The transient motion of the freshwater-seawater interface in coastal aquifers due to a constant lowering of freshwater levels at inland locations is simulated using a sharf-interface, finite-element model. The performance of the numerical model is compared with a Hele-Shaw model. The numerical model is then used to perform parametric studies on an advancing interface that considers hypothetical cases over a wider range of possible conditions. The advancement of the interface was found to be dependent on the rate, location, and period of freshwater level variations. A seaward moving freshwater divide is formed by the lowering of the freshwater level. When the advancing interface crosses the seaward moving divide, aggresive intrusion is likely because of the reversed freshwater gradient existing landwark of the divide. A safe zone for the freshwater extraction can be determined based on the movement of the interface and the freshwater divide. ¿ American Geophysical Union 1995.