RTD modeling


I have modeled a porous media system and I am looking to introduce a concentration variable in order to model a salt injection that would be used for a physical RTD (residence time distribution) study. Would I do this by simply implimenting a scalar field which holds this variable and coupling it with the main multiblock to provide the advective contribution? Also, how/where would I introduce Fick’s law to simulate diffusion?

If anyone could provide any pointers or experience they have had with this, any comments would be appreciated.