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2 Quantification of Environmental Transfer Processes
2.1 General Considerations
Uranium is transported through the environment by various physical, chemical and
biological processes. In terrestrial systems, uptakes from soil can result in both
radiological and chemical impacts on a wide variety of plants and animals. In the
aquatic environment, transport occurs mainly in surface waters, but sorption to suspended and bottom sediments also occurs. As in terrestrial systems, the uptake of
uranium into a wide variety of types of biota occurs. Also, some types of biota have
mixed terrestrial and aquatic habitats and take up uranium from both types of
environment.
2.2 Uptake and Retention in Soils and Sediments
In soils and sediments, a key consideration is the partitioning of uranium between
the soil solution and soil solids. Sorption of uranium onto soil solids is usually quantified by use of a distribution coefficient (K d value) defined as the ratio of the concentration in soil solids to the concentration in solution. This is an equilibrium
concept and is an approach often used for indigenous elements (Gooddy et al. 1995;
Sheppard et al. 2011), where it may give larger K d values than obtained in batch tests
with recently added radionuclides, because the indigenous element may be irreversibly sorbed or incorporated into the soil matrix and, therefore, not available for
exchange with soil solution.
Uranium sorption is affected by soil properties, mainly pH, content of amorphous iron oxides, soil texture, specific surface area, organic matter content, cation
exchange capacity and carbonate and phosphate status (EPA 1999; Payne et al.
2011). Although various regression relationships between K d and pH have been proposed (Echevarria et al. 2001; Sheppard et al. 2006), these explain a relatively small
fraction of the observed variance, and other soil properties need to be considered.
The IAEA (2018) suggests that if information on organic matter content and pH are
available, a lognormal distribution of K d values should be used, with the characteristics of the lognormal distribution being as listed in Table 1.
2.3 Uptake and Retention of Uranium in Terrestrial Plants
The IAEA (2010) describes the plant uptake of uranium from soil using the transfer
factor, F v , defined as the ratio of the dry weight concentration in the plants to the dry
weight concentration in a specified soil layer (to a depth of 0.1 m in pasture and
0.2 m for other crop types). The inadequacies of the F v approach are widely
M. C. Thorne
2 Quantification of Environmental Transfer Processes
2.1 General Considerations
Uranium is transported through the environment by various physical, chemical and
biological processes. In terrestrial systems, uptakes from soil can result in both
radiological and chemical impacts on a wide variety of plants and animals. In the
aquatic environment, transport occurs mainly in surface waters, but sorption to suspended and bottom sediments also occurs. As in terrestrial systems, the uptake of
uranium into a wide variety of types of biota occurs. Also, some types of biota have
mixed terrestrial and aquatic habitats and take up uranium from both types of
environment.
2.2 Uptake and Retention in Soils and Sediments
In soils and sediments, a key consideration is the partitioning of uranium between
the soil solution and soil solids. Sorption of uranium onto soil solids is usually quantified by use of a distribution coefficient (K d value) defined as the ratio of the concentration in soil solids to the concentration in solution. This is an equilibrium
concept and is an approach often used for indigenous elements (Gooddy et al. 1995;
Sheppard et al. 2011), where it may give larger K d values than obtained in batch tests
with recently added radionuclides, because the indigenous element may be irreversibly sorbed or incorporated into the soil matrix and, therefore, not available for
exchange with soil solution.
Uranium sorption is affected by soil properties, mainly pH, content of amorphous iron oxides, soil texture, specific surface area, organic matter content, cation
exchange capacity and carbonate and phosphate status (EPA 1999; Payne et al.
2011). Although various regression relationships between K d and pH have been proposed (Echevarria et al. 2001; Sheppard et al. 2006), these explain a relatively small
fraction of the observed variance, and other soil properties need to be considered.
The IAEA (2018) suggests that if information on organic matter content and pH are
available, a lognormal distribution of K d values should be used, with the characteristics of the lognormal distribution being as listed in Table 1.
2.3 Uptake and Retention of Uranium in Terrestrial Plants
The IAEA (2010) describes the plant uptake of uranium from soil using the transfer
factor, F v , defined as the ratio of the dry weight concentration in the plants to the dry
weight concentration in a specified soil layer (to a depth of 0.1 m in pasture and
0.2 m for other crop types). The inadequacies of the F v approach are widely
M. C. Thorne