Open water column

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) ... 

Since the open water column is nearly pure water (op =1, rs p = 0), for compounds with small to moderate solid-water distribution ratios (Kd < 10 m3kg ), the above equilibrium partition coefficient can be simplified to ... 

C p is aqueous concentration in the open water column. For a strongly sorbing chemical (Ksc/op of order 103), is of order 10"4, so the apparent diffusivity of the... 

As shown in Illustrative Examples 19.3 and 19.4, often it is not immediately known whether an exchange process is controlled by transport across a boundary layer or by transport in the bulk phase. In Illustrative Example 19.3 we look at the case of resuspension of particles from the polluted sediments of Boston Harbor. We are interested in the question of what fraction of the pollutants sorbed to the particles (such as polychlorinated biphenyls) can diffuse into the open water column while the particles are resuspended due to turbulence produced by tidal currents in the bay. To answer this question we need to assess the possible role of the boundary layer around the particles. 

Now we want to apply the box model approach to a two-box system which consists of a completely mixed water body in contact with a sediment box. Although the sediment column can hardly be visualized as being completely mixed, the concept of a surface mixed sediment layer (SMSL) introduced in the previous section is an approximate view of the sediments as mixed box. In fact, for strongly sorbing chemicals the diffusive penetration into the sediment column is so slow and the storage capacity of the top 1 to 2 cm so large, that the deeper parts of the sediments can be treated as sort of a permanent sink from which no feedback to the SMSL and to the open water column is possible. 

The model (Fig. 23.6) consists of three compartments, (a) the surface mixed water layer (SMWL) or epilimnion, (b) the remaining open water column (OP), and (c) the surface mixed sediment layer (SMSL). SMWL and OP are assumed to be completely mixed their mass balance equations correspond to the expressions derived in Box 23.1, although the different terms are not necessarily linear. The open water column is modeled as a spatially continuous system described by a diffusion/advection/ reaction... 

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 ... 

When calculating the characteristic coefficient, ku, kn, k2[, k22, of Box 23.3 you will find that the response velocity of the sediment reservoir is much smaller than that of the open water column. In order to predict the decrease of both concentrations, Cssc and C°p, you can assume a quasi-steady state between the two concentrations in which the system is controlled by the decrease of the slowly reacting sediment reservoir. 

Mount the upper end piece of the filling tube and immediately connect it to the pump, open the column outlet, and start the pump at a flow rate of 10 ml/min (delivering water or any desired buffer system of low ionic strength). Optionally, the column outlet can be additionally connected to a water jet pump, which has to be operated during the first 2 min of packing. The water... 

Sediment trap studies in the open ocean show that the flux of organic carbon at any depth is directly proportional to the rate of primary productivity in the surface water and inversely proportional to the depth of the water column (Suess, 1980) ... 

In cases where wells have a screened or open-hole interval partially submerged below the water table and a pump is going to be used for both purging and sampling, the pump should be placed at the midpoint between the top of the water table and the bottom of the screen. If different equipment is going to be used for purging and sampling of the well, the pump should be placed at the top of the water column. ... 

Baskaran and Santschi (1993) examined " Th from six shallow Texas estuaries. They found dissolved residence times ranged from 0.08 to 4.9 days and the total residence time ranged from 0.9 and 7.8 days. They found the Th dissolved and total water column residence times were much shorter in the summer. This was attributed to the more energetic particle resuspension rates during the summer sampling. They also observed an inverse relation between distribution coefficients and particle concentrations, implying that kinetic factors control Th distribution. Baskaran et al. (1993) and Baskaran and Santschi (2002) showed that the residence time of colloidal and particulate " Th residence time in the coastal waters are considerably lower (1.4 days) than those in the surface waters in the shelf and open ocean (9.1 days) of the Western Arctic Ocean (Baskaran et al. 2003). Based on the mass concentrations of colloidal and particulate matter, it was concluded that only a small portion of the colloidal " Th actively participates in Arctic Th cycling (Baskaran et al. 2003). 

Figure 2.4 Comparison of a typical separation on a packed and a open tubular column. The seunple (solvent extract of river water) is the same in both cases. (Reproduced with permission from ref. 135. Copyright Ann Arbor Science Publishers).

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