Gravity term effect on transport over a long period

Dear,

I want to model a 2D "diffusion driven" process in a homogeneous medium (pressure gradient=0, gravity along negative y). Qualitatively the solute is "simply" spreading over the whole domain. The hydraulic conductivity is rather low about 1e-10 m/s.

However, over a very long period, let's say 100 000 years (3.16E12 s), according to the generalized Darcy's law (see for example: https://en.wikipedia.org/wiki/Darcy%27s_law), I would expect the plume to move in the direction of the gravity because of the gravity term in the Darcy's law.

In practice I do not observe this phenomena in the FEFLOW results. Is my interpretation incorrect or did I miss to specify something in FEFLOW?

Best regards,

Thinking again about it, I guess that the answer is rather simple. The hydraulic-head has to take into account the height difference and consequently the gravity term...