Sediment/water boundary

The corresponding dynamic equations of the open water column are constructed from Eq. 22-6. They are completed by the sediment-water boundary flux derived in Eq. 23-38. We assume that the net vertical advection of water is zero. Thus, the vertical water movement is incorporated in the turbulent diffusivity, Ez. The assumption implies that if chemicals are directly introduced at depth z (term 1), they would not be accompanied by significant quantities of water. Typically, such inputs are due to sewage outlets (treated or untreated) into the lake. We get ... 

New production the NPP that is supported by nutrients from outside the estuary proper (e.g., river inputs) and production supported by nutrients regenerated from within the estuary (e.g., nutrient fluxes across the sediment/water boundary from remineralized organic matter in sediments), respectively. 

In addition, a fluffy layer sample of the 4 cm above the sediment/water boundary as well as a corresponding water sample representing the water phase directly above the sediments were obtained. Furthermore, the pore water of the first two sediment layers was separated and analysed. 

The role of the benthic interface is not to be defined exclusively by one transport direction. On the one hand, deposition and burial of marine sediments remove elements from the marine cycles over geological time periods. Yet, as the example of the carbon cycle shown above already demonstrated, an immense proportion of accumulated particles are subject to dissolution or microbial decomposition in the course of early diagenesis (cf. Chapter 9). Marine sediments therefore also act as a secondary source of remineralized dissolved components. The coexistence of these two fundamental, but oppositely directed mass movements constitutes one of the most essential phenomena at the seafloor. Next to studies on particle fluxes through the water column and element-specific accumulation rates, the quantification of benthic flux rates across the sediment/ water boundary represents the third pre-condition for obtaining a complete balance of the marine material cycles. 

Fig. 12.4 Effects of the depth resolution in pore water concentration profiles on calculating the rates of diffusive transport. Three samples drawn from surface sediments are shown to possess different resolutions (intervals 0.5 cm - dots, 1.0 cm diamonds, 2.0 cm - squares). All values are sufficient to plot the idealized concentration profile within the hounds of analytical error, yet very different flux rates are calculated in dependence on the depth resolution values. In the demonstrated example, the smallest sample distance indicates the highest diffusion (2.98 mmol cmA f ). As soon as the vertical distance between single values increases, or, when the sediment segments under study grows in thickness, the calculated export across the sediment-water boundary diminishes (2.34-t.64mmol cm yr ). In our example, this error which is due to the coarse depth resolution can be reduced by applying a mathematical Fit-function. A truncation of 0.05 cm yields a flux rate of 2.84 mmol cm yr. (The indicated values were calculated under the assumption of the presented porosity profile according to Pick s first law of diffusion - see Chapter 3. A diffusion coefficient of 1 cmA f was assumed. Adaptation to the resolution interval of 2.0 cm was accomplished by using a simple exponential equation).

Estuaries exhibit physical and chemical characteristics that are distinct from oceans or lakes. In estuaries, water renewal times are rapid (10 to 10 years compared to 1 to 10 years for lakes and 10 years for oceans), redox and salinity gradients are often transient, and diurnal variations in nutrient concentrations can be significant. The biological productivity of estuaries is high and this, coupled with accumulation of organic debris within estuary boundaries, often produces anoxic conditions at the sediment-water interface. Thus, in contrast to the relatively constant chemical composition of the... 

At the oxic-anoxic boundaries a rapid turnover of iron takes place. This oxic-anoxic boundary may occur in deeper layers of the water column of fresh and marine waters, at the sediment-water interface or within the sediments. 

Diffusive boundary iayer at the sediment-water interface measured with oxygen microelectrodes in Danish coastai sediments in a totai water depth of 15m. Source-. After Gundersen, J. K., and B. B. Jorgensen (1990). Nature 345, 604-607. 

Suboxic Diagenesis Metals remobilized from reducing sediments. Upward diffusive transport through pore waters supplies metals to nodule bottoms. Accretion is episodic, occurring only when the depth of the redox boundary rises close to the sediment-water interface. 100-200 Todorokite/Birnessite (low Cu and Ni content) 48% 20-70 60-200... 

Finally, nodules may not be growing under oxic conditions. The nodules found in oxic sediments may have formed at some earlier time when the redox boundary was closer to the sediment-water interfece. Changes in the position of the redox boundary are a consequence of changes in the flux of POM and bottom-water O2 concentrations. 

In coastal sediments where organic carbon concentrations are high, the redox boundary is at or near the sediment-water interfece. Under these conditions, denitrification acts as a sink for nitrate. In some settings, the rate of sedimentary denitrification is fast enough to drive a diffusive flux of nitrate from the bottom waters into the sediments. Remineralization of organic matter imder suboxic and anoxic conditions releases... 

In Equation (2.37), the first term on the right-hand side accounts for diffusion in the vertical direction the second term accounts for radial diffusion across the cylinder. The following boundary conditions apply. At the sediment-water and sediment-burrow interfaces, the concentrations are the same as in the overlying water ... 

There are many transport conditions where experiments are needed to determine coefficients to be used in the solution. Examples are an air-water transfer coefficient, a sediment-water transfer coefficient, and an eddy diffusion coefficient. These coefficients are usually specific to the type of boundary conditions and are determined from empirical characterization relations. These relations, in turn, are based on experimental data. 

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. 

A different situation is encountered at the bottom of a water body. The sediment-water interface is characterized by, on one side, a water column which is mostly turbulent (although usually less intensive than at the water surface), and, on the other side, by the pore space of the sediment column in which transport occurs by molecular diffusion. Thus, the turbulent water body meets a wall into which transport is slow, hence the term wall boundary (Fig. 19.3b). A wall boundary is like a one-sided bottleneck boundary, that is, like a freeway leading into a narrow winding road. 

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-... 

Figure 19.9 Schematic representation of the concentration profile of a compound across a wall boundary with phase change. For the case of the sediment-water interface, CA is the total (dissolved and sorbed) concentration of a chemical in the sediment column (sc) whereas C° represents the constant concentration in the overlying open water (op).

Table 19.1 Exchange of a Sorbing Chemical at the Sediment-Water Interface (Numerical example for penetration depth due to wall boundary flux).

The boundary condition at the sediment-water interface which relates the diffusion equations on both sides of the boundary, is given by ... 

To summarize for a sorbing chemical, mass exchange at the sediment-water interface can be treated like the exchange at a wall boundary with phase change. Sorption increases the specific and integrated mass exchange by the factor (l// )l/2 that is, it increases the capacity of the sediment to store the compound. At the same time, it slows down the speed at which the chemical penetrates the sediments (factor Note that the derived equations keep... 

These results indicate that the PCBs will be water-boundary layer limited in their release from the sediment bed for years after a step-function change in the overlying water column concentrations. 

Until now we have treated wall boundaries with constant concentration in the mixed system (system B in Fig. 19.8). Such situations are rare in nature. For instance, at the sediment-water interface of a lake the concentration of a chemical in the overlying water column is hardly constant during a period of several years. So we should find... 

The external processes (boundary fluxes) can be combined into four pairs of generalized exchange fluxes that is (a) input/output by streams, rivers, or ground-water, (b) air-water exchange, (c) sediment-water exchange, (d) exchange with adjacent water compartments. If the box represents a pond or lake as a whole, flux (d) does not exist. The fluxes into the system are controlled by external parameters such as the concentration in the inlets, the atmospheric and the sedimentaiy concentrations. These concentrations can be constant or variable with time. 

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