square meter is associated with 50 kg of soil (5 cm depth x 10" cm?/m? x 1073kg/cm3), a mass loading factor of 100 ve/m is equivalent to a resuspension factor of 2x 10°? m7!. The theoretical basis for the mass loading approach 1s described by Anspaugh (1974). Anspaugh et al. (1975) provide comparisons showing that predicted air concentrations based on Le = 100 ug/m? are in good agreement with measured air concentrations. where: kay 18 an air ~ vegetation deposition rate goefficient (m3/g-day), C, is the concentration of Pu in air (pCi/m”), hy is a weathering rate coefficient (day-*), Ag is a vegetation growth rate coefficient (day"!) We shall use the suggested mass loading factor to represent average conditions at NTS, but it must be noted that higher than average wind velocities (Shinn and Anspaugh, 1975) or mechanical disturbances such as plowing (Milham et al., 1976) could cause the mass loading factor to be temporarily much higher than 100 ug/m3. VEGETATION General Hypothesis As shown in Figure 1, vegetation may be contaminated externally by deposition of resuspended material or internally by uptake from soil, or by both processes simultaneously. Other mechanisms of external and internal contamination have been identified or postulated, but direct deposition from air and root uptake appear to be the processes most important to consider in attempting to develop a general model. Externally deposited material may be removed from plant surfaces by weathering, tf.e., the mechanical action of wind and rain, and it may be diluted by plant growth. Internally deposited material may also be diluted by growth but not by weathering. Processes which remove biomass from vegetation (e.g., grazing, eropping, root decay, dehiscence of aboveground parts, etc.) also remove plutonium. If they exceed growth rates, these processes may reduce the total amount of plutonium in the vegetation compartment of an ecosystem. Different plant species may vary widely with respect to their ability to retain externally deposited plutonium or to assimilate plutonium from foliar deposits or soil, and translocation within the plant may result in large differences regarding plutonium concentrations in different plant organs. In the present discussion, we do not attempt to distinguish one plant species from another. We assume that plutonium is uniformly distributed in edible plant materials, and that processes which remove biomass from the vegetation compartment have no effect on the concentration of plutonium in the remaining biomass. Differential equatdons expressing the principal processes described above can be written as follows: C - OL tA Hae dy,fat =k =kfC dy/dt =k C =k CO 632 - OL HWY ( 7) C 8) Yyo is the concentration in vegetation of externally deposity Pu(pCi/g), aq is the radioactive decay rate coefficient for Ipu (day7!y, Yy, 1s the concentration in vegetat‘on of internally deposited Bu (pCi/g), key is a soil ~ vegetation uptake rate coefficient (day-!), and C, is the concentration of Pu in soil (pCi/g). Equations (7) and (8) represent the external and internal components of plutonium in vegetation. The former is due to foliar deposition, the latter to root uptake. It is assumed that plutonium taken up via roots can be translocated to stems and leaves, but this rate is difficult to estimate. Consumers of vegetation are connected to both compartments simultaneously, and this is the same as summing the two components. Assimilation of externally deposited materials and their translocation to other parts of the plant has been demonstrated experimentally for various kinds of substances applied externally to foliage; but, in the case of plutonium (which is most probably deposited on foliage in the form of insoluble particles) foliar assimilation is assumed to be zero. A recent study (Cataldo, Klepper, and Craig, 1976) has demonstrated that translocation of foliarly deposited plutonium to roots and seeds can occur, However, the accumulation ratios observed in the absence of a solution vector (simulated rainfall) were on the order of 10-© €or both fresh and aged Pu0,, i.e., the observed concentrations in leaf tissue were about 200,000 to 500,000 times higher than the observed concentrations in seed and root tissue, respectively. While foliar deposition and roct uptake of plutonium have been studied separately in a variety of experiments, there is no reliable method for distinguishing between the two components in a plant sample 1f both are present. If plants are grown in contaminated soil or culture media and only the aerial parts are assayed, one may assume that the activity detected was internally deposited. If the aerial parts of plants are collected from a recently contaminated area which had no plutonium in the soil prior to the contaminating event, one may assume that all or nearly all the activity detected was externally deposited. If, on the other hand, plant samples are taken from an area which 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 (a) the biological availability of plutonium in contaminated soil may increase with time after environmental release so that successive crops of annuals would take up successively greater amounts of plutonium, (b} perennial plants may accumulate plutonium to a much greater extent than previously indicated by short-term uptake experiments, and (c) plant uptake may increase with decreasing plutonium concentration in the soil. Either or all of these