It is the most common mechanism of contamination of soil solutions in the vadose
zone, as well as, deeper regions of the saturated zones of the groundwater. As fluids move
downward under the influence of gravity, they dissolve materials to form leachates that
contain inorganic and organic constituents. As they reach the saturated zone of the
groundwater, the contaminants spread horizontally and vertically by joining the main cycles
of the geochemical flows. In the infiltration/rainfall, the colloidal size particles or elements
attached with mobile colloids would be transported to the groundwater through unsaturated
porous media. Several mechanisms are responsible for colloid transport in unsaturated zone
in addition to that of saturated zone, such as, liquid-gas interface capture, solid-liquid-gas
interface capture, liquid-film straining, and storage in immobile liquid zones.