TRANSURANIC RADIONUCLIDES IN ENEWETAK LAGOON 597 



The remaining 23 9 + 240p>^j -^^ ^^^^ particulate material is therefore associated with other 

 forms of suspended matter. Between 1960 and 1963, Johannes (1967) investigated the 

 composition of the suspended particles in the lagoon. Progressing from the eastern reef 

 toward the lagoon, suspended benthic algae and sediment particles became less abundant 

 with depth of the water as they settled to the bottom, and suspended macroscopic 

 organic aggregates, consisting largely of mucus released by coral, increased progressively 

 in size and number (Johannes, 1967). Often calcareous grains resuspended near the reef; 

 microorganisms, copepod fecal pellets, and other undifferentiated material were 

 incorporated with the aggregates. These materials and other particles produced in the 

 pelagic environment are the most important food components for lagoon zooplankton 

 and certain plankton-feeding fish (Gerber and Marshall, 1974). The small quantities of 

 plutonium ingested with tliis particulate debris are dispersed over the lagoon by these 

 organisms. Herbivorous fish play a role in the generation of particles in the water column 

 (Smith, 1973). These fish are not efficient assimilators; while satisfying their energetic 

 requirements, they disturb large quantities of material and release large amounts of 

 unassimilated material containing plutonium in their feces. Moriarty (1976) estimates 

 that a 200-g mullet, a species common to Enewetak, which feeds by scooping up bottom 

 material to sift and remove small algae, will pass 50 g of dry sediment through its gut per 

 day. 



Bottom particles from the northwest quadrant of the lagoon, where higliest 

 plutonium concentrations in sediment are found, usually have high plutonium concentra- 

 tions, which indicates that a fraction of the plutonium in the particulate phase may 

 originate from turbulent resuspension of the sediment components in deep (60 m) water. 

 The resuspended material and associated plutonium are probably not transported for any 

 distance in the lagoon. Previous results indicate that the material is redeposited in the 

 same general area of its origin. Only a few of the variety of active processes capable of 

 generating and moving particulate plutonium in the water mass have been considered. It is 

 remarkable that these and other processes resuspend so little of the plutonium inventory. 

 Barring catastrophic events, the present distribution and inventory of plutonium in the 

 sediments will be only slightly altered during the years by relocation of labeled material 

 from other regions in the lagoon. 



Laboratory studies with contaminated sediments and soils from Enewetak show that 

 plutonium is rapidly partitioned between the solid phase and solution and reaches 

 equilibrium after several days with an average distribution coefficient for plutonium of 

 1.8 X 10^. Table 6 shows this and some recent determinations of the distribution 

 coefficient for plutonium in laboratory and field experiments using a variety of 

 sediments. Considering the difference in the types of environmental samples represented 

 in Table 6, it is striking that the K^j for plutonium differs so little. 



Table 1 shows that the mean plutonium inventory associated with the sediment 

 components in the top 2.5 cm at Enewetak is 249 Ci. The lagoon sediment has an average 

 density of 1 .8 g/cm^ (Emery, Tracy, and Ladd, 1954) and occupies an area of 933 km^. 

 The mean depth of the lagoon is 47.4 m. With these data and the K^ for plutonium of 

 1.8 X 10^, one can construct a simple model to predict the average concentration 

 expected in the lagoon water by assuming that the plutonium in solution is in equilibrium 

 with that in the sediments. At any time the amount of plutonium in solution is limited by 

 the saturation of the solution under equilibrium conditions. The rate at which water and 

 its dissolved plutonium is flushed from the lagoon is balanced by input of uncon- 

 taminated ocean water, which is rapidly saturated with remobilized plutonium from the 



