Chapter 4 



Transport and Dispersal 



45 



dispersed by diffusion. Diffusion in the ocean 

 is caused by turbulence or eddies, and the 

 coefficient of eddy diffusivity is usually more 

 than a million times the corresponding molecu- 

 lar coefficient. The rate of eddy diffusion de- 

 pends on wind speed, current shear, density 

 gradient, gradient of the diffusing concentra- 

 tion, direction of diffusion, and the dimensions 

 of the phenomenon. The calculated rates de- 

 pend upon the magnitudes of eddy diffusivity 

 coefficients used, and they have been estimated 

 by a number of methods (Sverdrup et al., 1942, 

 p. 484-485; Munk, Ewing and Revelle, 1949). 

 Because of both the large number of variables 

 concerned and the present unsatisfactory state 

 of our quantitative knowledge of turbulence in 

 the ocean, it is difficult to predict the diffusion 

 of radioactive materials under any given cir- 

 cumstances. The most satisfactory approach at 

 present is to conduct diffusion studies and ex- 

 periments at the place and under the conditions 

 of contemplated release. The results are only 

 applicable to the particular areas. 



During the 1946 preliminary survey in Bikini 

 Lagoon, the state of turbulence was determined 

 by a variety of measurements, and the subse- 

 quent observed distribution of radioactivity was 

 in close agreement with the predicted values 

 (Munk, Ewing and Revelle, 1949) . A mean 

 value for the radius of the contaminated area 

 was 3 km., which approximately doubled be- 

 tween the first and second days after the burst. 

 The initial distribution of radioactivity as de- 

 posited by the atomic bomb was patchy, and 

 the turbulent eddies, which spread the con- 

 tamination over a larger area, did not appreci- 

 ably reduce this patchiness during the first 

 three days. 



Another pertinent study was made by Ketchum 

 and Ford (1952) who examined the rate of 

 dispersion of acid-iron wastes in the wake of 

 a barge at sea. Computed mixing coefficients 

 showed a tendency to increase with increasing 

 time, and thus with the dimensions of the mix- 

 ing field, and the radius of the contaminated 

 area was observed to double in time periods 

 ranging from 0.5 minutes to 35 minutes. It 

 should be noted that the scale of this phenom- 

 enon was about 10-- that of Munk, Ewing and 

 Revelle (1949) ; they show that the ratio of 

 lateral eddy diffusivity coefficient to the radius 

 of the area considered is relatively constant 

 over a range of radius between 10^ and 10^ cm. 



A large scale tracer experiment was carried 

 out in the Irish Sea prior to the discharge of 

 radioactive effluent (Seligman, 1955). During 

 each experiment, 10 tons of 6.7 percent fluores- 

 cein solution were introduced near the surface 

 during a 20-minute period, and the sensitivity 

 of subsequent detection was believed to be of 

 the order of 1 part in 10^. Maximum concen- 

 trations detected directly after release were 10~* 

 of the original concentration; 12 hours after 

 release, they were down to 5 x 10"^ of the 

 original concentration. The trial area was prob- 

 ably part of an eddy and was subject to tidal 

 mixing, so the results may not be generally 

 applicable. 



Exchange hetiveen near-surface and intermediate 

 xaaters 



Since the surface layer is separated from 

 deeper waters by a layer of rapid density in- 

 crease, and hence of great stability, vertical 

 transfer of materials across this layer by eddy 

 diffusion must be much less rapid than is ver- 

 tical diffusion in the upper layer. Thus radio- 

 activity introduced at the surface by fallout may 

 remain in the upper layer for a long time and 

 be diluted by only a small part of the total 

 volume of the sea. Conversely, radioactive ma- 

 terials introduced below the pycnocline should 

 only slowly contaminate the upper layer where 

 they are most likely to endanger human ac- 

 tivities. However, organisms and particles of 

 sufficient density may readily cross the pyc- 

 nocline, due both to gravity and to vertical 

 migrations. 



There are few observations which show di- 

 rectly the existence of cross-pycnocline exchange 

 on a local scale. In the western Pacific, both the 

 "Shunkotsu-Maru" survey (Japanese Fishery 

 Agency, 1955) and the "Taney" survey (U. S. 

 Atomic Energy Commission, 1956) reported 

 patches with significant concentrations of radio- 

 activity below the thermocline four months and 

 thirteen months, respectively, after mixed fis- 

 sion products were introduced at the surface in 

 the Marshall Island area. It is not known, how- 

 ever, whether this exchange was effected by 

 mixing processes, or by particulate or ecological 

 processes. 



Exchange of properties between the near- 



