Permeability radionuclide migration

Besides the composition of the soils, other factors have a major influence on the migration of radionuclides rainfall, the thickness of the soil layers, their permeability to water, and the nature of the layers underneath. For example, Cs is washed down quickly through layers of sand, but it will stay in layers of clay. Pu is sorbed by clay more strongly than by sand and may stay in soils for rather long times, if it is not dissolved by complexation or displaced by other compounds. 

FIGURE 14.21 Radionuclide dating can be successfully used in low-permeability soils and sediments (high in clay or organic matter) when there is little migration. 

Bentonite is the key material of the EBS, which retards the transport of radionuclides. This is because (1) it is an extremely low permeable material, which prevents movement of water and eventually radionuclides, (2) it seals cracks in the rock mass because of its swelling properties, and (3) it shows a high ability for cation adsorption, therefore it can retard the migration of radionuclides. 

Data from these loos indicate that verv low concentrations of radionuclides such as Xo, Xs, "Sb, and Ru were present in Well 199-N-9 soils above the water table, although the concentration increases markedly in soils at the water table. Wells 199-N-12 and 199-N-13 are more distant from the trench and had lower radionuclide concentrations in the unsaturated zone but also had elevated concentrations at the water table (within the saturated zone). These data indicate that extensive lateral migration of contaminants from the trench within the unsaturated zone is not apparent in these borings (DOE-RL 1990). Mechanisms that control the lateral migration of contaminants through the unsaturated zone include dispersion, diffusion, capillary flow, migration through a low-permeability zone because of increased moisture content and perched water, and the adsorption capacity of the soil. 

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