Chapter 4 



Transport and Dispersal 



47 



and (3) in coastal regions, particularly along 

 the western coasts of continents. Conversely, 

 such exchange is least likely in temperate and 

 tropical latitudes in the vast central regions 

 of the northern and southern oceans. 



Exchange betiveen the open sea and coastal areas 



In coastal areas or enclosed basins where 

 precipitation exceeds evaporation, there is a 

 seaward surface drift of diluted water and a 

 landward subsurface drift of water derived 

 from the open sea. If radioactive materials 

 were released in such a coastal area, the ma- 

 terial which remained in the surface layer would 

 be carried seaward, but the part of the material 

 which mixed or settled to the deeper water 

 would move toward shore and the estuaries of 

 rivers. Conversely, if radioisotopes were lib- 

 erated in the open sea, some would eventually 

 be carried inshore as a result of the coastal and 

 estuarine circulation. 



It is clear that the ultimate distribution in 

 coastal areas of radioactive materials added to 

 the sea would depend on the location of the 

 release, the vertical distribution of radioactivity 

 and density in the area of release, the length 

 of time required for the transport to the coastal 

 area or estuary, and the location of the source 

 sea water which provides for the counter drift. 

 The number of variables involved makes it 

 difficult to discuss the effects in general terms, 

 but it is worthwhile to note that the circulation 

 in coastal areas is rapid, and water bathing the 

 North Atlantic beaches is not uncommonly 90 

 per cent sea water even off large rivers such as 

 the Hudson and Delaware. 



An idea of the lengths of time involved in 

 the coastal circulation can be obtained from the 

 mean age of waters in various parts of the At- 

 lantic seacoast. Such mean ages are computed 

 from the volume of water contained in the 

 region and the estimated transport of water 

 through the region. The waters of the con- 

 tinental shelf from Cape Hatteras to Cape Cod 

 have a mean age of about 2\ years, those of 

 the Bay of Fundy about 3 months, and those of 

 Delaware Bay from the ocean to the height of 

 tide about 3-4 months (Ketchum and Keen, 

 1953, 1955). The source sea water for all of 

 these circulations is the "slope water" which 

 is formed between the Gulf Stream and the 

 edge of the continental shelf. 



A few data are available for confined basins 

 and seas from which estimates of the mean 

 age of the water can be derived. In most cases, 

 however, the sources of water entering into the 

 circulation are uncertain, and it should be em- 

 phasized that in all cases some of the waters 

 within the basin will be older or younger than 

 the mean age. 



The source waters of the Florida Current 

 are funnelled through the Caribbean Sea. The 

 mass transport is 26 million cubic meters a 

 second (Sverdrup et al., 1942, p. 638), so that 

 this current carries annually a volume of water 

 equivalent to one-sixth of the total volume of 

 the Caribbean. However, there is evidence that 

 the renewal of the deep water of the Caribbean 

 proceeds at a much slower rate than the six 

 year mean age that this ratio implies. Wor- 

 thington (1955) has calculated, on the basis of 

 loss of oxygen from this deep water during the 

 last 30 years, that the age of the deep water 

 in the various parts of the Caribbean may range 

 from 93-142 years. The mean age of the 

 waters above 2000 meters would be reduced to 

 about 5 years if the deepest \ of the volume 

 of the basin is isolated from the present 

 circulation. 



The same current passes through the Yucatan 

 channel into the Gulf of Mexico, before emerg- 

 ing as the Florida Current. No estimate of the 

 mean age of the waters of the Gulf of Mexico 

 is possible, however, since the current data in 

 the Gulf indicate an anticyclonic eddy in the 

 western portion, and suggest that the waters 

 of the Gulf of Mexico are drawn into the 

 Florida Current to only a slight extent (Die- 

 trich, 1939, Sverdrup et al., 1942, p. 642). 



The Black Sea probably contains the most 

 isolated and the oldest deep water to be found 

 anywhere in the oceans. Precipitation and run- 

 off exceed evaporation, and the surface waters 

 are dilute (salinities less than 18 per cent) and 

 isolated from the deep water by an intense 

 density gradient. The deep waters are anaero- 

 bic; hydrogen sulfide reaches large concentra- 

 tions below about 200 meters. The sill at the 

 Bosporus is only 90 meters below the surface 

 so that this deeper water is isolated from the 

 more rapid surface circulation. The inflow of 

 sea water is so small that it would take about 

 2500 years to replace the deep water in the 

 basin (Sverdrup et al., 1942, p. 651). The 

 mean replacement time for the surface layers 



