PLUTONIUM IN A GRASSLAND ECOSYSTEM 431 



TABLE 5 Plutonium Concentrations 



in Rocky Flats Vegetation 



and Litter Samples 



also present but in much lower numbers of individuals. Rather than study numerous plant 

 types individually, two plant-derived compartments were studied, litter and detritus and 

 standing vegetation. Although these compartments accounted for only a small fraction of 

 the total plutonium (about 0.2%). the study of those compartments helped derive some 

 concepts of plutonium transport. 



As with the soil, frequency distributions for vegetation samples were positively 

 skewed. Further, the hypothesis that plutonium concentrations in vegetation were 

 lognormally distributed could not be rejected (P > 0.05). Unexpectedly, the hypothesis 

 that plutonium concentrations in litter were normally distributed could not be rejected 

 (P> 0.05). 



Mean plutonium concentrations in Utter were liigher than those in vegetation 

 (Table 5). Concentrations of plutonium in Utter and vegetation were each inversely 

 correlated with distance east or south from the asphah pad (P < 0.01). 



The fact that litter had a higher mean concentration of plutonium than standing 

 vegetation is not surprising. This result reinforces the suggestion made above that soU 

 transport was the primary mechanism of plutonium transport. 



Plutonium in Animal Compartments 



Two animal compartments were studied, arthropods and small mammals. These 

 compartments together contained about 2 x 10~^ of the total plutonium estimated to be 

 in the studied areas. Nevertheless, the mobiUty of the animals makes them potential 

 transporters of plutonium, albeit relatively small amounts, off the site. 



As expected from the soU and vegetative sampling, the frequency distributions of 

 plutonium concentrations in smaU mammals were positively skewed, as indicated by the 

 histogram in Fig. 6. Not only were there many samples that had plutonium concentra- 

 tions below the detection Umit but also much of the total activity was supplied by 

 relatively few samples. Frequency distributions of plutonium concentrations in arthro- 

 pods were also positively skewed (Bly, 1977). Bly (1977) further indicated that 

 logarithmic transformations were useful in aUeviating the skewness. Therefore the 

 plutonium concentrations of the arthropod samples were probably lognormaUy distrib- 

 uted. 



Concentrations of plutonium in 23 groups of individual arthropods and in 

 small-mammal tissues were of comparable magnitude (Table 6). The smaU-mammal 



