Abstract
This paper deals with the phenomenon of seawater intrusion into a confined aquifer with a semi-permeable layer, using a hydrodynamic dispersion model. The numerical solutions by the finite-element method are obtained for various values of hydraulic conductivity in the semi-permeable layer and groundwater level at the inlet of the aquifer. Two values of the dispersion coefficient in the semi-permeable layer are adopted: one is the same value as that in the permeable layer (uniform dispersion coefficient); the other is one hundredth of that in the permeable layer (small dispersion coefficient).
The main results are as follows: For relatively high hydraulic gradients, the semi-permeable layer restrains saltwater intrusion into the lower permeable layer. In the semi-permealble layer, saltwater intrudes further inland in the case of the uniform dispersion coefficient, and retreats in the case of the small dispersion coefficient, in comparison with an aquifer without a semi-permeable layer and the upper or lower permeable layers. For relatively low hydraulic gradients, the semi-permeable layer restricts saltwater intrusion into the upper permeable layer, and expands saltwater intrusion into the lower permeable layer in the case of the small dispersion coefficient, though it hardly affects the concentration distribution in the case of the uniform dispersion coefficient. Each circulating current is found near the outlets of the upper and lower permeable layers. It should be noted that groundwater flows upward near the outlet of the semi-permeable layer, and its speed increases with an increase in the hydraulic gradient in the permeable layer.
