Plutonium in a Grassland Ecosystem 



CRAIG A. LITTLE 



Ttiis chapter is primarily concerned with plutonium contamination of grassland at the 

 U. S. Department of Energy Rocky Flats plant, which is located northwest of Denver, 

 Colo. Major topics include the definition of major plutonium-containing ecosystem 

 compartments; the relative amounts in those compartments: whether or not the 

 predominant isotopes, ^^^Pu and ^^^Pu, behaved differently; and what mechanisms 

 might have allowed for the observed patterns of contamination. 



Samples of soil, Utter, vegetation, arthropods, and small mammals were collected for 

 plutonium analysis and mass determination. Small aliquot s (5 g or less) were analyzed by 

 a rapid scintillation technique and by alpha spectrometry. 



Of the compartments sampled, greater than 99% of the total plutonium was 

 contained in the soil. The concentrations of plutonium in soil were significantly inversely 

 correlated with distance from the contamination source, depth of sample, and particle 

 size of the sieved soil samples. The soil data suggested that the distribution of 

 contamination largely resulted from physical transport processes. 



Concentrations of plutonium in litter and vegetation were inversely correlated to 

 distance from the source and directly correlated to soil concentrations at the same 

 location. Comparatively high concentration ratios of vegetation to soil suggested wind 

 resuspension of contamination as an important transport mechanism. 



Arthropod and small-mammal tissue samples were highly skewed, kurtotic, and quite 

 variable. Plutonium concentrations were lower in bone than in other tissues. Hide, 

 gastrointestinal tract, and lung were generally not higher in plutonium concentration than 

 kidney, liver, and muscle. All data tended to indicate that physical transport processes 

 were the most important. 



Median isotopic ratios of ^ ^ ^ Pu to^^ ^Pu by activity concentration in soil were 40 to 

 50. Litter and vegetation isotopic ratios were similar to those of soil. Arthropod and 

 small-mammal isotopic ratios were lower than those of soil, which implied that the two 

 isotopes were differentially incorporated into the animal bodies and ^^^Pu was taken up 

 at a higher rate. However, further investigations suggested that statistical bias may have 

 spuriously contributed to the lower isotopic ratios in small animals. 



Most of the world's agriculture occurs on land that, before tilling, was once covered by 

 stands of grasses. An important untilled tract of land contaminated with plutonium* is 

 the grassland immediately adjacent to and contained within the Rocky Flats plutonium 

 processing plant and associated buffer zone about 12 km northwest of Denver, Colo., 

 metropohtan area. Because Rocky Flats is a prime example of plutonium-contaminated 



*The word "plutonium" indicates ^ ^ ' '^ '^ " Pu in this chapter, unless otherwise noted. 



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