4.2.2 Diffusion
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.
4.3 Alteration, transformation, and initiation of chemical changes within the soil
Several chemical processes affect the concentration, specific form, rate of transport,
and ease of removal of inorganic substances from the subsurface. These processes include
(1) chemical speciation,
(2) oxidation/reduction,
(3) dissolution and precipitation of solid phases,
(4) ion exchange and adsorption onto the soil matrix, and
(5) transport of particles in the subsurface.
One of the difficulties in working with inorganic contaminants is that all of these
processes can be operating simultaneously. Therefore, it is sometimes difficult to determine
which is the most important at a particular site. The relative importance of these processes not
only varies from site to site but may vary from one area to another within a given site. The
relative importance of these processes also may change during the cleanup operation as
subsurface chemical conditions are altered.