Levels of tritium in soils and vegetation near Canadian nuclear facilities releasing tritium to the atmosphere: implications for environmental models

Abstract of the journal article published in Journal of Environmental Radioactivity
November 2014


Patsy A. Thompson
Canadian Nuclear Safety Commission

Nana-Owusua Kwamena
Canadian Nuclear Safety Commission

Michael Ilin
Canadian Nuclear Safety Commission

Monika Wilk
University of Ottawa

Ian D. Clark
University of Ottawa


Concentrations of organically bound tritium (OBT) and tritiated water (HTO) were measured over two growing seasons in vegetation and soil samples obtained in the vicinity of four nuclear facilities and two background locations in Canada.

At the background locations, with few exceptions, OBT concentrations were higher than HTO concentrations: OBT/HTO ratios in vegetation varied between 0.3 and 20 and values in soil varied between 2.7 and 15. In the vicinity of the four nuclear facilities OBT/HTO ratios in vegetation and soils deviated from the expected mean value of 0.7, which is used as a default value in environmental transfer models.

In this work, the activity concentration of OBT produced in the plant over the growing season ([OBT]plant) was compared to the activity concentration of OBT in the soil ([OBT]soil). The ratio of [OBT]plant to [OBT]soil ([OBT]plant/[OBT]soil) appears to be a good indicator of the long-term behaviour of tritium in soil and vegetation. In general, OBT activity concentrations in soils were nearly equal to OBT activity concentrations in plants in the vicinity of the two nuclear power plants. [OBT]plant/[OBT]soil ratios considerably below unity were observed at one nuclear processing facility, representing historically higher levels of tritium in the environment.

The results of our study reflect the dynamic nature of HTO retention and OBT formation in vegetation and soil during the growing season. Our data support the mounting evidence suggesting that some parameters used in environmental transfer models approved for regulatory assessments should be revisited to better account for the behavior of HTO and OBT in the environment and to ensure that modelled estimates (e.g., plant OBT) are appropriately conservative.

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