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sampled, the
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400
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(1959).

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Figure 10),
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1964]

WARTIN : RADIOECOLOGY AND STUDY OF ENVIRONMENTAL RADIATION

309

than 400 miles from ground zero was not significantly lower than that of
animals collected at a distance of 200 miles, but it was somewhat higher than
that of animals collected at a distance of only 50 miles from ground zero.

4, Discussion. These results provide evidence that the amount of fission
product activity deposited on plants in different parts of a local fallout field
is independent of total fallout deposited on soil but is closely related to the
distribution of particles < 44» in diameter. The assimilation of mixed fission
products by herbivorous mammals living in fallout contaminated environments is closely related to the ingestion of fallout contaminated plants.
Consequently, the intake of radioactive debris by herbivores is also independent (at least during thefirst few months after fallout) of total fallout
activity and soil contamination ; but it is closely related to plant contamimation, and therefore dependent on the distribution of < 44 fallout particles.
The distribution of a specific isotope is apparently dependent on the
physical and chemical properties of its precursors and their behavior during
particle formation and dispersal. The distribution patterns of radioiodine
and radiostrontium, as indicated by their accumulation in the thyroids and
skeletons of herbivores, appear to be similar and to correspond approximately to the distribution of the relatively small particles retained by
foliage. The differences between radioiodine and radiostrontium distribution patterns may be related to differences in the half-lives of their precursors and in the height at which they become associated with particles in
the rising fireball or stem of the mushroom cloud.
LONG-TERM ASPECTS OF REDISTRIBUTION AND CYCLING. 7. Soul, Following
the loss and replacement of contaminated foliage and the decay of shortlived fission products to an insignificant level of activity, most of the medium- and long-lived fission products in a fallout contaminated ecosystem
will be found on or in the soil. The soil acts as a fission product reservoir
and provides a continuing, usually low level, source of ionizing radiation
to which plants and animals may be exposed either externally or internally
or both. As mentioned earlier, the redistribution of external emitters by
environmental processes tends to dimimish with time.
In undisturbed soils, the downward movement of fallout materials deposited on the surface may be accomplished by the leaching and percolating
action of rain water. This is a relatively slow process, and most of the
activity deposited on undisturbed desert soils may remain, for a matter of
years, in the upper one or two inches of the soil profile. Most of the radioactivity of soils contaminated by fallout from the first atomic bomb explosion
was found, after 9 years, to be confined to the upper 2 inches. In leaching
experiments using water equivalent to 84 inches of rainfall only a fraction
of the activity from these materials was displaced downward, and the maximumdisplacement was only 0.5 of an inch (Olafson et al. 1957). Similarly,

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