PLUTONIUM IN A GRASSLAND ECOSYSTEM 425 



et al., 1971); Eberline modified a pyrosulfate fusion technique for the same purpose (Sill, 

 1969). Ion-exchange columns removed interfering elements and isolated plutonium from 

 the samples before alpha spectrometry analysis. Chemical recovery was measured by 

 adding ^^^Pu tracer to each sample. Agreement between homogenized split samples sent 

 to these laboratories was good (Little, 1976). In our laboratory a procedure was used that 

 incorporated harsh digestion of the sample by nitric and hydrofluoric acids, ion exchange, 

 organic extraction, and liquid scintillation spectrometry (Little, 1976). This method had 

 an estimated minimum detectable activity of 0.42 pCi (P < 0.05). Plutonium data in this 

 chapter are 2 39,240p^ unless otherwise noted. Plutonium-240 contributed about 20% on 

 the average to the alpha activity of ^^^'^'*°Pu. 



Plutonium Compartmentalization 



The inventories of plutonium in the principal compartments of the grassland ecosystem 

 were calculated. Compartments investigated were soil, in 3-cm increments from to 

 21 cm, litter, standing vegetation, arthropods, and small mammals. 



The compartmental inventories of plutonium were calculated by multiplying the 

 mean mass of each compartment (g/m^) by the mean plutonium concentration of the 

 compartment (|UCi/g). A total ecosystem inventory was calculated by summing over all 

 compartments. The compartmental fraction (unitless) of the total plutonium inventory 

 was calculated by dividing each compartmental inventory (^Ci/m^) by the total inventory 

 (AfCi/m^). 



The soil compartment had vastly the largest fraction of the total plutonium, 99.69% 

 (Table 2). As expected, the fraction of the total plutonium contained within a soil layer 

 decreased as the depth increased. The litter compartment comprised less than 1% of the 

 total plutonium (53 nCi/m^) in the study areas, and the vegetation represented only 

 about 0.01% of the total plutonium. By virtue of representing both low biomasses and 

 plutonium concentration, the animal compartments, arthropods and small mammals, had 

 extremely small fractions of the total plutonium, 6.8 X 10~^ and 33 x 10~^, 

 respectively. 



In summary, the .compartmentalization data indicated that greater than 99% of the 

 plutonium in the study area was located in the soil. At the time of sampling, nearly 

 one-half (49.7%) the total plutonium was in the top 3 cm of soil. In decreasing order, 

 smaller plutonium-inventory fractions were found in Htter, vegetation, arthropods, and 

 small mammals. The implication of these results is that, in the present state, transport of 

 plutonium is closely Unked to soil movement or erosion. Therefore efforts to prevent 

 plutonium transport off contaminated grasslands should be directed primarily at 

 minimizing soil transport rather than mobihzation by biota. 



Plutonium in Soil 



Analysis of the soil plutonium data suggested that two primary generalizations about 

 plutonium in soil could be stated. First, the plutonium concentrations in the soil samples 

 were highly variable. Second, the plutonium concentration in a soil sample was a function 

 of sample location, sample depth, and the soil particle composition of the sample. 

 Rationales for both of these conclusions are examined in some detail. 



Frequency distributions of plutonium in soil samples were positively skewed 

 (P < 0.05) with coefficients of variation (CV = standard deviation ^ mean) ranging to 



