UPTAKE OF NATURAL RADIONUCLIDES BY FIELD AND GARDEN CROPS

Abstract

Natural radionuclides of the uranium (U) and thorium (Th) decay series contribute to the radiation dose received by humans. In the context of U mining and refining, it is important to accurately assess the pathway for these radionuclides from soils to food plants. The Canadian Standards Association model of this pathway for dose estimation is based on the plant/soil concentration ratio (CR). In a controlled outdoor lysimeter setting, we measured CR values for experimentally applied U at 100 mg kg−1, 230Th at 8 kBq kg−1 and 210Pb (lead) at 20 kBq kg−1 using a sand, a loam, a clay and an organic soil in 12 soil and crop combinations. The overall geometric mean CR values were 0.013, 0.0022 and 0.0050 for U, 230Th and 210Pb, respectively. A second experiment using one soil and crop showed CR to be dependent on soil concentration for U, but not for Th and Pb. The final two experiments dealt with uptake of U, Th, Pb, Ra (radium), Cs (cesium), Po (polonium), and As (arsenic) from soils contaminated by a former Ra processing facility, both in a controlled lysimeter setting and in uncontrolled home-garden settings. The results generally agreed with those of the initial experiment, although the overall geometric mean CR for Th was higher at 0.02. Most of the recommended Th CR values for dose estimation are tenfold below those for U; our garden-setting results suggest they should be equal or higher. The effects on soil properties and crop type on CR values are discussed. Key words: Concentration ratio, uranium, thorium, lead