Critical Analysis of Data on Thorium Migration Parameters in the Soil-Plant System

This review critically examines published data on parameters characterizing the bioavailability of thorium in the soil-plant system, including thorium distribution coefficients in soil (K-d) and soil-to-plant concentration ratios for thorium (CR). The review notes that the behavior of thorium in the terrestrial environment is largely determined by its low mobility in soil. Concentration ratios of thorium accumulation by plants are relatively low: the highest CR values are observed for natural grass vegetation (n x 10(-2) kg/kg), while the lowest values are recorded for corn, white rice, and root crops (n x 10(-5) to n x 10(-4) kg/kg); vegetables, cereals, and fruits occupy an intermediate position in this row. The review also emphasizes the role of foliar contamination of plants and differences in the accumulation of thorium isotopes Th-228, Th-230, and Th-232 by plants. It is shown that the dependence of CR on the concentration of thorium in the soil is nonlinear, which limits the use of these data for radiological assessments without supplementary information.