Pore-water advection

Precht, E., and Huettel, M. (2003). Advective pore-water exchange driven by surface gravity waves and its ecological implications. Limnol. Oceanogr. 48, 1647—1684. 

Ehrenhauss, S., Witte, U., Buhring, S. I., and Huettel, M. (2004). Effect of advective pore water transport on distribution and degradation of diatoms in permeable North Sea sediments. Mar. Ecol. Prog. Ser. 271, 99-111. 

In the following, we will use data of Jorgensen et al. (2004) from the Black Sea as an example to illustrate the differences between the two approaches. Fig. 8.18 shows results from the deep sulfidic part of the Black Sea where macrofauna are unable to live and where bioirrigation is therefore absent (although current-induced advective pore water transport may take place near the sediment surface). Sulfate reduction rates were measured experimentally by the radiotracer method down to 20 cm depth in the sediment (Fig. 8.18 A). 

Figure 7. Sediment eontains derived both seavenged from the water eolumn during particle settling and contained in solid material. Ra produced in the sediments is highly soluble in pore waters and diffuses into the overlying water or is advected across the sediment-water interface by discharging groundwater. Rn is produced within the water column from dissolved Ra and within the underlying sediments.

In seawater, physical processes that transport water can also cause mass fluxes and, hence, are another means by which the salinity of seawater can be conservatively altered. The physical processes responsible for water movement within the ocean are turbulent mixing and water-mass advection. Turbulent mixing has been observed to follow Pick s first law and, hence, is also known as eddy diffusion. The rate at which solutes are transported by turbulent mixing and advection is usually much faster than that of molecular diffusion. Exceptions to this occur in locations where water motion is relatively slow, such as the pore waters of marine sediments. The effects of advection and turbulent mixing on the transport of chemicals are discussed further in Chapter 4. 

As continuing sedimentation increases the depth of a sedimentary layer relative to the seafloor, the overlying pressure increases because of the increased weight of the additional particles. The increased pressure leads to particle compaction if the pore waters can escape upward. Under these conditions, sedimentation generates an upward advective flow of pore water. This flow has the potential to transport solutes. 

Reaction rates of nonconservative chemicals in marine sediments can be estimated from porewater concentration profiles using a mathematical model similar to the onedimensional advection-diffusion model for the water column presented in Section 4.3.4. As with the water column, horizontal concentration gradients are assumed to be negligible as compared to the vertical gradients. In contrast to the water column, solute transport in the pore waters is controlled by molecular diffusion and advection, with the effects of turbulent mixing being negligible. 

The effect of pore-water advection on solute concentrations is given by... 

Sedimentation rates range from centimeters per year in coastal sediments down to millimeters per thousand years on the deep-sea floor. Thus, the effects of molecular diffusion are generally greater than that of advection in shaping pore-water concentration profiles. 

Combining the effects of molecular diffusion and pore-water advection yields a onedimensional advection-diffusion equation for a conservative solute ... 

These solutions to the one-dimensional advection-diffusion model can be used to estimate reaction rate constants Ck) from the pore-water concentrations of S, if and s are known. More sophisticated approaches have been used to define the reaction rate term as the sum of multiple removals and additions whose functionalities are not necessarily first-order. Information on the reaction kinetics is empirically obtained by determining which algorithmic representation of the rate law best fits the vertical depth concentration data. The best-fit rate law can then be used to provide some insight into potential... 

As we saw with the steady-state water-column application of the one-dimensional advection-diffusion-reaction equation (Eq. 4.14), the basic shapes of the vertical concentration profiles can be predicted from the relative rates of the chemical and physical processes. Figure 4.21 provided examples of profiles that exhibit curvatures whose shapes reflected differences in the direction and relative rates of these processes. Some generalized scenarios for sedimentary pore water profiles are presented in Figure 12.7 for the most commonly observed shapes. 

When sediment settles onto the seafloor, a considerable amount of sediment is trapped between the grains. As discussed in Chapter 12.2.2, pelagic sediments can initially have equal parts of pore water mixed with the solids. As burial progresses, compaction causes the upward vertical advection of pore water, thereby reducing the water content of the... 

A conceptual model of sedimentary nitrogen cycling. Dashed arrows represent pore water diffusion and advection. Dotted arrows represent sedimentation. Source-. After Burdige, D.J. (2006). Geochemistry of Marine Sediments. Princeton University Press, p. 453. 

Contaminated bed sediments exist at numerous locations in the United States and around the world. These result mainly from past indiscriminate pollution of our aquatic environments and consist of freshwater and marine bodies including streams, lakes, wetlands, and estuaries. The bed sediments contain many hydrophobic organic compounds and metal ions that in the course of time act as sources of pollutants of the overlying aqueous phase. There are a number of transport pathways by which pollutants are transferred to the aqueous phase from contaminated sediments. One of the lesser known, but potentially important, modes of transport of pollutants from bed sediments is by diffusion and advection of contaminants associated with colloidal-size dissolved macromolecules in pore water. These colloids are measured in the aqueous phase as dissolved organic compounds (DOCs). (These are defined operationally as particles with a diameter smaller than 0.45 micrometer.)... 

Cable, J.E., Martin, J.B., Swarzenski, P.W., Lindenberg, M.K., and Steward, J. (2004) Advection within shallow pore waters of a coastal lagoon, Florida. Ground Water 42, 1011-1020. 

Janssen, F., Faerber, P., Huettel, M., Meyer, V., and Witte, U. (2005a). Pore-water advection and solute fluxes in permeable marine sediments(I) Calibration and performance of the novel benthic chamber system Sandy. Limnol. Oceanogr. 50, 768—778. 

Other models directly couple chemical reaction with mass transport and fluid flow. The UNSATCHEM model (Suarez and Simunek, 1996) describes the chemical evolution of solutes in soils and includes kinetic expressions for a limited number of silicate phases. The model mathematically combines one- and two-dimensional chemical transport with saturated and unsaturated pore-water flow based on optimization of water retention, pressure head, and saturated conductivity. Heat transport is also considered in the model. The IDREAT and GIMRT codes (Steefel and Lasaga, 1994) and Geochemist s Workbench (Bethke, 2001) also contain coupled chemical reaction and fluid transport with input parameters including diffusion, advection, and dispersivity. These models also consider the coupled effects of chemical reaction and changes in porosity and permeability due to mass transport. 

The interpretation of pore-water concentration versus depth profiles of O2 and NO in oxic sediments is based on a one-dimensional, steady-state model in which the production or consumption of a solute in a sedimentary layer is balanced by transport into or out of the layer by solute diffusion and burial advection. In mathematical form. 

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