We derive a new model for seawater intrusion phenomena in free aquifers. It com- bines the efficiency of the sharp interface approach with the physical realism of the diffuse interface one. More precisely, a phase field is used as an intermediate variable for including the exchanges between the characteristic layers of the aquifer (salt water, fresh water, unsaturated zone). The three- dimensional problem is reduced to a two-dimensional model involving a strongly coupled system of PDEs of parabolic type describing the evolution of the depths of the two free surfaces. The model is discretized by finite elements in space and by a semi-implicit Euler method in time. Numerical simulations present some physical predictions from the model.