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.