The re-evaluation was based on a lysimeter study carried out at the Health Protection Agency (HPA) in 2008. The lysimeters used in the study contain radionuclides which have been present for 25 years. The results from the study are therefore representative of soil to plant transfer factors for a long term contamination situation, where the soil has been repeatedly cultivated and where the soil and the radionuclide content have been subject to significant weathering.
The main aim of the study was to determine soil to plant transfer factors to identify if there had been any significant changes since the last study carried out with these lysimeters 10 years previously. The HPA’s dynamic food chain model FARMLAND contains default soil to plant transfer factors. The results of this study have been compared with the default values in the model.
The measured soil to plant transfer factors were, in general, in good agreement when compared to the compiled values reported by the International Atomic Energy Agency (IAEA) and previous studies carried out at HPA. However, differences have been observed for uptake of americium-241 and strontium-90, where soil to plant transfer values were below or on the lower limit of the range of values published by IAEA and the previous studies carried out at HPA.
In contrast, the soil to plant transfer factor for caesum-137 in cabbage was approximately a factor of 2 higher than the average value of previous studies carried out at HPA and a factor of 4 higher than the upper limit of the range reported by the IAEA. These differences will be investigated as part of a further study at HPA. The current default values used in the FARMLAND model to carry out radiological dose assessments remain valid.