Critical analysis of data on thorium migration parameters in the soil–plant system

This review critically examines information on parameters characterizing the bioavailability of thorium in the soil–plant system, including thorium distribution coefficients in soil (Kd) and soil-to-plant concentration ratios of thorium (CR). It evaluates data from international projects summarizing information on thorium migration in the environment, as well as current research on thorium accumulation by plants. The review notes that the behaviour 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×10–2 kg/kg), while the lowest values are noted for corn, white rice, and tubers (n×10–5 — n×10–4 kg/kg). The review also highlights the role of foliar contamination of plants and differences in the accumulation of thorium isotopes 228Th, 230Th, and 232Th by plants. It concludes 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.