4 70 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



between the two components in a plant sample if both are present. If plants are grown in 

 contaminated soil or culture media and only the aerial parts are assayed, we can assume 

 ihat the activity detected was internally deposited. If the aerial parts of plants are 

 collected from a recently contaminated area that had no plutonium in the soil before the 

 contaminating event, we can assume that all or nearly all the activity detected was 

 externally deposited. If, on the other hand, plant samples are taken from an area that was 

 contaminated years ago, it is likely that most of the plutonium contained therein is due 

 to external contamination by resuspended soil particles and that only a small fraction is 

 due to internal contamination by root uptake. 



Recent evidence (Romney et al., 1975; Wildung and Garland, 1974) suggests the 

 possibility that (1) the biological availability of plutonium in contaminated soil may 

 increase with time after environmental release and that successive crops of annuals thus 

 would take up successively greater amounts of plutonium; (2) perennial plants may 

 accumulate plutonium to a much greater extent than previously indicated by short-term 

 uptake experiments; and (3) plant uptake may increase with decreasing plutonium 

 concentration in the soil. Either or all of these factors could have the effect of making 

 biological transport of plutonium to man progressively more important relative to 

 physical transport and inhalation. These considerations are interesting because most 

 assessments of the potential hazards of environmental plutonium attribute little 

 importance to biological transport and ingestion compared with physical transport and 

 inhalation. 



Foliar Deposition. The estimation of deposition velocity, V^, i.e., the ratio of surface 

 deposition rate to air concentration, was discussed earlier as applied to soil surfaces. 



Deposition velocities have also been determined experimentally for different kinds of 

 plants and several kinds of vegetation with respect to various aerosols and particulates. In 

 this study, however, it was more convenient to base estimates of air-to-plant deposition 

 rates on the product of air concentration, soil/air deposition velocity, and a plant (or 

 vegetation) interception factor. The plant interception factor is defined as the amount 

 initially deposited per gram (dry weiglit) of plant material divided by the amount initially 

 deposited per unit area of soil surface. 



Much of the available information concerning die interception of airborne radionu- 

 clides by plants has come from studies made in the fallout fields produced by nuclear test 

 detonations at NTS. Romney etal. (1963) summarized early studies in the vicinity of 

 NTS. They found that levels of fallout deposition on plants varied with respect to (1) 

 downwind distance from the detonation point, (2) lateral distance away from the midline 

 of the fallout field, (3) a variety of morphological features associated with different plant 

 species, and (4) the level of fallout deposition on soil surfaces near the contaminated 

 plants. Although there was no constant relationship between fallout concentrations on 

 plants (disintegrations per minute per gram) and on soils (disintegrations per minute per 

 square meter), there was a good correlation between radioactivity in plant samples and in 

 the fraction of fallout samples with particle sizes less than 44 jum. Apparently, the larger 

 particles were deposited close to the detonation point and were not as readily intercepted 

 by plants as were the smaller particles, which were deposited farther downwind; or, if 

 they were intercepted, they were apparently more easily removed by weathering 

 processes. In the laboratory, Romney et al. (1963) found that 50 to 90% of the gross 

 radioactivity on fallout-contaminated plants could be removed by washing with water or 

 a wetting agent, such as versene. These and similar studies demonstrated the possibility of 



