532 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



complex interaction of food, water, and sediment, depending on trophic level and feeding 

 type. 



Another problem in using concentration factors in field studies is that concentrations 

 of elements in water or particulate materials are highly variable and organisms under 

 study usually are constantly moving both horizontally and vertically in the ocean. 

 Concentrations of elements in organisms represent an integration of environmental levels 

 to which the organisms have been exposed, and environmental levels at the point of 

 capture may not actually reflect the history of exposure. Therefore there is no known 

 bioavailable fraction or constant environmental level that can serve as a realistic value on 

 which we can base concentration factors. For these reasons we believe that the use of 

 concentration factors in aquatic studies should be approached more cautiously in the 

 future than in the past. 



Biological Transport of Transuranics in the Ocean 



Lowman, Rice, and Richards (1971) have discussed the relative importance of biological 

 vs. physical processes in the distribution of radionuchdes and trace metals in the ocean 

 and have included an excellent bibhography on the subject. It has become increasingly 

 apparent that the biological processes operative in the mixed layer of the ocean greatly 

 influence the vertical transport of materials from it and that biological activity near the 

 sediment— water interface may be a key mechanism for transport of recently deposited 

 materials to depth in the sediment. Although we have chosen to address only the data 

 relating to transuranics in this regard, the bulk of the evidence for vertical biological 

 transport of materials from the mixed layer to depth has come from measurements of 

 trace metals (Kuenzler, 1969; Fowler et al., 1973; Small and Fowler, 1973; Small, Fowler, 

 and Keckes, 1973), natural radionuchdes (Broecker, Kaufman, and Trier, 1973; Cherry 

 et al., 1975; Beasley et al., 1977), artificial radionuchdes (Osterberg, Carey, and Curl, 

 1963), and synthetic organic compounds (Elder and Fowler, 1977). Vertical mixing of 

 sediments by biological activity has been well established (Davidson, 1891; Dapples, 

 1942; Emery, 1953; Gordon, 1966; Glass, 1969; Rhoads, 1974), and this process 

 obviously will affect the vertical distribution of radionuclides associated with sediments. 



Bowen, Wong, and Noshkin (1971) first demonstrated that plutonium subsurface 

 maxima occur in the upper 1000 m of the ocean and attributed its removal from the 

 mixed layer to biogenic particle fluxes. Subsequently Noshkin and Bowen (1973) 

 proposed a model to explain both the vertical distributions observed in the water column 

 and the small but measurable amounts of plutonium found in deep-sea sediments as a 

 function of fallout delivery. This heuristic model invokes a mixed-particle population, 

 30% sinking at 392 m/yr, 40% sinking at 140m/yr, and 30% sinking at 70 m/yr; no 

 assumptions concerning the exact nature of the particulate matter involved are required. 



Direct evidence for the association of plutonium and americium with particulate 

 matter in the upper mixed layer of the oceans has been confirmed by several 

 investigators. Livingston and Bowen (1976b) have found that as much as 70% of fallout 

 plutonium can be removed by Milhpore filtration of open North Atlantic surface seawater 

 (presumably 0.45 )um); in coastal waters near Woods Hole, Mass., something over 90% of 

 the plutonium is associated with the particulate phase. Evidence is accruing which would 

 suggest that americium is also associated with particulate matter, although the number of 

 analyses are fewer than those for plutonium. Silker (1974) has measured both the soluble 

 and particulate plutonium in Pacific surface waters and has found that some 55 ± 7% of 



