Pore water vertical profiles

Fig. 19. Comparison of the one- and two-dimensional models for Mn distribution in the top 0-18 cm of sediment at NWC. The production rate in both cases is that found for core NWC-4. The anoxic precipitation rate is assumed to be zero. The effective cylinder geometry used in the two-dimensional model is that determined for NH4 in Part I r, = 0.14 cm, rj = 4.5 cm. The basal gradient is constrained to be zero. The diffusion geometry created by irrigated burrows results in a vertical pore-water solute profile exhibiting apparent precipitation.

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).

Parker and Lenhard (1989) and Lenhard and Parker (1988) have developed equations that relate the apparent product thickness measured at a well under equilibrium conditions with the product and water saturations in a vertical column of soils adjacent to the well. By integrating the product saturation curve with respect to elevation, an equivalent depth of LNAPL-saturated pores is obtained. This process has been implemented in a computer program called OILEQUIL. The result is reported as a total oil depth in a vertical profile. The water and oil saturation curves with elevation can also be produced and printed in graphical or tabular form. 

The results of concentration measurements are presented as vertical profiles similar to those for the water column, with the vertical axis representing increasing depth below the sediment-water interfece. Depth profiles of concentrations can be used to illustrate downcore variations in the chemical composition of pore waters or in the solid particles. Dissolved concentrations are typically reported in units of moles of solute per liter of pore water. Solid concentrations are reported in mass/mass units, such as grams of carbon per 100 grams of dry sediment (%C) or mg of manganese per kg of dry sediment (ppm Mn). 

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. 

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. 

Numerous measurements of pore-water chemistry have been made in LRL throughout the experiment (4, 17, 59). Typical vertical pore-water profiles (Figure 6) indicate that the sediments are acting as sinks for sulfate... 

Within ocean sediments, the decay of uranium and thorium isotopes leads to the creation of Rn, which is released to sedimentary pore waters and subsequently diffuses into the over-lying seawater. Near the seafloor, excess Rn can be seen against the background of a natural standing stock of this isotope in the water column, which is produced by in situ decay of Ra, a long-lived and relatively uniformly distributed isotope. Because of its short half-life, the existence of this excess isotope some several hundred meters above the seafloor implies a significant flux into the bottom waters, and the shape of the profiles has been modeled as a vertical diffusive balance with radioactive decay of radon and in situ... 

Figure 15.14 gives some pore water profiles from Lake Greifen. The idealized redox sequence leads to a picture of vertically separated processes (see Section 8.6). 

In the unsaturated zone, water movement is caused by both gravity and by pore water pressure differences arising from variations in the water content from one location to another water may even move vertically upward through the soil profile if evaporation or plant roots remove it from the near-surface soil. Water flow is impeded, however, by the fact that water can only move via the relatively thin film of water coating the particles. Such flow contrasts with water flow in the saturated zone, where water can move through the entire pore volume and occupy the full cross-sectional area of the pore spaces. 

Fig. 42. (a) A representative average microenvironment predicted for station NWC illustrating radial NH4 concentration profiles around the idealized central burrow at three depths. The production rate of NH/ is that in Fig. 36, r, = 0.14 and = 4 cm. (b) The average vertical pore-water gradient in 1-cm increments predicted by the microenvironment of (a). See Eqs. (6.22) and (6.26). 

A two-dimensional transport-reaction model incorporating both radial transport into burrows and vertical diffusion is presented. This model is capable of predicting both the form and magnitude of pore-water profiles extraordinarily well at all stations. A one-dimensional model in which an effective transport coefficient is used to account for the influence of reworking and burrow construction on solute movement is far less satisfactory in predicting the observed profiles. 

Fig. 22. Pore-water Fe vertical concentration profiles (continuous line) predicted from the three-zone, one-dimensional model compared with the measured profile (solid bars). Model values are listed in Table VII.

Studies of pore waters have become a standard tool for understanding the biogeochemical processes that influence sediments, and considerable efforts have been invested during the past several decades to develop techniques to collect samples, evaluate whether vertical profiles exhibit artifacts introduced during collection and handling, and develop approaches to model the observed profiles and obtain quantitative estimates of reaction kinetics and stoichiometry. Usually, modeling approaches assume steady-state behavior, but when time-dependent constraints can be established, nonsteady-state approaches can be applied. 

FIGURE 10.29 Vertical profile of pore water Mn(II) and manganese extracted with CDB at the Skagerrak site. (Redrawn from van der Zee and van Raaphorst, 2004.)... 

This is a convenient point at which to introduce the concept of water in the ground, water table, pressure and pressure measurement and some simple hydrostatics - buoyancy, Archimedes - and ideas of surface tension. This then leads to the logical partition of total stress (which is what we discover that we have been thinking about in applying considerations of equilibrium to the vertical profile of stress in the ground) between pore pressure and effective stress supported by the soil particles. I do not think it is necessary to dwell on putative proofs of the Principle of Effective Stress. It can be treated as a moderately well non-falsified conjecture which has demonstrated its worth over many decades. In early year teaching it is helpful to convey certainties even if we expect to encourage students to query them later on. 

In summary, the lanthanides undergo large scale diagenetic reactions under sub-oxic and anoxic conditions. Pore water concentrations increase greatly over those of oxic seawater. Large cerium anomalies develop and large scale fractionation occurs as the strictly-trivalent lanthanides are added to and removed from pore waters. These features develop in vertical profiles within sediments and in surface sediments exposed to seasonally varying redox conditions. 

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