MODEL FOR ESTIMATING Pu TRANSPORT AND DOSE 465 



TABLE 2 (Continued) 



*Based on data from Gilbert (1977, p. 425). 



fMean ± standard error (SE). SE = s/{n)'2= standard error. 



Losses from Soil Compartment. As suggested by Fig. 1, plutonium can be transferred 

 from the soil compartment to compartments representing other ecosystem components. 

 It can also be removed from the soil of a given area by water or wind erosion. Percolation 

 into the profile could remove plutonium from the surface, where it is most susceptible to 

 resuspension, and could, if the soil were plowed and rainfall were plentiful, transport 

 some plutonium below the root zones of crop plants. 



Owing to the extreme variability of plutonium concentrations in soil samples taken 

 from the same general area and to the arbitrary nature of soil-compartment boundaries 

 (usually specified by a depth measurement), it would be difficult to design field studies to 

 estimate the overall rate of plutonium loss from the soil compartment. In fact, no such 

 studies have been undertaken in the field or in the laboratory, and we have no basis for 

 assuming that the average concentrations of plutonium in the soils of large contaminated 

 areas will decrease significantly in the next lOOyr or so. Consequently the soil 

 concentrations given in Table 1 will be treated as constants for the areas indicated; i.e., 

 the soil compartment is assumed to be a continuous and constant source for plutonium 

 transfer to other compartments. In the absence of any evidence that the rate of 

 plutonium loss is, in fact, significantly greater than the rate of loss due to radioactive 

 decay, the equation for the soil compartment is 



Cs = Cs(0)exp(-XAt) 



(2) 



where Cs = average concentration of plutonium in the surface soil of a contaminated area 

 at time t (pCi/g) 



Cs(0) = initial concentration as given in Table 1 



Xa = radioactive decay rate of ^^''Pu (7.7829 x 10"^ day^') 



t = time (days) 



