Turnover and Elimination of Radioactive Contaminants

Radioactive isotopes within the bodies of individual plants and animals, whether introduced intentionally for study or inadvertently taken up from contaminated habitats, can also be particularly useful in determining the rates and patterns by which such contaminants are eliminated from the body and thereby help to predict the time which would be required for such organisms to return to background levels following contamination events. The rate at which contaminants are eliminated from an organism's body is the sum of the rate at which the isotope itself physically decays to a stable form plus the rate at which the organism's own physiological processes absorb, metabolize, and eventually eliminate the molecules in which the radioactive isotopes have been incorporated. These rates are quantified as the half-life which is the amount of time required for half of a given amount of isotope or half of the organism's total body burden of the contaminant to be eliminated. When measured in a captive individual under laboratory conditions, the latter is termed the physiological or biological half-life and is usually much shorter than the physical half-life with which the isotope alone decays. However when measured in an individual organism ranging freely in its natural environment, the rate measured is considered to represent the ecological half-life which may differ significantly from the biological half-life measured under captive conditions. The differences between an organism's biological and ecological half-life may be quite large and reflect differences in the types and amounts of food eaten, activity levels, and particularly in the case of poikilothermic vertebrates such as fish, amphibians, or reptiles, the temperatures experienced in the environment. During cold winter months for example, the ecological half-life of dormant reptiles can be much longer than when they are more active during warm summer temperatures. Although often challenging to measure under field conditions, new field techniques using radiotelemetry tracking, allowing organisms to be captured repeatedly for measurement of their isotope burdens, are now providing opportunities to quantify ecological half-lives in a variety of species and thus gain a more complete understanding of how environmentally important radioactive contaminants may cycle within and eventually be eliminated from their bodies under natural conditions. In some cases, approaches such as these have been able to use these radioecological studies to gain a better understanding of the cycling patterns and rates of non-nuclear contaminants such as heavy metals or chlorinated hydrocarbons in these same organisms in the same natural habitats. In these cases radioecological study has proved to be able to contribute to a better understanding of environmental toxicology in general. This is particularly true in those cases where it is the basic movement and/or behavior of the organism itself which is the critical factor in determining the uptake, concentration, or elimination of the contaminants from the system in question whether these contaminants are nuclear or non-nuclear in character.

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