Radioactivity in the Marine Environment (1971) / Chapter Skim
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PHYSICAL PROCESSES OF WATER MOVEMENT AND MIXING
PHYSICAL PROCESSES OF WATER MOVEMENT AND MIXING
Pages 90-136
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From page 90... ...
340 2467 I 340 2 I 4 C 8 I 344 6 6 390 2 4 6 * MO 2 FIGURE 4 Vertical salinity curves for a series of oceanographic stations along a meridional section through the Atlantic (see legend of Figure 3)
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From page 91... ...
, which they proposed as a primary source of deep water in the North Atlantic. Figure 6, for the Pacific, does not indicate any similar source region.
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From page 92... ...
»USHNtLL 70°S eo° 50°40° SO* tO° J0° 50° 60° N 1000 2000 3000 4000 5000 FIGURE 7 Vertical density section along the western trough of the Atlantic, corresponding to temperature-salinity sections of Figure 5.
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From page 94... ...
Observational evidence for deep currents certainly supports the idea that Sverdrup's initial estimate of 4 sv for the North Atlantic rate of supply to the deep water is too small. If this is true, then the compensational flow northward across the equator from the South Atlantic must be proportionately greater than indicated in Figure 9.
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From page 95... ...
, particularly in the North Atlantic. At 2,000 m, a southward flow exists beneath the Gulf Stream and also along the coast of South America to 25°S, and current speeds are generally less than 20 cm/sec.
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From page 96... ...
(Reprinted with permission from Defant, 1961.) support for the southward set of the deep currents along the western boundary of the North Atlantic is provided by the direct observations of Swallow and Worthington (1961)
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From page 97... ...
is as follows: The term in parentheses is a wind stress torque per unit area; the term/w is the vorticity tendency term associated with stretching or shrinking of the water column; and the term on the left is the planetary vorticity tendency term, which is associated with displacement of the water column to a different latitude. If z is taken at the variable elevation of the seabed [z = -D(x,y)
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From page 98... ...
• Currents - - Vertical convection >34 8 S%o <348S%o / Cold water upwelling Velocities in depth:2,4,7 17 cm/sec (Geostroph1c components) Exaggeration of depth ~ I300X FIGURE 1 3 Schematic block-diagram of the surface currents, of salinity distribution, and of the deep sea circulation of the Atlantic Ocean.
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From page 99... ...
A value of about 40 sv is obtained. This upward transport must be balanced by sinking in the deep and intermediate water source regions (in the North Atlantic and the Antarctic region)
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From page 100... ...
Apparently, the reason for this excess overturn rate is that although the model is confined to a basin whose size and surface temperature distribution are similar to those of the North Atlantic, no allowance has been made for interchange with the remaining 8/9 of the world ocean. Accordingly, the estimated residence time in deep water of only about 100 years, implied by the Bryan and Cox model, is probably low by a factor of nine.
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From page 101... ...
We shall discuss the two approaches separately. RADIALLY SYMMETRICAL SOLUTIONS OF HORIZONTAL DIFFUSION DUE TO HORIZONTAL TURBULENCE The following discussion considers primarily the twodimensional horizontal diffusion of a radioactive substance introduced instantaneously at a point in the sea.
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From page 102... ...
. An ever-increasing power of dispersion is characteristic of the horizontal diffusion due to oceanic turbulence.
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From page 103... ...
It goes without saying that the radioactive decay does not affect the variance of the concentration distribution in a patch. The proposed solutions can also be classified into categories on the basis of a characteristic parameter involved in the solution.
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From page 104... ...
fl c o 777 J J s rt o o o o o 2 XXX X X c O t- CTN -- ( -.
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From page 105... ...
, using a few available observations, presented a "diffusion-velocity spectrum" that indicates a weak dependence of the diffusion velocity on the scale of diffusion. Constancy of each characteristic parameter would be desirable for the purpose of practical application of the proposed solutions to the horizontal diffusion of radioactivity from a source; actually, however, they vary with the intensity and structure of turbulence in the sea under consideration.
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From page 106... ...
