Sediment columns

Schematic diagram of a piston corer in operation. The weight of the corer is sufficient to cause its penetration into the sediment, while the upward motion of the piston allows water pressure to help force the sediment column into the barrel of the corer.

The size of particles is calculated from the height of fall and time elapsed, according to Stokes law, but allowance must be made for the decrease in height of sedimentation column after each sample is withdrawn. The initial concn in the suspension is calculated from the weight of powder and the volume of liq in which... 

These instruments, sometimes referred to also as sediment accumulation devices, weight the sediment as it accumulates on a weigh-pan at the base of the sedimentation column. The methods are cumulative ones. With the development of sensitive electro balances, the cumulative sedimentation technique is generally easier to perform and more accurate than is the incremental technique. The powder may be dispersed initially in the bulk of the fluid or added instantaneously at the top. An advantage of this type of equipment is the absence of the conical base, needed in sediment extraction devices, upon the walls of which some sediment may adhere. The danger of particles sticking to the vertical walls is however still present... 

Flux of dissolved material and water into sediment, contributing to the growth of the sediment column. 

It is recommended to use a borer of diameter greater than 5 cm to avoid column compaction. The sediment columns are mixed and homogenized and a subsample is taken for laboratory analysis. To investigate the vertical distribution of a chemical for... 

To investigate a vertical distribution of a chemical, a sediment column is divided into sections with appropriate thickness. The sediment column taken in a pipe should be refrigerated in an ice-cooled container, transported to the laboratory, and removed carefully on to a clean tray so that there is as little disturbance as possible to the soil core structure. In the case of a column in which there is little soil moisture and it tends to collapse, the soil should be pushed out to each required thickness and carved off. It is also possible to take a sediment column up to a 30-cm depth using a pipe that is connected to cylinders (5-cm height) with sealing tape. In this case, the sample in each 5-cm fraction can be obtained as it is, after removing the tape. 

Once the radionuclides reach the sediments they are subject to several processes, prime among them being sedimentation, mixing, radioactive decay and production, and chemical diagenesis. This makes the distribution profiles of radionuclides observed in the sediment column a residuum of these multiple processes, rather than a reflection of their delivery pattern to the ocean floor. Therefore, the application of these nuclides as chrono-metric tracers of sedimentary processes requires a knowledge of the processes affecting their distribution and their relationship with time. Mathematical models describing some of these processes and their effects on the radionuclide profiles have been reviewed recently [8,9,10] and hence are not discussed in detail here. However, for the sake of completeness they are presented briefly below. 

Solution of equation (10) which involves sedimentation in the presence of mixing and that of equation (11) which contains the sedimentation term only, are exponential in nature. The major conclusion which arises from this is that the logarithmic nature of the activity-depth profiles by itself is not a guarantee for undisturbed particle by particle sediment accumulation, as has often been assumed. The effects of mixing and sedimentation on the radionuclide distribution in the sediment column have to be resolved to obtain pertinent information on the sediment accumulation rates. (It is pertinent to mention here that recently Guinasso and Schink [65] have developed a detailed mathematical model to calculate the depth profiles of a non-radioactive transient tracer pulse deposited on the sediment surface. Their model is yet to be applied in detail for radionuclides. )... 

However, as far as we know, the distribution of LAS biodegradation intermediates according to depth in the sediment column has been determined only in marine sediments [58]. This study was performed in a saltmarsh channel (Sancti Petri Channel, Cadiz Bay, Spain), receiving untreated urban wastewater effluents. In this zone the benthic organisms are very scarce [59], and the capacity for irrigation of... 

The vertical distribution of LAS in the sediment column has been characterised in various lakes [21,22], where evidence has been found of its degradation in the top 5 cm, but not at greater depths where the conditions usually are anoxic. Amano et al. [23,24] have simulated the temporal variation of the concentration of LAS in the surface layer of sediments and have estimated the flux across the water-sediment... 

The proportion of long-chain LAS homologues is greater in the solid phase (solids in suspension and sediment) than in water and greater than in commercial LAS. For LAS clear vertical trends in distribution can be observed both in the water and sediment columns, with relatively enriched concentrations in the surface microlayer and sediment top layers. 

