The seawater encroachment in a coastal aquifer is studied by means of a mathematical model. In some aquifers the dispersion amalgamates the encroaching seawater and the discharging freshwater to produce an extensive zone of diffusion. The coupled nonlinear conservation equations of mass and of salt are formulated and solved by the finite element method with the aids of iteration and underrelaxation. The numerical procedure is verified by comparing solutions with known results in the literature. In this study the seawater encroachment in the Biscayne aquifer at Cutler area, Florida, is modeled; the numerical results are in good qualitative agreement with the field data obtained by Kohout.