Molecular diffusion is a process that takes place independently of groundwater movement. It is based on the inherent thermal movement of the molecules, Brownian motion, and leads to an equalisation of concentration. Only when the concentration of a substance is the same at all points in the groundwater does diffusion come to a standstill.

Consideration of diffuse mass tranport

The mass flow due to molecular diffusion j_diff is described by Fick's 1st law:

with

d_m = molecular diffusion coefficient [m²/s].

c = concentration gradient

Here, the mass flow is proportional to the concentration gradient as a result of Brownian motion. Analogous to Darcy's law, the negative sign results from the fact that the mass flow from high to low concentrations runs counter to the concentration gradient c.

Hydromechanical dispersion