchange. No other reactions are assumed which would affect the solution concentrations. The model also has the capacity to simulate conservative solute transport. A complete documentation of the computer model and a detailed description of the numerical solution of both the groundwater flow equation and the solute-transport equations are presented. The model was successfully applied to an actual field problem of ammonium restoration for a pilot scale uranium solution mining operation in northeast Colorado near the town of Grover. The computer model is offered as a basic working tool that should be readily adaptable to many other field problems. The model should have wide applicability by regulating agencies, mining companies and others concerned with groundwater restoration for in situ solution mining.