MODEL FOR ESTIMATING Pu TRANSPORT AND DOSE 469 



growth. Internally deposited material can also be diluted by growth but not by 

 weathering. Processes that remove biomass from vegetation (e.g., grazing, cropping, root 

 decay, and dehiscence of above-ground parts) also remove plutonium. If they exceed 

 growth rates, these processes can reduce the total amount of plutonium in the vegetation 

 compartment of an ecosystem. Different plant species can vary widely with respect to 

 their ability to retain externally deposited plutonium or to assimilate plutonium from 

 foliar deposits or soil, and translocation witliin the plant can result in large differences 

 regarding plutonium concentrations in different plant organs. In this 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 equations expressing the principal processes described above can be 

 written: 



''^ = kavCa - (>^\v + K + >^a) Yve (7) 



dt 



dyvi 

 dt 



ksvCs-(X, + AA)yvi (8) 



where yve = concentration in vegetation of externally deposited plutonium (pCi/g) 

 kgv = air-to-vegetation deposition rate coefficient (m^/g • day) 

 Ca = concentration of plutonium in air (pCi/m^ ) 

 X\v = weathering rate coefficient (day~^ ) 

 Xg = vegetation growth rate coefficient (day~^ ) 

 Xa = radioactive decay rate coefficient for ^^^Pu(day~^) 

 yvi = concentration in vegetation of internally deposited plutonium (pCi/g) 

 ksv = soil-to-vegetation uptake rate coefficient (day"') 

 Cs = concentration of plutonium 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 have 

 been demonstrated experimentally for various kinds of substances applied externally to 

 foUage, but, in the case of plutonium (which is most probably deposited on foUage 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~^ for both fresh and aged PuOa ; i.e., tlie observed concentrations in leaf tissue were 

 about 200,000 to 500,000 times higher than the observed concentrations in seed and root 

 tissue, respectively. 



Althougli foliar deposition and root uptake of plutonium have been studied 

 separately in a variety of experiments, there is no reliable method for distinguishing 



