The molecular diffusion is a process that occurs independently of the groundwater movement. It is based on the thermal motion of the molecules, the Brownian movement, and leads to a concentration compensation. Only when the concentration of a substance is equal at all points in the groundwater, the diffusion stops.

 

Diffuse mass transport

 

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

 

with

 

d_m = molecular diffusion coefficient [m²/s].

 

Here, the mass flux due to the Brownian movement is proportional to the concentration gradient. Similar to Darcy´s law the negative sign results from the fact that the mass flows from high to low concentrations against the concentration gradient c.

 

Hydromechanical dispersion