MODEL FOR ESTIMATING Pu TRANSPORT AND DOSE 475 



In general, plant/soil ratios for ^"'^Pu, which are based on plant and soil samples 

 collected under field conditions, range from 1 to 10~^, whereas ratios based on 

 laboratory studies, which preclude external contamination, range from 10"^ to 10~^. 

 Considering the situation at NTS, we believe it is reasonable to assume that 

 approximately 99% of the plutonium associated with the vegetation compartment is due 

 to external contamination and that no more than 1% is due to root uptake. 



Data on the plant/soil ratio for other transuranium elements are very limited. 

 Romney et al. (1975) measured ^'*' Am concentrations of the vegetation from the areas 

 at and near NTS. Grouped according to species and location, the mean plutonium/ 

 americium ratio in vegetation ranged from 2.0 to 28.3, with typical values being about 

 10. Similar groups of soil (Gilbert et al., 1975) ranged from 5.3 to 26, with typical values 

 also being about 10. These analyses indicate that the long-term plant/soil ratio for 

 americium is not significantly different from that for plutonium.* The data on the 

 short-term plant/soil ratio indicate significant differences that may be related to the 

 solubility of the element. Price (1973b) measured the uptake of ^^^Np. ^^^Pu, ^"^^ Am, 

 and ^'*'*Cm by tumbleweed and cheatgrass from various solutions applied to the soil. The 

 americium uptake was about 2 to 30 times as great as that of plutonium, curium uptake 

 was about 2 to 40 times as great as that of plutonium, and neptunium uptake was about 

 100 to 1000 times as great as that of plutonium. Bennett (1976) summarized much of the 

 short-term data and concluded that americium and curium uptakes were about 10 to 30 

 times as great as that of plutonium. 



Variation of Plant I Soil Ratio. Data presented by Romney et al. (1975) also demonstrate 

 that the mean concentrations of plutonium in soils and plants decrease with increasing 

 distance from ground zero locations, whereas the vegetation/soil ratios within sampling 

 strata show a tendency to increase. Tamura (1976) provides a graph of soil activity vs. 

 distance from ground zero and fits the data to a power curve of the form y = ax"^, where 

 y is soil activity and x is distance from ground zero. This is an interesting notion to 

 pursue because we expect both soil concentration and particle size to decrease with 

 increasing distance from ground zero. If foliar retention is greater for small particles, we 

 would expect vegetation/soil ratios to increase as particle size and soil contamination 

 decrease with increasing distance from ground zero. We have no comparable curve for 

 vegetation, but we assume it would be of the same form. On the basis of this assumption, 

 the vegetation/soil ratio could be expressed as a function of distance from ground zero as 

 follows: 



yv = avx ^^ 





(10) 



*Gilbert et al. (1975, p. 407), using Double Tracks data, show that the average vegetation/soil 

 ratio tor americium is about 1 .5 times the average vegetation/soil ratio for plutonium; but, considering 

 the range of ratios contributmg to the averages, we are not persuaded that 1 .5 is significantly different 

 from 1.0. At other sites the ratios were not different. 



