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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

Ingestion of soil, inadvertent or otherwise, is an important route of exposure for contaminants that are not geochemically or biologically mobile. There is little known about the bioavailability of these contaminants, especially when the contaminants are sorbed onto native soil particles. We investigated this with in vitro acid‐extraction and enzymolysis experiments and with in vivo single and chronic exposure studies with mice (Mus musculus). The only anion studied was 125I, and soil in the diet had no effect on the carcass 125I content. The bioavailability of the cations tested decreased in the order of 134Cs > 203Hg > 115Cd = 210Pb, and the effect of soil in the diet on concentrations in the carcass decreased in the same order. Soil in the diet significantly decreased the bioavallability of 134Cs, by more than four‐fold, whereas the effect on 210Pb was only ≈1.1‐fold and was not significant. The results of the in vitro digestions ordered the elements in the same way as observed in the in vivo analyses. These results indicate that for contaminants that are not very mobile and are sorbed onto native soil particles, the presence of soil in the diet does not markedly affect bioavailability in the gut.

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