MIGRATION OF PLUTONIUM FROM FRESHWATER ECOSYSTEMS 635 



10- 



•••.0 



10' 



> 



■D 



u 



a 



10' 



10' 



10^ 



POND PROFILE 



■WATER; 



ORGANIC FLOC 

 VJ Ml XTU RE •■■>•■.';<> 



.UPPER 

 5 cm 



;-i^;:V-:- SEDIMENT'-' " ;V:::' I LOWE^ 



COMBINED 

 10 cm 



^ 



239,240 



Pu 



238 



Pu 



A 



UPPER 

 5 cm 



LOWER 

 5 cm 



COMBINED 

 10 cm 



Fig. 3 Distribution of plutonium isotopes in the upper 10 cm of U-Pond sediments. The 

 broken horizontal line indicates the pond-wide mean for both plutonium isotopes in a 

 10-cm core. 



Horizontal distributions of plutonium in the sediments are spatially and temporally 

 random, but several vertical profiles indicate that plutonium is most heavily concentrated 

 in the upper 5 cm (Emery et al., 1976). When three sediment cores were analyzed for 

 plutonium concentrations in the upper 5 cm, lower 5 cm, and combined 10 cm, it was 

 found that most of the plutonium was located in the upper layer (Fig. 3). In these 

 samples the plutonium concentration in the upper 5 cm was about 4 x 10^ pCi/g, 

 whereas that in the lower 5 cm was only about 5 X 10' pCi/g. The combined 10-cm core 

 showed a plutonium concentration of about 2 x 10^ pCi/g. It should be noted, however, 

 that the concentrations in these samples did not resemble the mean plutonium 

 concentrations in samples of 10-cm sediment cores. The difference of plutonium 

 concentrations between the upper and lower sections of the sediment core in Fig. 3 

 appears to be exaggerated beyond the normal range, but a vertical reduction of plutonium 

 in the top 10 cm of sediments is indicated. 



This vertical distribution of plutonium appears to be largely the result of the rapid 

 accumulation of sedimenting seston discussed earUer. Seston has shown the highest 

 concentrations of plutonium in any subcompartment sample, often greater than 1 X 10' 

 nCi/g. However, this material settles to the bottom and is captured by a layer of floe. This 

 accumulation of seston, decomposing plant material and a mixture of older sediments, 

 contains the largest portion of the pond's inventory. 



