AbstractA numerical model (NEWVAR) to simulate the transient movement of a discrete interface between salt water and fresh water has been developed. NEWVAR is designed to allow the analysis of a regional two‐dimensional ground‐water flow in coastal aquifers. The numerical solution permits the prediction of both regional fresh‐water levels and two‐dimensional fresh‐water/salt‐water interface by using nested square meshes.The numerical solution is based on the finite‐difference method; the Gauss‐Jordan direct method is used for solving steady‐ and unsteady‐state linear equations. Different procedures are used to avoid numerical difficulties in the transient position of the interface toe for two‐dimensional areal flow.The numerical solution was tested against the analytical ones for the cases of an advancing interface and of a floating fresh‐water lens over sea water. These tests showed good agreement, thus verifying the finite‐difference approximation. The results of an application of this model to a real aquifer are discussed in a companion paper entitled: “A Compatible Single‐Phase/Two‐Phase Numerical Model 2. Application