Lakes and oceans

Water is constantly evaporated from rivers, lakes, and oceans, and released from vegetation through evapo-transpiration. Water vapor travels through the atmosphere, eventually forming small droplets or ice crystals in clouds. Some particles grow sufficiently... 

Molecules am act one another. Fiuni that simple fact spring fundamentally important consequences. Rivers, lakes, and oceans exist because water molecules attract one another and form a liquid. Without that liquid, there would be no life. Without forces between molecules, our flesh would drip off our bones and the oceans would be gas. Less dramatically, the forces between molecules govern the physical properties of bulk matter and help to account for the differences in the substances around us. They explain why carbon dioxide is a gas that we exhale, why wood is a solid that we can stand on, and why ice floats on water. At very close range, molecules also repel one another. When pressed together, molecules resist further compression. 

Cold, wet times in Greenland occurred with very cold, dry, windy conditions in Europe and North America along with very warm weather in the South Atlantic and Antarctica. This weather is indicated by studies of high mountain glaciers, the thickness of tree rings, and the types of pollen and shells found in mud at the bottoms of lakes and oceans. 

In Fig. 1.1 we already encountered a scheme of some of the chemical, biological, and physical processes which regulate the concentration of trace elements in the water column of lakes and oceans. When these trace elements are introduced into a lake by riverine and atmospheric input, they interact... 

Another danger is the waste mercury that has been deposited by industries and agricultural chemicals in the lakes and oceans of the world. Several decades ago most of the nations of the world approved an international ban on dumping mercury into our waterways and oceans. The problem is that smaller ocean plants and animals consume mercury. Larger flsh consume... 

Many important processes in the environment occur at boundaries. Here we use the term boundary in a fairly general manner for surfaces at which properties of a system change extensively or, as in the case of interfaces, even discontinuously. Interface boundaries are characterized by a discontinuity of certain parameters such as density and chemical composition. Examples of interface boundaries are the air-water interface of surface waters (ocean, lakes, rivers), the sediment-water interface in lakes and oceans, the surface of an oil droplet, the surface of an algal cell or a mineral particle suspended in water. 

Lakes and oceans are often vertically stratified. That is, two or more fairly homogeneous water layers are separated by zones of strong concentration and density gradients. In Chapter 21, two- and multibox models will be developed to describe the distribution of chemicals in such systems. In these models, volume fluxes, Qex, are introduced to describe the exchange of water and solutes between adjacent boxes (Fig. 19.5). Qex has the same dimension as, for instance, the discharge of a river, [L3TT ]. The net mass flux, LFnet, from box 1 into box 2 is given by ... 

Wall boundaries are defined by an abrupt change of diffusivity D(x) from a large value allowing virtually complete homogeneity to a value that is orders of magnitude smaller (Fig. 19.3b). Examples are the sediment-water interface in lakes and oceans, a spill of a nonaqueous-phase liquid (NAPL) exposed to air, or the surface of a natural particle suspended in water. In this section we deal with flat wall bound-... 

The structure of turbulence in the transition zone from a fully turbulent fluid to a nonfluid medium (often called the Prandtl layer) has been studied intensively (see, for instance, Williams and Elder, 1989). Well-known examples are the structure of the turbulent wind field above the land surface (known as the planetary boundary layer) or the mixing regime above the sediments of lakes and oceans (benthic boundary layer). The vertical variation of D(x) is schematically shown in Fig. 19.8b. Yet, in most cases it is sufficient to treat the boundary as if D(x) had the shape shown in Fig. 19.8a. 

This corresponds to an annual evaporation rate of 0.25 to 2.5 m yr 1, which is in accordance with observed evaporative loss rates of water from lakes and oceans (Sverdrup et al., 1942 Miller, 1977). 

In the epilimnion/hypolimnion two-box model the vertical concentration profile of a chemical adopts the shape of two zones with constant values separated by a thin zone with an abrupt concentration gradient. Often vertical profiles in lakes and oceans exhibit a smoother and more complex structure (see, e.g., Figs. 19.1a and 19.2). Obviously, the two-box model can be refined by separating the water body into three or more horizontal layers which are connected by vertical exchange rates. 

Radioactive or stable isotopes of noble gases are also used to determine vertical turbulent diffusion in natural water bodies. For instance, the decay of tritium (3H)— either produced by cosmic rays in the atmosphere or introduced into the hydrosphere by anthropogenic sources—causes the natural stable isotope ratio of helium, 3He/ 4He, to increase. Only if water contacts the atmosphere can the helium ratio be set back to its atmospheric equilibrium value. Thus the combined measurement of the 3H-concentration and the 3He/4He ratio yields information on the so-called water age, that is, the time since the analyzed water was last exposed to the atmosphere (Aeschbach-Hertig et al., 1996). The vertical distribution of water age in lakes and oceans allows us to quantify vertical mixing. 

Another procedure is based on the measurement of the radioactive isotope radon-222 (half-life 3.8 days), the decay product of natural radium-226. At the bottom of lakes and oceans, radon diffuses from the sediment to the overlying water where it is transported upward by turbulence. Broecker (1965) was among the first to use the vertical profile of 222Rn in the deep sea to determine vertical turbulent diffusivity in the ocean. 

As pointed out by Peeters et al. (1996), based on their own experiments and on the reinterpretation of published field data, the adequate model to describe horizontal diffusion in lakes and oceans is the shear diffusion model by Carter and Okubo (1965). The model is described in Box 22.4. The most important consequence of this model is that the 4/3 law and the equivalent t3-power law for c2(t) expressed by Eq. 22-42 are replaced by an equation which corresponds to a continuous increase of the exponent m from 1 to 2 (Box 22.4, Eq.l) ... 

As shown by Peeters et al. (1996), horizontal diffusion experiments in lakes and oceans can be best described with the shear-diffusion model of Carter and Okubo (1965). The model yields the following relation between cloud size, ct2, and time, t ... 

Box 23.1 Linear One-Box Model for Well-Mixed Volumes of Ponds, Lakes, and Oceans... 

In Section 23.1, this procedure will be applied to just one completely mixed water body. This control volume may represent the lake as a whole or some part of it (e.g., the mixed surface layer). Section 23.2 deals with the dynamics of particles in lakes and their influence on the behavior of organic chemicals. Particles to which chemicals are sorbed may be suspended in the water column and eventually settle to the lake bottom. In addition, particles already lying at the sediment-water interface may act as source or sink for the dissolved chemical. In Section 23.3, two-box models of lakes are discussed, particularly a model consisting of the water body as one box and the sediment bed as the other. Finally, in Section 23.4, one-dimensional vertical models of lakes and oceans are discussed. 

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