160 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



readily mobilized by citrate or resin equilibrations where the pH is maintained at natural 

 values. A small fraction (0.08%) is soluble in dilute sodium bicarbonate. On the basis of 

 chemical extractions with dilute nitric acid and strong carbonate solutions and by 

 plant-extraction data, the Th and Pu appear to behave similarly, which suggests similar 

 chemical associations in the soil. Since the soil itself is largely alluvium, the extractable 

 Th is probably associated with colloidal surfaces of secondary minerals. The intrusion of 

 Pu may have resulted in similar associations. 



For this soil and for the type of contamination that was believed to have occurred, it 

 appears reasonable to conclude that 30+ yr after deposition in soil Pu is not assimilated 

 by vegetation to a greater extent than natural Th. To the extent that Th is recognized as 

 an element which does not readily transfer in biological food chains, a similar behavior 

 for Pu should be observed many centuries after its release to the biosphere. 



Physical Characterization 



Observations of Particle-Size Association ofPu in Contaminated Sites 



As noted in an earlier section, plutonium can actually be released into the environment 

 by several modes. In addition, the size and character of the plutonium as it is being 

 introduced are subject to change as it interacts with environmental material. After the 

 interaction, some of the properties of the plutonium can be controlled by the matrix 

 properties. 



Studies of the association of plutonium with soil and sediment particles have been 

 reported by Tamura (1976) for several contaminated sites. He reported that, in the safety 

 shot sites of NTS, the plutonium found in the soils surrounding the site was primarily 

 associated with coarse-silt (50 to 20 ;um) and fine-sand size (125 to 50 /jm) fractions. The 

 activity/size ratio of the coarse-silt fraction of two samples reported by Tamura (1976) 

 was approximately 7.7 and of the clay size was approximately 0.5. Since the size 

 segregation was based on the density of silicate particles (2.65 g/cm^ density), it could 

 not be ascertained whether the plutonium particles were of the designated sizes or 

 whether the plutonium particles were of a finer size and attached to the silicate surfaces. 

 These size associations are consistent with the findings of Mork (1970), who reported in 

 his studies of NTS soils that the major portion of the activity was associated with 

 particles larger than 44 iim. 



In contrast to the NTS soils, the Pu in the bottom sediment originating from a 

 waste-transfer line leak at ML was primarily associated with particles less than 2 ^um in 

 diameter (Tamura, 1976). Interestingly, MuUer and Sprugel (1977) reported that small 

 amounts of plutonium released from stacks at ML and absorbed by soils were also 

 primarily associated with the <2-jum soil particles. They also found that fallout 

 plutonium in the environs of ML showed the same pattern of concentration in the various 

 size fractions. 



A sample from the floodplain soil at ORNL showed that the plutonium distribution 

 followed the soil-particle size distribution (Tamura, 1976). This distribution would 

 indicate that the plutonium in the floodplain was part of settling sediment particles that 

 had reacted with the plutonium farther upstream. As noted earlier, upstream of the 

 floodplain is a waste pond that contains plutonium in the bottom sediment; the overflow 

 from this pond empties into the creek flowing to the floodplain. The activity/size ratio of 

 the floodplain sample was 1.40 in the clay size and 0.97 in the coarse-silt fractions; in 



