RADIAL REACTIVE SOLUTE TRANSPORT IN AN AQUIFER-AQUITARD SYSTEM

The purpose of this study is to investigate radial reactive transport in an aquifer-aquitard system considering the important processes such as advection, radial and vertical dispersions for the aquifer, and vertical advection and dispersion for the aquitards. We solved the coupled governing equations of transport in the aquifer and the aquitards by honoring the continuity of concentration and mass flux across the aquifer-aquitard interfaces and recognizing the concentration variation along the aquifer thickness. This effort improved the averaged-approximation (AA) model, which dealt with radial dispersion in an aquifer-aquitard system by including the aquitard diffusion but excluding the aquitard advection, and employed a thickness-averaged aquifer concentration. To compare with our new solution, we expanded the AA model by including the aquitard advection, and the results shown the AA solution and the expanded AA solution overestimate the concentration at the upper aquifer-aquitard interface, while they underestimate the concentration at the lower aquifer-aquitard interface when upward advections exist in both aquitards. The expanded AA solution and the AA solution have considerably overestimated the mass in the upper aquitard over the time of study when an upward advection exists in the upper aquitard. The rates of dimensionless mass change in the upper aquitard calculated from the new solution and the expanded AA solution increase with the dimensionless time following sub-linear fashions, while the one from the expanded AA solution is universally greater than that calculated from the new solution at any given time. The times corresponding to the peak values of the residence time distributions (RTDs) for the AA solution, the expanded AA solution, and the new solution of this study are almost the same, and all RTDs exhibit asymmetric distribution with long tails. The RTDs curves seem to follow the Maxwell-Boltzmann distribution closely with some skewness towards the right side when plotting the dimensionless time in a logarithmic scale. In addition, we developed a finite-element COMSOL Multiphysics simulation of the problems to overcome the difficulty of the finite-difference MT3DMS program, and found that the COMSOL solution agreed with the new solution very well.