From the leading edge toward the tail, the dye was located at successively deeper levels. It is evident that the apparent elongation of the dye patch is a result of the vertical shear in the horizontal mean flow.
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From page 107... ...
However, any gradient of mean velocity combined with turbulent mixing leads to an effective diffusion. Thus, the shear effect (in a broad sense)
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From page 108... ...
A long time after release, a very much elongated patch will line up in the direction of mean flow, and, at the same time, the combined effect of the shears and of the diffusion due to random eddies accelerates, to a great extent, the rate of dispersion of radioactive material around the moving center of mass. For an initial period of diffusion, however, t-i 02 .
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From page 109... ...
This implies that the shear-diffusion model describes more explicitly the physical processes of horizontal diffusion from a source than the radially symmetrical model does. CONTINUOUS RELEASE FROM A FIXED POINT As mentioned earlier, the solution for an instantaneous source is not necessarily fundamental to the construction of the solution for a continuous source.
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From page 110... ...
Plume-dispersion models in widespread use employ instantaneous patches as elementary components. The ideal plume model is assembled by superposition of an infinity of overlapping instantaneous patches, each released from a fixed origin and each translated by the mean velocity, U
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From page 111... ...
represents the frequency function associated with the meandering of the central line of a plume about the x-axis, i.e., the direction of the overall mean velocity, um. Thus far, very few experimental and theoretical results concerning the frequency function appear to have been reported.
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From page 112... ...
showed experimentally that the standard deviation, ag, increases linearly with the distance from the source. This implies that the overall mean concentration along the x-axis, without radioactive decay, still decreases inversely with the distance, with an effective diffusion velocity larger than that of the ideal plume.
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From page 113... ...
physical processes in the sea, such as diffusion and advection. We may hope that as tracer measurement techniques improve, our understanding of the worldwide distributions of these radioactive tracers will also improve, thus permitting us to focus our attention on smaller scale processes.
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From page 114... ...
In the situation represented in Figure 25, the radiocarbon and water molecules in the deep ocean can be exchanged only with molecules in the mixed layer; therefore, if such an ocean could somehow be created, the residence times of the deep ocean water and of the radiocarbon molecules would be identical. If one now ventilates a portion of the deep water by exposure to the atmosphere, such as occurs at high latitudes, the radiocarbon age of the deep water will be reduced, since the radiocarbon molecules in the deep sea can now be exchanged with those of both the mixed water just above and the atmosphere.
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From page 115... ...
For comparison, even though they do not utilize radioactive isotope data, Montgomery's (1959) results for the residence times of subtropical surface water, based on his salinity model, are included.
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From page 116... ...
Pacific and Indian Common Water North Atlantic Deep Water, Antarctic Bottom Water, Pacific and Indian Intermediate Water 1,200 0.6 Bolin and Stommel (1961) Atlantic Antarctic Intermediate Water Antarctic Surface Water, Atlantic Deep Water, Atlantic Surface Water 100-400 0.3-1.0 Broecker (1966)
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From page 117... ...
Evidence from analysis of natural tracers indicates that the mean residence time for the Deep Water in the North Atlantic is at least 300 years, and for the entire North Atlantic Deep Water, including its penetration through the South Atlantic, the residence time has been shown to be 600 to 800 years. Any tracer added to the sur
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From page 118... ...
Thus, radionuclides injected into the source waters of the North Atlantic Intermediate Water within the last decade should be highly diluted at intermediate depths in the middle and low latitudes of the Atlantic. The problem we face in explaining the relatively high 90Sr radioactivity observed by Bowen, therefore, is not that there are no mechanisms that would ultimately bring radionuclides originally injected into the surface layers as fallout to depths between 500 and 2,500 m at middle and low latitudes, but rather that the radioactivity observed appears to be much higher than would be expected from known residence times for the intermediate and deep water.
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From page 119... ...
* •-Ck f 1961 1962 I963 1964 Year 1965 I966 FIGURE 27 Average of surface 90Sr activity in the North Atlantic by 6-month intervals.
