Role of soil-to-leaf tritium transfer in controlling leaf tritium dynamics: Comparison of experimental garden and tritium-transfer model results

Abstract of journal article published in Journal of Environmental Radioactivity
November 2017

Author(s):

Masakazu Ota
Japan Atomic Energy Agency

Nana-Owusua Kwamena
Canadian Nuclear Safety Commission

Steve Mihok
Canadian Nuclear Safety Commission

Volodymyr Korolevych
Canadian Nuclear Laboratories

Abstract:

Environmental transfer models assume that organically bound tritium (OBT) is formed directly from tissue-free water tritium (TFWT) in environmental compartments. Nevertheless, studies in the literature have shown that measured OBT/TFWT ratios are variable and higher than expected. The importance of the soil-to-leaf HTO transfer pathway in controlling the leaf tritium dynamics is not well understood. A model intercomparison of two tritium transfer models (CTEM-CLASS and SOLVEG-II) was carried out with measured environmental samples from an experimental garden plot set up next to a tritium-processing facility. The garden plot received one of three different irrigation treatments – no external irrigation, irrigation with low-tritium water and irrigation with high-tritium water.

The contrast between the results obtained with the different irrigation treatments provided insights into the impact of soil-to-leaf HTO transfer on the leaf tritium dynamics. Concentrations of TFWT and OBT in the garden plots that were not irrigated or irrigated with low-tritium water were variable, responding to the arrival of the HTO-plume from the tritium-processing facility. In contrast, for the plants irrigated with high tritium water, the TFWT concentration remained elevated during the entire experimental period due to a continuous source of high HTO in the soil. Calculated concentrations of OBT in the leaves showed an initial increase followed by quasi-equilibration with the TFWT concentration. In this quasi-equilibrium state, concentrations of OBT remained elevated and unchanged despite the arrival of the plume. The results from the model inter-comparison demonstrate that soil-to-leaf HTO transfer significantly affects tritium dynamics in leaves and thereby OBT/HTO ratio in the leaf regardless of the atmospheric HTO concentration, if there is elevated HTO concentrations in the soil.

The results of this work indicate that assessment models should be refined to consider the importance of soil-to-leaf HTO transfer to ensure that dose estimates are accurate and conservative.

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