The fate of the radioactive isotopes in deposited fallout depends on the physical 

 properties of the fallout and the chemical behavior of the nuclides. Surface bursts in 

 the kiloton range, over continental soils, yield predominantly siliceous radioactive 

 particles (3, 83, 92). Particles from tower bursts in the same energy range reflect 

 the incorporation of tower materials (83). Megaton bursts over coral islands have 

 produced primarily calcareous particles (92). It has been reported that the dust from 

 the Castle Bravo burst of 1954 was mainly calcite. Presumably, aragonite was evap- 

 orated, re crystallized as calcite, and precipitated as aggregates (44, 126). 



This wide range in gross chemical composition, considered along with the 

 observed range of particle sizes, leads to the conclusion that the biological availa- 

 bility of the constituent radioactive isotopes cannot be predicted for a particular ma- 

 terial without some knowledge of its characteristics. The solubility in distilled 

 water of selected particles from a continental detonation ranged from 0. 28 to 1.2 per 

 cent of the total radioactivity. One to 74 per cent was dissolved in 0. 1 N HCl (83, 

 51). In another study (8), it was found that the solubility in 0. 1 N HCl of deposited 

 particles from four tower shots ranged from 20 to 30 per cent and that of airborne 

 particles from 65 to 85 per cent. 



Some of the nuclides of agricultural importance, notably strontium- 90 and 

 cesium-137, may be partially depleted in the local and close-in fallout. This frac- 

 tionation results from the fact that precursors of these nuclides are noble gases 

 early in the condensation of fallout particles (136, p. 72). 



A major part of the biological experimentation with fallout constituents has 

 been conducted with soluble sources of the respective isotopes. Consequently, the 

 observed effects exceed those that would be obtained from the same amount of the 

 isotope in the less soluble fallout. It is presumed that the use of soluble sources 

 generally provides maximal effects. 



In addition to the variability in the composition and solubility of fallout, the 

 soil and plant aspects of the food chain contaxnination are complicated by variations 

 in soil properties and differences in the structure and physiology of plant species. 

 This will be the subject of discussion in the following sections. 



