River-lake interface

Seven types of aquatic interfaces, with differing horizontal and vertical gradients in hydrochemical and physical characteristics, can occur in floodplain lakes river-lake interface, direct rain and through-fall—lake interface, upland stream—lake interface, lake-lake connections, epilimnion-hypolimnion interface, lake—sediment porewater interface, and lakeshore—ground-water interface. 

Before discussing in detail any of the fate and transport processes occurring in surface waters, the major characteristics of surface waters must be defined. As illustrated in Fig. 2-1, rivers and streams are relatively long, shallow, narrow water bodies characterized by a pronounced horizontal movement of water in the downstream direction. Often the water flow is sufficiently turbulent to erode the stream channel and carry sediment for considerable distances. Due to this movement of sediment, some river channels are constantly shifting in geometry. Compared with rivers, lakes tend to be deeper and wider and are not dominated by a persistent downstream current (Fig. 2-2). Lakes are often vertically stratified for part of the year, with two distinct layers of water whose temperatures and chemistries are markedly different. Estuaries (Fig. 2-3), the interfaces between rivers and the ocean, also are often vertically stratified, due to the denser saline seawater sinking beneath the freshwater discharged from the river. Estuaries have tides due to their connection to the ocean, and they tend to be rich in nutrients. 

The bed surface, in the context of this section, is considered to be nonmobile porous material made of various size solid particles. The particles typically range in diameter from a few millimeters to less than a micron. The various bed types generally reflect the fluid dynamic nature of the water column above the bed. Table 12.7 contains typical porosity values for sediment. Section 12.2.1 contains an overview description of the types of aquatic streams and currents above the beds. These aquatic systems include rivers, lakes, estuaries, shelf, and marine environments. Unlike the air-water interface, the sediment-water interface has a single fluid (i.e aqueous phase) on either side. Water, the continuous phase, exists from within the column above, through the imaginary interface plane and into the porous bed where it is termed porewater. The interface plane is not a sharp one. It can be considered a thin mixed layer of finite thickness in the context of mass-transfer modeling (DiToro, 2001). Visual and physical examination of thin-sliced (0.1mm) layers of a frozen core sample from a lake sediment bed microcosm showed the presence of a finite flocculent layer positioned between the water side and the particles on the bed surface (Formica et al 1988). Little is known about this layer from a mass-transfer perspective, it will not be considered further. Mass transport in those bed surface layers at and below the first layer of solid particles will be the subject of this section. 

Under equiUbrium or near-equiUbrium conditions, the distribution of volatile species between gas and water phases can be described in terms of Henry s law. The rate of transfer of a compound across the water-gas phase boundary can be characterized by a mass-transfer coefficient and the activity gradient at the air—water interface. In addition, these substance-specific coefficients depend on the turbulence, interfacial area, and other conditions of the aquatic systems. They may be related to the exchange constant of oxygen as a reference substance for a system-independent parameter reaeration coefficients are often known for individual rivers and lakes. 

It is common observation that a liquid takes the shape of a container that surrounds or contains it. However, it is also found that, in many cases, there are other subtle properties that arise at the interface of liquids. The most common behavior is bubble and foam formation. Another phenomena is that, when a glass capillary tube is dipped in water, the fluid rises to a given height. It is observed that the narrower the tube, the higher the water rises. The role of liquids and liquid surfaces is important in many everyday natural processes (e.g., oceans, lakes, rivers, raindrops, etc.). Therefore, in these systems, one will expect the surface forces to be important, considering that the oceans cover some 75% of the surface of the earth. Accordingly, there is a need to study surface tension and its effect on surface phenomena in these different systems. This means that the structures of molecules in the bulk phase need to be considered in comparison to those at the surface. 

Table 5.1 shows that, with the boundary conditions present in most environmental flows (i.e., the Earth s surface, ocean top and bottom, river or lake bottom), turbulent flow would be the predominant condition. One exception that is important for interfacial mass transfer would be very close to an interface, such as air-solid, solid-liquid, or air-water interfaces, where the distance from the interface is too small for turbulence to occur. Because turbulence is an important source of mass transfer, the lack of turbulence very near the interface is also significant for mass transfer, where diffusion once again becomes the predominant transport mechanism. This will be discussed further in Chapter 8. 

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. 

The three main processes by which atmospheric deposition of pesticides to the lakes occurs are wet deposition (rain and snow), dry deposition (particulates), and air-water gas exchange. For many of the banned pesticides, gas exchange across the air-water interface, in particular, is often the dominant deposition process, when compared with precipitation and dry particle exchange [42-44], The physicochemical considerations, as well as descriptions of calculation models for atmospheric deposition to lakes, rivers, and the oceans, have been reviewed extensively [45-47]. 

Metabolic and Energetic Coupling between Terrestrial + Land-Water Interface with Pelagic of Lakes and Rivers... 

This bacterial production occurs in the pore fluids of sediments and in stagnant basins (seas, lakes, rivers and fiords). At the interface between anoxic and oxic waters the H2S can be oxidized. This oxidation is frequently coupled to changes in the redox state of metals (1.2) and non-metals (2). Another major interest in the H-jS system comes from an attempt to understand the authigenic production of sulfide minerals as a result of biological or submarine hydrothermal activity and the transformation and disappearance of these minerals due to oxidation (4). For example, hydrothermally produced H2S can react with iron to form pyrite, the overall reaction given by... 

Gardner et al. [9] recognised that because component separations on size exclusion columns with distilled water are affected by chemical physical interactions as well as component molecular size, distilled water size exclusion chromatography will also fractionate dissolved metal forms. These workers interfaced distilled water size exclusion chromatography with inductively coupled argon plasma detection to fractionate and detect dissolved forms of calcium and magnesium in lake and river waters. 

Considerable interest has arisen in the environmental problems associated with the disposal of solid waste again this interfaces closely with the aquatic environment through leaching of organic compounds from landfills — both as solutes and as particulate material — into watercourses, rivers, and lakes. There has been considerable interest in the bioremediation of sites that have been contaminated with both municipal and industrial solid waste. A chapter on bioremediation is therefore provided for two additional reasons (1) in important respects, bioremediation involves an extension of the principles outlined in Chapters 3, 4, 5 and 6, and (2) it illustrates that many of the principles developed within the aquatic environment that were the subject of previous chapters can be applied with suitable and relatively minor modification to the terrestrial environment. 

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