211
of the Directive into the French legislation led to a regulatory text defining a provisional EQS of 0.3 μg L
−1
for uranium to use in evaluating increments on the natural
background (MEDAD 2007).
These examples illustrate that EQSs for uranium have largely been specified by
national or regional authorities and have tended to rely on evaluations of the available environmental and toxicological information identified as relevant to the specific situation or range of situations of interest. Generic, international
recommendations, as are being developed for radiotoxic substances, have not been
developed for uranium as a chemically toxic substance.
Finally, it is noted that Mathews et al. (2009) have presented a method for relating chemical concentrations (μg [U] L
−1
) and dose rates (μGy h
−1
) for a specific set
of reference organisms. This allows a determination as to whether chemical or
radiological toxicity is more limiting for depleted, natural, low-enriched and highenriched uranium in various ecological contexts.
4 Conclusions
Models for representing the transport of radionuclides and other contaminants
through both terrestrial and aquatic environments have been developed over several
decades, and a mature assessment methodology exists for developing both conceptual and mathematical models that can be applied either generically or in specific
environmental contexts (Thorne 2012). Whether simplified 1D models or complex
3D models are employed, there remains substantial reliance on empirically determined parameter values, such as distribution coefficients for soils and sediments,
concentration ratios for plants and transfer factors or concentration ratios for animals. For uranium, an extensive database of such information exists. Examination
of this database reveals strong dependencies of distribution coefficients on pH and
both soil texture and organic matter content. In turn, availability in soil solution can
affect plant uptake, but it is generally observed that uranium is bioexcluded from
plants rather than being bioaccumulated. The database of information related to
transfer factors to terrestrial animals is rather limited, but there is consistency
between estimates based on equilibrium and kinetic analyses, and again uranium
tends to be bioexcluded rather than bioaccumulated, in part due to limited absorption from the gastrointestinal tract. In terms of biomarkers, earthworms may prove
to be a useful organism in which histopathological changes could provide an indication of soil quality.
In freshwaters, the uptake of uranium by sediments is strongly affected by concentrations of calcium and magnesium, and complexation with carbonates at high
pH can also be of significance. Uptake is greater in freshwater algae, plankton and
invertebrates than it is in freshwater fish. In the latter, uptake is thought to be mainly
through the gills. In the marine environment, concentrations of uranium in seawater
do not vary very much geographically, and there are no identified cases of uranium
buildup in food chains.
Assessment Modelling and the Evaluation of Radiological and Chemical Impacts…
of the Directive into the French legislation led to a regulatory text defining a provisional EQS of 0.3 μg L
−1
for uranium to use in evaluating increments on the natural
background (MEDAD 2007).
These examples illustrate that EQSs for uranium have largely been specified by
national or regional authorities and have tended to rely on evaluations of the available environmental and toxicological information identified as relevant to the specific situation or range of situations of interest. Generic, international
recommendations, as are being developed for radiotoxic substances, have not been
developed for uranium as a chemically toxic substance.
Finally, it is noted that Mathews et al. (2009) have presented a method for relating chemical concentrations (μg [U] L
−1
) and dose rates (μGy h
−1
) for a specific set
of reference organisms. This allows a determination as to whether chemical or
radiological toxicity is more limiting for depleted, natural, low-enriched and highenriched uranium in various ecological contexts.
4 Conclusions
Models for representing the transport of radionuclides and other contaminants
through both terrestrial and aquatic environments have been developed over several
decades, and a mature assessment methodology exists for developing both conceptual and mathematical models that can be applied either generically or in specific
environmental contexts (Thorne 2012). Whether simplified 1D models or complex
3D models are employed, there remains substantial reliance on empirically determined parameter values, such as distribution coefficients for soils and sediments,
concentration ratios for plants and transfer factors or concentration ratios for animals. For uranium, an extensive database of such information exists. Examination
of this database reveals strong dependencies of distribution coefficients on pH and
both soil texture and organic matter content. In turn, availability in soil solution can
affect plant uptake, but it is generally observed that uranium is bioexcluded from
plants rather than being bioaccumulated. The database of information related to
transfer factors to terrestrial animals is rather limited, but there is consistency
between estimates based on equilibrium and kinetic analyses, and again uranium
tends to be bioexcluded rather than bioaccumulated, in part due to limited absorption from the gastrointestinal tract. In terms of biomarkers, earthworms may prove
to be a useful organism in which histopathological changes could provide an indication of soil quality.
In freshwaters, the uptake of uranium by sediments is strongly affected by concentrations of calcium and magnesium, and complexation with carbonates at high
pH can also be of significance. Uptake is greater in freshwater algae, plankton and
invertebrates than it is in freshwater fish. In the latter, uptake is thought to be mainly
through the gills. In the marine environment, concentrations of uranium in seawater
do not vary very much geographically, and there are no identified cases of uranium
buildup in food chains.
Assessment Modelling and the Evaluation of Radiological and Chemical Impacts…