Assimilation of thorium isotopes into terrestrial vegetation.

Vegetation and root zone soil from 17 locations within and near the boundaries of a former thorium processing plant were collected and analyzed for 232Th, 228Th and 230Th in order to evaluate the degree of movement of these radionuclides from soil to root tissue, as well as the extent of their translocation into above-ground plant compartments. Concentration ratio (CR) values ranged as follows: 232Th: 0.0003-0.3218 (shoots), 0.0065-0.9057 (roots); 228Th: 0.0038-0.3300 (shoots), 0.0183-0.7335 (roots); 230Th: 0.0007-0.4560 (shoots), 0.0076-1.252 (roots). Root/shoot ratios were 1.03-85.6 (232Th), 1.54-45.8 (228Th) and 1.36-54.5 (230Th). In nearly all instances, the CR values for 228Th exceeded those for 232Th, with the average 228Th/232Th CR ratios being 3.36 and 1.68 for shoots and roots, respectively. No discernable difference was observed between 230Th and 232Th concentration patterns. The increased 228Th/232Th CR ratio observed is presumably due to the intake of 228Ra into the plant structure, leading to subsequent production of 228Th, through decay, within the plant. It is estimated that approximately 80% of the resident 228Th in above-ground vegetation originates through this mechanism.