Problem with transport of diluted species mass conservation

Dear Comsol Community,

My model is a 2d axisymmetric geometry, composed by one fluid layer and many porous layers.

I'm modeling both laminar flow and transport of diluted species. This chemical species flows with the solvent into the fluid layer, then, by both convection and diffusion, passes through the porous layers, in radial direction.

My problem is that the concentration in the fluid layer remains uniform. This is quite strange, because:

1) physically, the concentration should increases a little bit, near the boundary between the fluid region and the first porous layer,
2) the mass decreases: this should imply that the species concentration increases, but this is not happening

Does anyone of you have suggestions for this? I'm not familiar with Transport of diluted species

Thank you for your attention,

Kramer