[Vou. 91 ‘he problems ate and biod in the bioThe fallout , or animals tal processes lich they are l cycles. VIMALS HH ial ecosystem. 1962) is deof radionuthe general ind relative -d on soils, rs which innt routes of edicting, on he probable stances intermliving parts 1964] \WARTIN : RADIOECOLOGY AND STUDY OF ENVIRONMENTAL RADIATION 299 future concentrations of radionuclides in or on soils, in or on plants, and in various animal tissues and animal products. The general purpose of such studies is to provide data and to develop concepts which can be used in making realistic quantitative evaluations of the potential biological hazards of fallout under a wide variety of environmental circumstances. The problem of evaluating biclogical hazards due to environmental contamination has been arbitrarily but conveniently divided into two parts (Dunning and Hilkin 1956, Lindberg and Larson 1956). The ‘‘acute’’ or immediate and short-term hazards arising from environmental contamination by fallout are associated primarily with external emitters and secondarily with internal emitters. The ‘‘chronic’’ or long-term hazards are associated primarily with internal and secondarily with external emitters. The exposure of plants and animals to ionizing radiation from external emitters in a fallout contaminated environment mayberelatively high at the time of fallout, but it usually decreases in proportion to the radioactive decay of fission products. The early decay rate of mixed fission products in fallout debris (ca. t-?) approximates a 10-fold decrease in activity for every 7 days after contamination. If the level of environmental contamination at D+7 days is 100%, it will be approximately 10% at D+14 days, 1.0% at D+ 21 days, and 0.1% at D+ 28 days. This rapid, early decay rate of fresh fallout is due to the predominance of short-lived fission products. Hence, the acute or short-term biological hazards of fallout are associated with: (a} the exposure of plants and animals to relatively high levels of ionizing radiation from external emitters most of which have short radioactive half-lives, and (b) the assimilation of these short-lived isotopes by plants and animals. The chronie or long-term biological hazards of fallout are associated with : (a) the continuing exposure of plants and animals to lowlevels of ionizing radiation from long-lived fission products which are trapped in the environment, (b) the persistence of medium and long-lived radionuclides in plants and animals and especially in or on soils, and (c) the continuing availability of radionuclides in or on the soil for the recontamination of plants and for biogeochemical cycling through consumer and decomposer food chains. INITIAL DISTRIBUTION OF FALLOUT PARTICLES. The most important factors which influence the formation, dispersal, and geographical distribution of fallout particles were described in preceding sections. In studies of the oecurrence, redistribution, and cycling of fallout materials, it is often necessary to begin by determining the physical and chemical properties of fallout particles and their distribution to soil, plants, and animals in given areas. 1. Soil. The total amount and particle size composition of fallout deposited in different parts of a local fallout field can be estimated by assays of fallout collected on trays of polyethylene pellets (Romney et al. 1959) placed, in open areas, on the soil surface. Unit area soil samples can also be