A three-dimensional miscible transport model for seawater intrusion in a phreatic aquifer with a transition zone is presented. This model considers many important factors, such as the effect of variable density on fluid flow, the effect of precipitation infiltration and phreatic surface fluctuation on the process of seawater intrusion, the existence of great discharge pumping wells, etc. The difficulty in solving this problem can be tackled by the presented numerical method and iteration technique. This model is used to describe seawater intrusion in Huangheying, Longkou, People's Republic of China. The simulated values agree very well with the field data (e.g., the total mean values of the absolute error of Cl- concentration are 46.13 mg/L and 55.67 mg/L, respectively, and those of the water head are 0.24 m and 0.08 m, respectively). ¿ American Geophysical Union 1995