Water moving through soil pores travels at different speeds due to different pore sizes and the tortuous paths taken by the water. Therefore a pulse of chemical travelling with the water has its leading and trailing edges spread out. This spreading is called dispersion. Chemicals also move from a region of higher concentration to one of lower concentration due to molecular motion. This is called diffusion.
Diffusion is usually much less important than mass flow.
Feel free to use them to gain insight into these complex processes. The user can specify soil and chemical properties for the system. This applet calculates and displays the concentration distribution resulting from only diffusion of the chemical. The mathematical equations used in the model are also presented. The depth to which the chemical moves is relatively small.