How to set-up a model that reacts only on the surface

I am trying to set-up a model that simulates a leaching experiment. The material is very impermeable (flow is not supposed to go through) and based on existed data, it probably only reacts on the surface. I know TOUGHREACT requires some porosity and permeability for the flow to go through the media. Is there a way or any tricks to make it possible?