AIRBORNE PLUTONIUM 289 



radioelements. such as iodine (Markee, 1971). Our studies indicate that the fate of 

 transuranic elements following foliar interception is influenced by particle size (mass). 



The importance of the foliar-entry pathway compared to root absorption for 

 worldwide fallout was recognized long ago (Chamberlain, 1970; Russell, 1965). In later 

 studies of particles of probably wind-resuspended origin, 87% of the ^''Sr, 81% of the 

 '^^Cs, and 73% of the ''^'^Ce in forage plants were derived from foliar contamination 

 (Romney et al., 1973). This was shown by comparing plants grown inside plastic 

 enclosures with those grown with no cover at the Nevada Test Site. Plutonium may 

 behave similarly, but unfortunately there are no well-controlled field observations. 

 Recently, the importance of foliar-to-root pathways in the plant was defined in the 

 Liquid Metal Fast Breeder Reactor final environmental statement (U. S. Atomic Energy 

 Commission, 1974). Despite inconsistencies with respect to other dose-assessment codes, 

 risk tends to be minimized by specifying extremely conservative limits at points where 

 radioelements actually enter the human body, i.e., air and food. As a matter of systema'tic 

 practice, an improved quantitative understanding of the basic environmental processes is 

 required. This becomes important in situations where new technology may lead to 

 different physical (size) and chemical characteristics of the source term for release, 

 especially in nuclear-fuel reprocessing plants, and where comparatively large increases in 

 the handling of radioelements are projected for the future (Energy Research and 

 Development Administration, 1975). 



The following discussion reviews current knowledge regarding the retention and 

 absorption potential of foliar surfaces and describes the fate of transuranic particles 

 following plant-foliage interception as deduced from the extrapolation of information on 

 the behavior of other particles and the limited information on plutonium. 



The Problem of Retention of Particles on Foliar Surfaces 



The retention of particles on foliar surfaces depends on many parameters associated with 

 the foliar surface and the physical aspects of the particle. Leaf factors affecting the 

 efficiency of particle entrapment include components of the leaf that affect roughness 

 (Holloway, 1971), namely, venation, surface features of epidermal cells, nature of the 

 cuticle surface, nature and frequency of trichomes, and the microstructure of surface 

 wax. Each of the microtopographical features of the leaf may contribute to the 

 entrainment and retention of particles. Other factors affecting retention include surface 

 stickiness from organic and inorganic secretion, leaf wetness and charge attraction 

 between particles, and surface waxes or components. In addition, retention is dependent 

 on particle size, particle density, wind speed within the boundary layer, and, when a 

 particular element comprising or contained in a particle is being considered, solubility. 



Available information on foliar retention is sometimes disconcerting and contra- 

 dictory when one tries to reconcile the retention and behavior of relatively insoluble 

 particles with early fallout data on soluble or volatile fission products. Early fallout work 

 with respect to fission products has been reviev/ed by Chamberlain (1970) and Russell 

 (1965). In general, these reviews indicate a retention half-time of 10 to 14 days for 

 soluble fission products, with losses resulting from reentrainment of carrier particles, 

 sloughing of surface wax (Moorby and Squire, 1963), and rainfall (Middleton, 1959). 

 Except for radioiodine (Markee, 1971), such a short retention half-time is probably 

 characteristic only of large aerosol particles, as described later. 



