contamination resulting from dry deposition and/or wet deposition, with subsequent calculation of retention times based on soluble or readily leachable components found in fallout particles such as '?!1, 137¢,) and °sy. Resuspension data from the NTS provides a basis for much of the information we have concerning the interception of plutonium contaminated soil particles and their retention. The behavior of the transuranic elements in the environment and their potential for transfer in the food chain has been the subject of extensive study over the past 25 years. Although there is a general understanding of many problems concerning atmospheric transport, and the behavior of plutonium in specific ecosystems (NTS, Rocky Flats, etc.), little is known of the controlling mechanisms’ influence on the bioavailability of plutonium and the other transuranics, and their subsequent transfers along the food web to man. With the current stratospheric depletion of fallout plutonium (Bennett, 1976), the inhalation route to man is greatly reduced. This would then suggest that the major sources of transuranic elements would be from resuspension of fallout contaminated soils on a global basis, resuspension from highly contaminated local sources, accident situations, and lowlevel releases from nuclear facilities. A frequent practice in present radiological safety estimations is to discount foliar sorption and emphasize the soil to root pathway for food chain entry of transuranic and other radiocelements (Vaughan ef al., 1976). Typical dose assessment codes assume a rapidly declining exponential loss of material from leaves (Soldat, 1971). This is certainly not a general situation. Tt does not apply to the behavior of plutonium aerosols to be described herein, and probably applies only to very large particles and to certain gaseous radioelements like iodine (Markee, 1971). The relatively greater importance of the foliar entry pathway compared to root absorption for worldwide fallout was, of course, recognized long ago (Chamberlain, 1970; Russell, 1965). In later studies, for particles probably of wind resuspended origin, it has been shown that 872% of the Sst, 81% of the '?’Cs, and 73% of the '**Ce in forage plants was derived from foltar contamination (Romney ef al,, 1973). This was done by comparing plants grown with and without plastic enclosures at the NTS site. Currently, the relatively greater importance of foliar to root pathways to the plant has been incorporated in the LMFBR Final Environmental Statement (USAEC, 1974). Despite the inconsistencies in many other dose assessment codes, risk tends to be minimized because of extremely conservative limits specified at points where radioelements enter the human body. However, as a matter of systematic practice, an improved quantitative understanding of the basic environmental processes is required. This becomes especially important in situations: 1) where new technology may lead to different physical (size) and chemtcal characteristics of the source term for release, especially in nuclear fuel reprocessing plants; and 2) where comparatively large increases in the handling of radioelements are projected for the future (ERDA, 1975). The following discussion will briefly review what is known concerning the absorption and adsorption potential of foliar surfaces, and attempt to by plant describe the fate of transuranic particulates on interception s and the foliage, by extrapolation from the behavior of other particulate limited information on plutonium. CANOPY INTERCEPTION correlations of fallout in the HERMES Model (Soldat, 1971) air to piant representing the sr, Cs and Iz have led to an average estimator, 0.25, surface deposijnterception fraction, i.e., the fraction of each month's initially retained by tion from air and from sprinkler irrigation that is Such a value . agricultural vegetation, before leaching by rainfall to marked variarepresents an operational definition, in practice subject tion depending on specific circumstances of exposure. interception has In field observations, another way of considering canopy This coefficient been to calculate the interception coefficient, f. ) was found to vary (sometimes erroneously termed "interception fraction" 1000-fold in various field situations (Bloom et al., 1974). 2 cm’ /g £ = d/dt {c\) + c, Vy> deposition on plant where d/dt (C_) = instantaneous rate of radioactivity leaf, pCi/g dry/sec v Cy = air concentration, pCi/cm 3 V5 = deposition velocity, cm/sec n clearly establishes A range this wide for estimates of initial depositio foliar that aerosol polydispersity and other unassayed variables control Probably chief among deposition and retention, as measured operationally. these variables is the particle size distribution, since deposfiton known particle sizes velocities can be estimated with greater accuracy for eous Values determine? for air concentration and instantan (USAEC, 1974). generally represented deposition rates, in past field observations, have Leaf type and degree of mineralian unknown distribution of particles. likely to he the zation may also affect these estimates, but they are only minor variables. BOUNDARY-LAYER PARAMETERS AFFECTING DEPOSITION basis in the Although particle deposition fs an aerodynamic problem with a n of particles physical sciences, in real-world situations, the interactio a modification of with a dynamic surface such as a plant canopy “requires (Slinn, to data" predictive formulae which are mostly just empirical fits e due to Several of the factors which become difficult to quantitat 1975). , eddy their ever-changing nature in the boundary layer include turbulence 333