Such contaminations have evidently been caused by the diverse activities of modern man. The dispersion and redistribution of these contaminations, apart from the losses incurred in transport, are explained by certain purelyabiotic factors (such as transport by atmospheric and oceanic cur rents, and ion exchange in soils), as well as by some powerful biotic factors (biogeochemical and radioecological).
It should be emphasized that these biotic factors may also affect the re distribution of the radioactive isotopes that exist in nature. Thus, when radioisotopes are absorbed from superficially deposited ore layers by the root systems of plants, a considerable part of the substances finds its way to the epigeal organs. From the fallen leaves and needles, and the dead branches and stems, the isotopes are transmitted to the forest litter and plant debris of meadows, from which they are leached by atmospheric precipitations, or absorbed by the microorganisms decomposing the litter, as well as by mushrooms, various worms, insects, and other animals.
It is possible that the same may happen with the radioactive excretions and carcasses of such animals, although not usually at the same site, but rather at some distance from the ore-bearing sectors. The same can also be said of phytophagous animals, including the multitude of insects, and of the insectivorous and phytophagous birds which, with their movements, flights, and migrations over the centuries could contribute to the spatial redistri bution of various elements, the changing of boundaries of geochemical provinces, and the dissemination of surface radioactive ores /24, 25/.
It was experimentally demonstrated /26/ that various plants can release through root exudates some of the radioactive isotopes absorbed by any of their organs including leaves. From the soil, these radioactive isotopes are soon taken up by the root systems of neighboring plants (of the same or different species of trees or grasses), and, in the same manner, continue their migration. In this way, the initial concentration of radioisotopes is steadily lowered as a result of dispersion in increasingly larger volumes of soil, and in the biomass of the rhizosphere microorganisms, and plants. This is one type of ecological interrelationship of organisms, causing biological migration of radioisotopes.
Similar in principle, although more diversified, are the ecological paths of migration, redistribution, concentration, and dispersion of radio isotopes released from industrial and military sites, contaminating the surrounding areas— their soils, plants, water bodies, and atmosphere. The paths of ecological food chains have been studied more thoroughly and can be followed more easily. Thus, radioactive substances migrate from the soil through plants to man. A more intricate chain involves the passage of radioisotopes from plants to animals, and then to man via meat and dairy products. In other known chains, the radioisotopes reach man through predatory and nonpredatory fishes that had consumed radioactive fishes, plants, minute aquatic organisms (plankton), periphytic organisms from silts, and benthos, which, in turn, had fed on organisms with stored up radioactivity, or had extracted the radioisotopes directly from the water or from organic substances.
Although this concept is almost classical, it cannot encompass all actual possibilities since in addition to the alimentary path there are also other different means of uptake of radioisotopes by organisms in any eco logical chain. For instance, radioisotopes may be taken up by direct con sumption of contaminated water, or of fish which have drunk it or passed it through their gills, or from contaminated water (including atmospheric precipitation) that has moistened the surface of leaves or of animals, or from depositions on the body and inhalation of radioactive aerosols. The food chains in nature are usually much more diversified and complex than outlined in the examples mentioned above, and may vary tremendously for the same organism according to different conditions, season, developmental phase, physiological state, etc. Depending on season and other conditions, many fishes and terrestrial animals may also vary abruptly, by expanding or narrowing down the specific food composition and other sources of nutrition, as well as by altering the frequency of their feeding. These changes complicate, sometimes extremely, the outline of their ecological, i, e., radioecological interrelationships /27/.