Effect of Elevated Velocity of Particles in Groundwater Flow and Its Role in Colloid-facilitated Transport of Radionuclides in Underground Medium

Colloid-facilitated transport of radionuclides by groundwater can increase the level of ecological hazard from radioactive contaminant migration in geological medium. The reason for this is that the migration velocity of a radioactive colloid can be higher than that of radionuclides carried by the groundwater as a solute. On the basis of their field and laboratory studies, a few researchers have concluded that the velocity of the colloid can even exceed the interstitial velocity of the groundwater by a few times. A theoretical analysis of this effect is carried out in this article. The analysis is based on the assumption that the only mechanism responsible for the effect is caused by a redistribution of the colloid in a cross section of the groundwater flow in a representative volume to such domains of the cross section where the local velocity of the flow is higher than the average velocity over the whole cross section. This redistribution can be caused by drift forces arising as a result of the Magnus effect. The influence of these drift forces on a particle's movement is considered in two extreme cases; that of relatively large and relatively small colloid particles. Particles are considered relatively small if the thermal motion of water molecules exerts a decisive influence on the particles' movement. Otherwise the particles are considered as being relatively large. It is shown that in the case of relatively large particles this redistribution can be caused by the instability of their movement. The redistribution in the case of relatively small particles can be caused by an influence of the drift forces on characteristics of Brownian motion. It follows from the results of the theoretical analysis that an influence of the drift forces in both cases does not lead to an increase in the particles' migration velocity at near-horizontal direction of the groundwater flow. Data from experimental studies of the elevated velocity of colloids in porous medium are analyzed. It is shown that some findings of colloidal migration velocity exceeding the interstitial velocity of the groundwater are a result of misinterpretation of experimental results.