A numerical model for simulating the transport of reactive ground water contaminants

Abstract

A contaminant discharged into an aquifer can have a significant impact on the quality of the subsurface water resources. Transported by the groundwater, the contaminant can pollute withdrawal sites at pumping wells, and may reappear at the surface, emerging from springs and seepage faces. Due to the relative inaccessibility of the groundwater and the complexity of the processes involved, it is difficult to predict the extent and impact of the contamination using solely experimental techniques. Numerical modelling has been found to be one of the most effective techniques for predicting the effects, extent and impact of groundwater contamination, remediation and protection of groundwater resources. In this study, a numerical model for simulating the flow and transport of reactive groundwater contaminants has been developed. The flow properties were derived from the Darcy and mass conservation equations that gave velocity distributions required by the contaminant transport equations to determine the spatial and temporal variation of concentration. The governing equations were discretized by integrated finite differences method. The transport equation was solved by operator-splitting technique. The kinetic reaction part was implemented by the linear integrated method. The model was validated using analytical solutions available in literature for uniform velocity cases. The various modules for solving the governing equations were implemented in the object-oriented computer codes. A comparison between model results and analytical solutions show that the numerical solution traits the analytical solution with increasing time. This is attributed to the propagation of the truncation error associated with the numerical method used. Experiments on hydrodynamic control and varying aquifer properties have been done to further show the models usefulness in water quality management. From the hydrodynamic experiment, it was established that it is possible to effectively use a pump-treat and inject strategy to protect a production well from an up-gradient source. Another experiment conducted using the model aimed at predicting the rate of contaminant transport through different aquifer materials. Aquifer materials were characterized by using the values of their hydraulic conductivities obtained from literature. Breakthrough curves were obtained for both continuous and accidental pollution cases. It was found that the rate of contaminant transport process increases with increase in the value of hydraulic conductivity. The numerical model developed will aid in understanding the subsurface flow and transport processes and may also have pedagogic uses especially in the field of groundwater hydrology and engineering.