Concentration Constrain on Transport of Dilute Species Simulation

Hi,

I am doing a simple simulation of ion diffusion and drift using the Transport of Dilute Species module and the Electrostatics module. I was wondering, is there a way to restrict the concentration or to place a constrain on the concentration so that inside the surface (I'm doing a 2D simulation) over which the ions are diffusing and drifting the concentration does not exceed a certain value (that I would input)? I want to do this basically because as the ions drift to one side of the surface and reach the side, the concentration increases extremely fast and there seems to be no way to restrict how much the concentration can be in a given place.

Thank you for any help you can provide me.