The paper determines the location of the steady state interface between fresh and saltwater in a plane coastal aquifer. The lower boundary is totally impervious while the upper one is impervious below land and semi-pervious below the sea allowing an outflow through this part of the upper boundary. The model equations reduce to two boundary value problems, one valid in x < 0 and the other in x > 0 here x is measured along the aquifer with the origin at the coast. In each region unknown boundaries have to be determined as part of the solution using boundary and continuity conditions. Two cases are presented using the Dupuit approximation. One where the solution can be written down in terms of elementary functions and the other in which we have to use a phase space analysis.