How to calculate the mechanical dispersion in TOUGHREACT

Hello. I am inquiring about how to know the mechanical dispersion in the PetraSim/TOUGH.

When I input the diffusion coefficient and tortuosity/porosity, I just can obtain the so-called effective diffusion coefficient. If I want to get the hydrodynamic dispersion, I must know the mechanical dispersion ( hydrodynamic dispersion = diffusion + mechanical dispersion). Then I can apply the hydrodynamic dispersion coefficient to the advection-dispersion equation. So how can I get the mechanical dispersion coefficient ?

Thank you.