34 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



compartments, the physical size of the compartments, and the processes and rates 

 controlling movements of materials between compartments. Knowledge of ecosystem 

 structure and function is useful in predicting the transport of any insoluble element 

 where physical and biotic transport processes dominate. Quantification of physical 

 and biotic processes controlling soil and sediment transport provides a basis for predicting 

 the behavior of elements tiglitly bound to them. 



Ecological Effects of Transuranic Elements in the Environment 



The main purpose of the studies described in this volume is to provide information that 

 can be used to help predict the consequences of transuranium elements in the 

 environment. Such consequences include possible harmful effects on man and other 

 species from current and potential levels of these elements in the biosphere. The 

 prediction of consequences requires detailed knowledge of source terms, environmental 

 transport, biological uptake, and biological effects expected from uptake. This review 

 emphasizes effects that might be expressed in species populations, such as mortality and 

 natality, and resulting perturbations in population density and community composition. 



It is clear that effects induced by transuranic elements at the population or 

 community level have not been measured directly because environmental levels have not 

 been sufficient to produce obvious changes. Subtle changes in populations or communi- 

 ties are readily masked by natural variations, and ecologists are ordinarily unable to 

 measure small perturbations and identify their causes. However, indirect calculations and 

 extrapolations to low doses can be used to infer ecological consequences of transuranic 

 elements presently in the environment. The task of predicting ecological impacts of a 

 given level of transuranic elements in a particular environment is not simple. How the 

 contaminant will behave; i.e., how it will be distributed among the various ecosystem 

 components; how this distribution will change with time; and what physical, chemical, 

 and biological factors will affect the distribution, must be understood. There is also tlie 

 question of doses to critical biological tissues. Most transuranic elements are alpha 

 emitters that exliibit generally heterogeneous distributions in tissues, and this makes the 

 calculation of effective doses difficult. Finally, the relation of effective doses to biologic 

 effects must be understood. 



The bank of data from which ecological impacts of the transuranic elements can be 

 predicted is limited. For instance, the relationships of tissue concentrations of transuranic 

 elements to concentrations in soil, air, or water are accurately known for only a few 

 ecosystems. Current data pertain to plutonium and americium; research on other 

 transuranic elements only recently has been initiated. Our experience with transuranic 

 elements in the environment has been too brief to allow us to predict long-term behavior 

 confidently. Another problem is tliat the microdosimetry of transuranic elements has 

 been well studied in only a few laboratory animals. Finally, predictable dose— effect 

 relationships exist mainly for plutonium in laboratory animals. Thus there is considerable 

 uncertainty as to dose— effect relationships for all the transuranic elements in plants and 

 aquatic organisms. 



To measure direct relationships between amounts of transuranic elements and effects 

 would require purposely contaminating ecosystems at levels permitting direct observa- 

 tions of biological effects. In practice, however, this approach is not feasible for 

 ecosystem-scale investigations, and such studies have not been done. Ecosystems have 

 been contaminated with transuranic elements through mishaps or experiments for other 