Transformation of Fe(II,III) at an oxic-anoxic boundary in the water or sediment column (Modified from Davison, 1983)... 

Example 5.3 Find the sedimentation rate from the following data. The excess °Th activity varies with depth of an ocean sediment column as follows ... 

Many investigations have reported the presence of zeolites at the deep ocean bottom (Biscaye, 1965 Heath, 1969 Bonatti, 1963 Sheppard and Gude, 1971 Jacobs, 1970 Morgenstein, 1967 among others). Most of the alkali zeolites are represented except the silica-poor species natrolite and analcite. Rex and Martin (1966) indicate that detrital potassium feldspar is not stable under ocean floor conditions. Zeolites are found in most ocean basins where wind-carried volcanic ash predominates over detrital river-born clay mineral sediments. In these sediments phillipsite is particularly evident and it is known to continue to grow in the sediment column to depths of more than a meter (Bernat, t al.,... 

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. 

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).

Imagine that system B is the water column of a lake and system A is the pore space of the lake sediments. In B, mixing is by turbulence and fairly intensive while in system A transport is by molecular diffusion. The above case corresponds to a situation in which at time t the concentration of a compound in the water suddenly rises to the value Cg. Then Eqs. 19-25 and 19-26 describe the cumulative and incremental mass flux of the compound into the infinitely deep sediment column. 

In Section 19.2 we treated the phase problem by choosing a reference system (for instance, water) to which the concentrations of the chemicals in other phases are related by equilibrium distribution coefficients such as the Henry s law constant. Here we employ the same approach. The following derivation is valid for an arbitrary wall boundary with phase change. The mixed system B is selected as the reference system. In order to exemplify the situation, Fig. 19.9 shows the case in which system A represents a sediment column and system B is the water overlying the sediments. This case will be explicitly discussed in Box 19.1. 

For the special case of the sediment-water interface, DA is determined by the aqueous diffusivity, the sediment structure (porosity, tortuosity, pore size), and the sorption property of the chemical. Let us demonstrate this by applying the theory of transport of sorbing chemicals in fluid-filled porous media, which we have derived in Chapter 18.4 and Box 18.5, to the special case of diffusion in the sediment column. Since for this particular situation the fluid in the pore space is water, the subscript f (fluid) is replaced by w (water) while the superscripts sc and op mean sediment column and open water. 

Note that for the total (dissolved and particulate) concentration, Ct, the abrupt change of the solid-to-water-phase ratio, rsw (Eq. 9-15), at the sediment surface acts like a phase change. The numerical example given in Table 19.1 demonstrates that the transition from the open water column of a lake or the ocean to the sediments involves an increase of rsw by 5 to 6 orders of magnitude. Typically, in the open water, rs p is of order 10 3 kg m-3 while in the sediment column lies between 102 and 103 kg nr3. Thus, at equilibrium the total (dissolved and sorbed) concentration per unit bulk volume on either side of the interface for compounds with small to moderate solid-water distribution ratios (Ki <10 m3kg ) is approximatively given by (see Box 19.1, Eq. 4) ... 

The notation used was introduced in Chapter 9 and in Box. 18.5. Note that compared to the latter the subscript f (fluid) is replaced by w (for water). The superscripts sc and op mean sediment column and open water . 

That is, for the particular case of the sediment column, DA is diffusivity of the total concentration, Ct, for which the second Fick s law is given by Eq. 10 of Box 18.5. From this equation we see that DA adopts the form ... 

A = sediment column, B = open water above sediment). 

Water temperature in a lake fluctuates annually around a mean value. How far into the sediment column do these temperature changes penetrate Use the tools developed for the wall boundary model to make a first estimate and assume that the sediment is mostly pure water. 

Due to the funnel-like shape of the lake basin, only the fraction (AJAa) of the flux is received by the hypolimnion while the rest of the material is added to the sediment column which is directly exposed to the epilimnion. Thus, the corresponding input into the hypolimnion is ... 

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