Transport of diluted species physics for diffusion through two layers

Hi,

I have two layers of diffusive material, each having different diffusion co-efficient. There is a concentration gradient between bottom and top layer. So, i know the top and lower concentration for the layers. I have incorporated transport of diluted species physics in the two domain. I am not sure what boundary condition should i apply on the interface between two layers. Any heads up on this problem?

-Shailesh