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Colloidal transport through soils has been shown in recent years to facilitate the movement of pollutants previously thought to have very limited mobility in the subsurface, increasing the risk of drinking groundwater contamination. Recent studies showed that colloid deposition/release was related to the ionic strength. An interesting question, therefore, is how road salt affects the transport of colloids. The aims of this work are: (1) to investigate the effects of chemical transient flows on the attachment efficiency in the deposition and release of model colloids in saturated and unsaturated porous media (2) to view secondary minimum deposition and release of suspended particles on collector surface in real time using bright field microscope. The experiments were carried out in duplicate or triplicate under laboratory conditions glass columns filled with clean angular translucent quartz sand. Polystyrene latex particles were added in salt solutions followed by DI water.

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Colloid transport during initial phases where the ionic strength was kept constant were similar to previous observation in the literature where increasing ionic strength resulted in more colloid retention and less breakthrough. The change from the ionic solution to DI resulted in release of colloids that became greater with increasing ionic strength. The colloid peak concentration for the highest ionic strength solution was about two times to the initial colloid concentration. Visual observations showed the removal of colloids after the introduction of the DI water and dissection experiments and found most of colloids in the stagnant zones could be easily removed after washing at dissection part, but other colloids remained on the surface of the grains and could not be removed. We pose that the strongly attached colloids were in the primary minimum while the colloids that could be easily removed were in the secondary minimum. The change from secondary minimum to primary minimum was caused by capillary force associated with the meniscus pushing the colloids through the energy barrier between the second and primary minimum. Colloid flushes are the greatest in the spring and fall and it will be dangerous when the soil do not dry out. More experiments under field conditions need to be performed in future studies.