Numerical studies of tide-induced groundwater level change in a multi-layered coastal leaky aquifer system

A coastal leaky aquifer system is numerically investigated. The system consists of an unconfined aquifer, a confined aquifer and a semipermeable confining unit between them. Previous analytical studies for such a system used the flow direction assumption [e.g., Jiao and Tang, 1999; Li and Jiao, 2001a and 2001b; Li and Jiao (in review)] that the horizontal flow in the semipermeable layer and the vertical flow in the confined aquifer are negligible. In the current numerical study, the flow direction assumption is not used, both the vertical and horizontal flows in the semipermeable layer and the confined aquifer are taken into account. The watertable in the unconfined aquifer is controlled by the spatially one-dimensional Boussinesq equation that includes the leakage occurring at the interface of the unconfined aquifer and the semipermeable layer. An iterative algorithm is used to solve the nonlinear system. The numerical solution is compared with the previous analytical solution to examine the validity of the flow direction assumption. The comparison shows that the errors of the analytical solution induced by the flow direction assumption are negligible for certain parameter value ranges of real aquifer systems.