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From page 120... ...
have not penetrated in measurable amounts below a depth of 300m. Estimates of Vertical Velocity and Vertical Diffusivity through the Thermocline Based on the Temporal Change in the Vertical Distribution of Carbon-14 Produced by Weapons Tests The use of the distributions of 14C and stable carbon in box models to estimate residence times of the various subdivisions of the oceans, as described earlier in this chapter, involved the use of measurements made prior to large weapons tests and subsequent correction to post-weapons-test values in order to remove the effect of the added flux of bombproduced 14C from the data.
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From page 121... ...
It is evident from the vertical profiles of 14C concentration for 1959, 1965, and 1966 that the added bomb-derived flux across the surface considerably exceeded the flux of natural radiocarbon. It is also evident that the bomb-produced 14C added to the surface layers of the ocean had not penetrated below 300 m in this area of the northeast Pacific as of October 1966.
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From page 122... ...
Since the horizontal motion that must carry the adcending water back to regions of sinking is concentrated most strongly in the upper 200 m, we assume for our immediate purpose that the vertical velocity is constant from a depth of 1,000 m up to a depth of 150 m, and then decreases in magnitude to zero at the surface. The vertical density gradient in this region is approximately constant through the depth range of 160 to 300 m, and the vertical diffusivity might also be expected to be constant through the same depth range.
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From page 123... ...
Consequently, it is to be expected that some radioactive materials from man's peaceful uses of nuclear energy will enter the near-shore and estuarine waters; it is necessary to ensure that man's use of these waters and their products will not be limited by the introduction of these radioactive materials. Knowledge of the physical processes of movement and turbulent diffusion in estuaries has grown significantly in the last 15 or 20 years.
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From page 124... ...
Dividing this accumulation of water derived from land drainage by the annual volume rate of flow from all rivers discharging from the adjacent coast gave a mean residence time for this segment of the continental shelf of about 1! £ years.
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From page 125... ...
The effectiveness of the dilution of the ascending plume is enhanced by discharging the waste stream through a multiport diffuser, designed so that the ascending plumes overlap slightly when they reach the surface. This method produces an elongated volume source in the surface waters, which is then further subjected to the physical processes of movement and dispersion in the estuary.
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From page 126... ...
If the method of introduction produces sufficient initial mechanical dilution to significantly reduce the density difference between the waste stream and the estuarine waters, further dilution by mixing is enhanced. In the surface layers, the diluted effluent will be extended into an elongated horizontal plume by the prevailing tidal currents.
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From page 127... ...
This is particularly true for problems where all three spatial dimensions are important, since proven alternative means of solution are not yet available. SUMMARY In this chapter the physical processes that result in movement and mixing of radioactive materials within the ocean are described.
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From page 128... ...
1963. Deep currents in the North Atlantic Ocean.
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From page 129... ...
1959. Salinity and the residence time of subtropical oceanic surface water, p.
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From page 130... ...
1962. Deep current observations in the western North Atlantic.
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From page 131... ...
Marine environments can be conveniently categorized into two major domains: the coastal ocean and the open ocean. The coastal ocean includes estuaries, lagoons, the water over the continental shelves, and many marginal seas.
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From page 132... ...
Reactive elements generally have relatively short residence times in the oceans, while chemically inert species generally have much longer residence times, assuming a steady-state system in which the amount of an element entering the marine environment is compensated by the transfer of an equivalent amount from seawater to the sediments. The residence time, T, is defined by the relationship T = A/(dA/dt)
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From page 133... ...
Residence Time in Ocean (yr)
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From page 134... ...
Principal Dissolved Species Concentration Dissolved in Residence Stream Waters Time in Ocean (Mg/liter)
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From page 135... ...
The discharge of a river quickly mixes with a volume of seawater several times as large to form a low-salinity surface layer that flows into the coastal ocean, mixing continuously. Such discharges can be identified on the basis of salinity and other parameters.
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From page 136... ...
Although materials are lost to the sediments or to the surface layers of the open ocean, the circulation of the surface and subsurface waters tends to retain some chemical species in the coastal ocean. Dissolved oxygen in the near-bottom waters or in the sediment may be depleted or even completely exhausted, depending on the supply rate of dissolved oxygen relative to the rate of consumption in the decomposition of organic matter formed in the photic zone.
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