It is a process that causes the spread of a constituent mass within the medium under
gradient of concentration. Diffusion is the process by which a contaminant in water will
move from an area of greater concentration toward an area where it is less concentrated.
Diffusion will occur as long as a concentration gradient exists, even if the fluid is not moving,
and as a result, a contaminant may spread away from the place where it is introduced into a
porous medium