Radionuclides in the Aquatic Environment

Radionuclides differ from other nuclei in that they emit ionizing radiation—alpha particles, beta particles, and gamma rays. The most massive of these emissions is the alpha particle, a helium nucleus of atomic mass 4, consisting of two neutrons and two protons. The symbol for an alpha particle is shown as the product of Reaction 4.10. An example of alpha production is found in the radioactive decay of uranium-238  

This transformation occurs when a uranium nucleus, atomic number 92 and atomic mass 238, loses an alpha particle, atomic number 2 and atomic mass 4 (a helium atom nucleus), to yield a thorium nucleus, atomic number 90 and atomic mass 234. 

Beta radiation consists of either highly energetic, negative electrons or their positively charged counterparts, called positrons  

A typical beta emitter, chlorine-38, may be produced by irradiating chlorine with neutrons. The chlorine-37 nucleus, natural abundance 24.5%, absorbs a neutron to produce chlorine-38 and gamma radiation  

FIGURE 4.15 A heavy nucleus, such as that of may absorb a neutron and break up (undergo fission), yielding lighter radioactive nuclei. A stable nucleus may absorb a neutron to produce a radioactive nucleus. 

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The fate of trace metal radionuclides in the aquatic environment and their participation in the biogeochemical cycle depend strongly on the chemical and physico--chemical form in which radionuclides are introduced in natural waters. The abundance of natural humic substances and their ability to form metal complexes and to adsorb on suspended matter and sediment makes these substances especially important in transport, availability and accumulation of trace metal radionuclides in natural water environments. In that sense complexatlon of di- and tri-valent metal radionuclides with humic and fulvic acids of different origin was studied. The sorption properties of natural suspended matter and undissolved humic acid for the sorption of some radionuclides was also studied. 

Of the following, the statement that is untrue regarding radionuclides in the aquatic environment is (a) they emit ionizing radiation, (b) they invariably come from human activities, (c) radionuclides of life elements, such as iodine-131, are particularly dangerous, (d) normally the radionuclide of most concern in drinking water is radium, (e) they may originate from the fission of uranium nuclei. 

Actinides in the environment can be classified into two groups (i) the uranium and thorium series of radionuclides in the natural environment and (ii) neptunium, plutonium, americium and curium which are formed in a nuclear reactor during the neutron bombardment of uranium through a series of neutron capture and radioactive decay reactions. Transuranics thus produced have been spread widely in the atmosphere, geosphere and aquatic environment on the earth, as a result of nuclear bomb tests in the atmosphere, and accidental release from nuclear facilities (Sakanoue, 1987). Most of these radionuclide inventories have deposited in the northern hemisphere following the tests conducted by the United States and the Soviet Union. 

Figure 1. Based on these values. Figure 1 includes the theoretical relationship between Kd and [NH4 ] according to equation (8). The interpretation of the radiocesium KdS in Figure 1 clearly shows that Cs in the aquatic environment obeys ion-exchange theory. The ion-exchange model allows the Kp of this radionuclide in sediment transport models to be predicted from environmental variables (i.e. the quantity of FES in the sediments and the pore-water concentration), rather than to be erroneously treated as a constant.

Polonium is found in the natural environment, especially in uranium and thorium ores. Of seven natural radionuclides of polonium, Po is the most important. It is an alpha emitter with energy of 5.305 MeV and half-life of 138.376 days [24]. Polonium is a very radiotoxic element and undergoes strong bioaccumulation in land and aquatic organisms [1]. 

The IAEA campaign corroborated the extensive data already available and provided additional scientific information. The activity concentrations of radionuclides in the terrestrial and aquatic environments are generally low and comparable with reported concentrations of the same radionuclides at similar atolls where no nuclear weapon testing took place. 

Santschi, P.H. et al. (1988) Chernobyl radionuclides in the environment Tracers for the tight coupling of atmospheric, terrestrial, and aquatic geochemical processes. Environ. Sci. Technol. 22. 

Edible tissues from terrestrial animals, aquatic animals, and fowl are measured for radionuclide content to estimate the radiation dose to humans from food consumption. Organs and tissue that concentrate radionuclides, such as radioiodine in thyroids or radiostrontium in bones and teeth, are analyzed as indicators of such radionuclides in the environment. Domestic animals are sampled at the normal time of slaughter. Wildlife samples can be obtained from hunters or by special collections. Because radionuclide concentrations in animal tissues vary widely, depending on species, feed or inhalation intake, location, and individual metabolism, multiple animals must be sampled to establish a normal range of radionuclide concentrations. Radionuclides that emit gamma rays can be measured in live animals by a technique similar to whole-body counting in humans. 

Abstract The environmental distribution of radionuclides, released from nuclear facilities and other sources, and the principles of the emergency countermeasures for radiation protection of the public and workers are discussed in this chapter. The concentration levels of radionuclides in various aquatic and terrestrial environments and the exposure levels of the population due to the various sources of radiation (natural and artificial radionuclides, cosmic radiation, diagnostic medical examinations, atmospheric nuclear tests, etc.) are presented. 

Similarly, the methodologies implemented in state-of-the-art models for predicting the physical processes of radionuclide migration through the aquatic environment are not discussed here. A review of the methodologies, including transport due to water currents, diffusion, settling, and resuspension has recently been published by Monte et al., where the models are briefly described, model parameter values reviewed, and values recommended (Montea et al. 2009). 

A number of natural and artificial alpha radionuclides are used, or could be used, as indicators for studying geochemical and biological processes in the natural aquatic environment. The concentrations of these radionuclides in natural components are very low. Thus, high-quality analytical procedures are needed for the measurement of radionuclides in environmental samples. Until now a large number of radioanalytical methods for either a single radionuclide or a limited number of radionuclides have been described in the literature. However, only a few methods are available for multiradionuclide determination.29-32... 

Mathematical Models. The accumulation of an element by any pathway can involve a number of different processes. If the rate-determining process can be described mathematically, a model can be developed to predict changes in concentration with time and location. A considerable effort has been made to develop models to predict the distribution of radionuclides released into the environment (15). The types of models developed to predict concentrations of radionuclides in aquatic organisms include equilibrium (J, 17, 18) and dynamic models (j, 20). 

Much information of relevance for the environment of the two atolls on the distributions and amounts of residual radionuclides is available in the results of the French monitoring programmes. For the reasons given above, the lAC decided to conduct an independent environmental sampling and surveillance campaign at the two atolls. The survey campaign was conducted at the atolls in July 1996. The campaign comprised a terrestrial part co-ordinated by the IAEA Seibersdorf Laboratory and an aquatic part co-ordinated by the lEA Marine environment Laboratory in Monaco. 

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