Radionuclides through soil, migration

A number of methods have been proposed for predicting the migration of radionuclides through the soil. Most methods have used a concept of layers of soil, one upon another, with the radionuclide migrating from the top layer downward. 

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. 

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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