438 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Summary 



The soil to a depth of 21 cm contained more than 99% of the plutonium estimated to be 

 in the studied areas of the Rocky Flats grassland. Litter contained a larger fraction of the 

 total plutonium (~10~^) than vegetation (~10~'*), arthropods (~10~^), or small 

 mammals (~I0~^). These results implied that soil— plutonium relationships and 

 soil-management practices are very important at contaminated sites. 



Plutonium-concentration frequency distributions for soil samples were positively 

 skewed and characterized by CV's that were generally greater than 100%. Plutonium 

 concentrations in surface (0 to 3 cm) soil were inversely related to distance from the 

 plutonium source, the former oil -barrel storage area. Soil— plutonium concentrations 

 tended to decrease as depth increased and tended to increase as the soil particle size 

 decreased. This latter result suggested that plutonium— soil interaction was a surface- 

 attachment mechanism. 



Mean concentrations of plutonium were higher in litter than in vegetation. Frequency 

 distributions of plutonium concentration were normal in litter and lognormal in 

 vegetation. In a manner similar to soil, plutonium concentration both in litter and in 

 vegetation was also inversely related to distance from the barrel storage area. Plutonium 

 concentrations in plant-derived compartment samples were also significantly correlated to 

 plutonium concentration in surface soil at the same locations. 



Plutonium frequency distributions in arthropods and small mammals were also 

 positively skewed. Plutonium concentrations in bone samples were lower than those in 

 the other tissues sampled, namely, GI tract, hide, kidney, liver, lung, and muscle. 



Concentration ratios of litter, arthropods, and small mammals relative to soil 

 indicated that litter had the highest value. The other compartments, in descending order, 

 were vegetation (3.4 X 10"^), small mammals (7.8 x 10~^), and arthropods 

 (6.8 X 10'^). The relatively liigh CR's suggested that most of the contamination of 

 vegetation resulted from surficially attached plutonium— soil particles as opposed to root 

 uptake. All the above data strongly indicate that in the grassland soil is by far the most 

 important compartment insofar as plutonium content and transport are concerned. The 

 primary conclusion is that, if transport of plutonium is to be avoided, then transport of 

 soil should be avoided. Therefore soil stabilization should be promoted by maximizing 

 vegetative cover growth and minimizing mechanical disturbances. 



Isotopic ratios of ^^^Pu to ^^^Pu were calculated for soil, litter, vegetation, 

 arthropod, and small-mammal samples processed by commercial laboratories. The soil 

 results indicated that the median ratio was about 50. Litter and vegetation IR's were 

 similar to IR's in soil. The IR's of small-mammal tissues and arthropods were likely lower 

 than those of soil. 



The meaning of the lower IR's in animal compartments was clouded by the fact that 

 the frequency distributions of the ^^^Pu and ■^''^Pu concentrations, from which the 

 ratios were formed, were censored. Further, the '^^^Pu concentration distribution was 

 censored to a much larger degree than was the ^^^Pu distribution. This situation may 

 have the effect of spuriously decreasing the mean or median ratio if the ratios are formed 

 before the average is calculated. An estimation procedure was used to calculate the mean 

 of both ^^^Pu and ^"'^Pu by taking into account the degree of censorship. Although 

 most small-mammal compartments may not be amenable to such a procedure, the ratio in 

 hide was calculated to be about 37. This value was within the 95% confidence interval of 

 most of the soil IR's. Without further analysis, the hide data suggested that the IR may 



