Nuclide specific transfer of radium from soils to plants

For a comprehensive assessment of radiation exposure from naturally occurring radionuclides in soils, the ingestion pathway of foodstuffs grown on such soils may be a major contributor. Often, only measurement results of the radionuclide vector of the soil are available for an initial dose estimation. For such calculations, however, the activity concentrations in plants are crucial. These can generally be estimated from the soil data using soil-to-plant transfer factors. Typically, these transfer factors are used on an elemental basis, i.e., not specific to individual nuclides of the same element. The reason behind this approach appears to be based on limited measurement availability for several nuclides of the same element. Thus, the transfer factors have preferably been determined only for the more easily measurable nuclide of an element but mostly find application for all nuclides of this element.

However, for nuclides of an element that display significant differences in their radiotoxicity, potential but previously unconsidered radionuclide-specific differences in the soil-to-plant transfer factors can result in a significant under- or overestimation of the ingested dose. This is especially true for radium, as ²²⁸Ra causes a dose that is approximately 3 to 6 times higher than that of ²²⁶Ra. Therefore, in this contribution we will analyse the validity of equal soil-to-plant transfer factors for ²²⁶Ra and ²²⁸Ra based on a large, representative data set of a recent total diet study and data available in the literature. First results suggest that on average there is a change in the radium activity ratio during the transfer of radium between soils and plants, with ²²⁸Ra being approximately nearly twice as effectively taken up by plants compared to ²²⁶Ra.