Chapter 4 



TRANSPORT AND DISPERSAL OF RADIOACTIVE ELEMENTS IN THE SEA ' 



Warren S. Wooster, Scripps Institution of Oceanography, La Jolla, California 



and 

 BosTWiCK H. Ketchum, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 



The fate of radioactive elements in the sea 

 differs from that of non-radioactive elements 

 since they are subject to radioactive decay. 

 Otherwise, concentrations of radioactive ele- 

 ments are changed by the same physical and 

 biological processes as are those of other iso- 

 topes in the same physical state. Thus the fate 

 of radioactive material introduced into the sea 

 depends on: 



1. What is introduced — the nuclide, its radio- 

 active properties (half -life, nuclear reaction, 

 kind and energy of radiation), its physical state 

 in sea water (whether particulate, colloidal or 

 ionic) and its chemical properties (including 

 its role in biological processes). 



2. Where it is introduced — position and depth 

 with respect to the density and velocity struc- 

 ture of the sea. 



This paper describes the physical processes 

 whereby radioactive elements in true solution 

 are diluted by mixing and are carried from one 

 part of the ocean to another. Although all parts 

 of the open ocean appear to be in continuous 

 motion and in communication with each other, 

 the rates of this motion and exchange cover 

 such a wide range that it is convenient to con- 

 sider separately the questions of near-surface 

 vertical and horizontal exchange, intermediate 

 and deep circulation, and the exchange between 

 the deep sea, coastal areas and enclosed basins. 



Near-surface circulation 



In middle and low latitudes the surface layer 

 of the ocean, from 10 to 200 meters thick, is 



'^ Contribution from the Scripps Institution of 

 Oceanography, New Series, no. 903. Contribution no. 

 870 from the Woods Hole Oceanographic Institution. 

 This paper, in part, represents results of research car- 

 ried out by the University of California under con- 

 tract with the Office of Naval Research. Reproduction 

 in whole or in part is permitted for any purpose of 

 the United States Government. 



separated from the colder deep waters by a 

 layer of rapid density change and great sta- 

 bility, the pycnocline or thermocline. This 

 intermediate layer varies in depth and stability 

 from time to time and from place to place. At 

 times there are two such layers, the seasonal 

 thermocline and a deeper main thermocline. 

 The surface layer is often called the "stirred" 

 or "mixed" layer ^ because of its relative uni- 

 formity in temperature and in concentrations of 

 dissolved substances. 



It is believed that radioactive material in- 

 troduced into this surface layer will be rapidly 

 distributed vertically throughout the layer. The 

 general uniformity of concentrations within this 

 layer suggests that forces are present which tend 

 to bring it about. Because density increases 

 only slightly with depth through the layer, 

 little energy is required for vertical stirring. 



Some evidence of the rapidity of vertical 

 mixing in the upper layer is given by Folsom 

 (Revelle, Folsom, Goldberg and Isaacs, 1955), 

 who observed that when fission products were 

 introduced at the surface in an area where the 

 surface layer was about 100 meters thick, the 

 lower boundary of the radioactive water reached 

 the bottom of this layer in about 28 hours. 

 Within this period of time radioactivity had be- 

 come uniformly distributed vertically through- 

 out the layer. 



Rapid vertical mixing in the upper layer is 

 brought about primarily by the following two 

 processes: 



1. Convection: When the density of surface 

 water is sufficiently increased, owing to either 



2 A distinction is made here between stirring and 

 mixing. In stirring, one causes relative motion of 

 different parts of the liquid, and the average value 

 of the gradient is increased. Mixing then takes place, 

 the gradients disappearing and the liquid becoming 

 homogeneous (Eckart, 1948). 



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