L[Vou. 91 sampled, the her than preime, the con‘ontaminated background NTIUM 400 after fallout (1959). vy days after ind zero. In jack rabbits ero than at Figure 10), jack rabbits o. The conice of more 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,