as a function of residence time on foliage of bushbean, using a simulated rainfall of 0.4 cm in 7 minutes. The particles deposited onto the foliage had aerodynamic sizes (AMAD + GSD) of 1.274 um + 1.63 and 0.734 pm + 2.16 for freshly prepared and water-aged oxides, respectively. The count modeg for the log normal distributions were 6.142 and 0.019 jm, respectively. These latter values represent the particle diameter (absolute size) with the highest frequency within the family of particles. The percentage of plutonium retained on foliage following mild leaching ranged from 92-997, These data are qualitatively similar to those obtained for submicronic Pb particles (Carlson et al., 1975}, and contrary to data obtained with larger simulated fallout particles (Witherspoon and Taylor, 1969, 1970, 1971). Both the fresh and hydrated oxide exhibit a reduced leachability with increased residence time on the leaf. The retention mechanism may be related to physical entrapment of the submicron particles in small fissures on the leaf surface or to charge adsorption. The inability to readily remove plutonium from foliar surfaces has been noted by several authors (Hanson, 1975; Romney et al., 1975; and Tranzo, 1968); however, the mechanisms controlling retention are unclear. Little (1973) employed weakly acidic solutions to study the physical processes of fon exchange involved in the retention of heavy metals such as Pb on foliage. A comparison of the leachability of foltfar plutonium using a synthetic rainwater with and without 0.1% HNO, is shown in Table 1. Leaching with acidic solution results in a moderate increase in insoluble plutonium leached from leaves contaminated with fresh Pu02, while there is a substantial increase exhibited with leaves contamined with the hydrated oxide. The large increase seen in the soluble components may result from a solubilization of noncrystaliine plutonium on the surface of the particles. The increased leachability of the hydrated oxide, as compared with the fresh oxide may be related to the larger surface area available for reaction (0.019 um as compared to 0.142 pm diameter of the fresh oxide). Even though much of the foliar deposited plutonium is unavailable for leaching with weakly fonic, pH 5.8 solution, the increased removal of both soluble and insoluble components with acidic solutions may Indicate that a portion of submicron particulates intercepted by foliage may be held on the leaf surface by charge phenomena and by physical entrapment, and not necessarily buried in waxy plates. A comparison of the leaching behavior of plutonium for two-plant species with varying surface roughness is shown in Table 2 (Cataldo, unpublished). Plants were leached with 800 ml of solution, with Leachate being collected in 50 ml fractions. Since total plutonium in the leachate decreased logarithmically, with plutonium activity in the last few leachate fractions approaching background levels, reported retention values represent Pu not readily leachable. a = 3% zgoe& 3 al 238bu - OXIDE (FRESH 6 F 2= ob L ao Q = a oS = o = ab 1 7 28ou - OXIDE (HYDRATED) TOTA ae C3 sowie (3 INSOLUBLE 1 7 a at TIME OF LEACHING FOLLOWING CONTAMINATION, DAYS Fig. 1. lLeachability of plutonium from bushbean foliage. Sets of four plants each were leached at 1, 7, 14, or 21 days following contamination. x + SEM (n=4). 336 Scanning electron micrographs of the leaf blades show bushbean leaves to have moderate to low surface relief, while sugar beet possesses leaves which are relatively flat. Difference in surface microtopography between these two species related primarily to patterns of wax deposition and the presence of trichomes in bushbean. In bushbean, surface wax is laid down in such a way as to form high longitudinally oriented ridges with deep crevasses, with the surface of the waxy plates being relatively smooth. By comparison, the surface wax of sugar beets form relatively shallow irreguiar convolutions, with the surface of the wax deposits being rougher than in the case of bushbean. The trichomes of bushbean are ¥ 150 um high and are spaced © 190 um apart, providing additional surface relief. It is this microtopgraphy, and its effect on particle entrapment and leaf surface wettability which may provide a basis for understanding the processes involved in particle retention, In general, the leaching data for sugar beet and bushbean suggests that retention of particles on foliar surface is a function of both surface roughness and particle size. In the case of the larger fresh oxide Particles (count mode v 0.142 pm), substantially more of the Pu is leachable from smooth leaf surfaces under beth leaching conditions. This may, in fact, be the result of physical entrapment of particles in comparatively deep fissures or crevices contributing to surface roughness in bushbean, especially if it is assumed that a particle must be suspended in a water droplet to be removed from the leaf surface. Similarly, the effect of acid leachate may be in alleviating the attractive forces holding particles 337