Radium, thorium, and the light rare earth elements in soils and vegetables grown in an area of high natural radioactivity

A study is in progress in Brazil to assess the soil-to-plant concentration ratios (CR) of the naturally occurring radionuclides 226Ra, 228Ra, 232Th, 230Th, and 228Th and the light rare earth elements (REE) La, Ce, and Nd. Thorium serves as an analog for Pu(IV) and La or Nd as analogs for Am and Cm(III). A near-surface deposit of Th (∼30,000 tons) and REE (>100,000 tons) exists at the center of the plateau near the summit of a small hill. No trends have been observed between farm soil concentrations and proximity to the ore body, substantiating earlier conclusions of the relative immobility of these elements from the deposit. New analytical procedures are presented for the sequential determination of isotopic thorium and the light REE in large biological samples. Typical radiochemical yields are 50 to 70% for Th and 80 to 100% for the REE. Preliminary analyses of seven vegetable types indicate that concentrations of Th, La, Nd, Ce, and Sm are quite variable between and among species. Average concentrations in plant tissues generally reflect soil abundances as: Ce > La > Nd > Sm ∼ Th. Mean CRs are 604, 270, 24, 17, 8 and 1 for 228Ra, 226Ra, La, Nd, Ce, and Th, respectively. Plant uptake of elements with stable oxidation states of II, III, and IV decreases as Ra(II) > REE(III) > Th(IV), which may reflect the availability of these elements in soil. 33 references, 4 figures, 4 tables