Sediment-Water Exchange

Note In Chapter 23 we will further elaborate on the sediment-water exchange flux, especially in Box 23.2. and Table 23.6. 

Note The contribution of particle resuspension followed by desorption to the total sediment-water exchange will be further discussed in Chapter 23 (Box 23.2 and Table 23.6). 

Exchange at the Sediment-Water Interface Box 23.2 Model of Sediment-Water Exchange PCBs in Lake Superior (Part 2)... 

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. 

Table 23.6 Characteristic Quantities of the Sediment-Water Exchange and SMSL Model (Fig. 23.4)...

Specific sediment-water exchange rates sedex — sedex / (yr1) 0.29 0.22... 

In what sense is the 1DV lake model more than one-dimensional, in spite of the fact that the resulting differential equation is one-dimensional Why is this important for describing sediment-water exchange ... 

Illustrative Example 24.3 A Spill of Atrazine in River G Box 24.2 Sediment-Water Exchange in Sandy Sediments... 

To summarize, ksedex can be either derived from an explicit model of sediment-water exchange (for instance, Eq. 6 of Box 23.2, or Eq. 24-29b) or treated as an empirical parameter to be determined from field data. Note that in any case, fcsedexis proportional to/w of the chemical. Strongly sorbing chemicals are mainly removed on settling particles, while for weakly sorbing chemicals diffusion at the sediment surface is more important. The formalism of Eqs. 24-28 and 24-29 is applied in Illustrative Example 24.3. 

Eylers (1994) has described a further mechanism of sediment-water exchange which is especially important for sandy river beds. The bottom of such rivers is often shaped by ripples and dunes which lead to horizontal pressure gradients. As a result, river water is forced through the pore space of the sediments where chemicals are exchanged. The mathematics of this process is summarized in Box 24.2. 

Note All these elimination rates are fairly small. If River G had a sandy bed with ripples, then according to Box 24.2 the sediment-water exchange by pore water flow could become the largest process. 

Explain the difference between diffusive sediment-water exchange and the sediment-water interaction caused by ripples on a sandy river bottom. 

Emerson, S., Jahnke, R., and Heggie, D. (1984) Sediment water exchange in shallow water estuarine sediments. J. Mar. Res. 4, 709-730. 

Jprgensen, B.B., and Boudreau, B.P. (2001) Diagenesis and sediment-water exchange. In The Benthic Boundary Layer (Boudreau, B.P., and Jprgensen, B.B., eds.), pp. 211-244, Oxford University Press, New York. 

Recycling of N and P occurs in the water column and at interfaces between the water and substrata such as profundal sediments. In Lake Calado, regeneration of ammonium and phosphorus is dominated by planktonic heterotrophs less than 53 pm in size (Table 14.7, Lenz et al. 1986, Fisher et al. 1988a, Fisher et al. 1988b, Morrissey and Fisher 1988). Sediment-water exchange is smaller than planktonic processes, but is substantial... 

Vanduyl, F. C., Vanraaphorst, W., and Kop, A. J. (1993). Benthic Bacterial Production and Nutrient Sediment-Water Exchange in Sandy North-Sea Sediments. Mar. Ecol. Prog. Ser. 100, 85—95. vanDuyl, F. C., Duineveld, G. C. A., and Kop, A. J. (1997). Short-term variabUity in pelagic—benthic exchange of phytopigments and their relations to benthic bacterial variables in the North Sea. Aquat. Microb. Ecol. 13, 47—61. 

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