Surface mixed sediment layer

Figure 23.4 Surface mixed sediment layer (SMSL) of thickness zmjx above a permanent sediment. The processes are A = particle settling, B = transfer into permanent sediment, C = diffusive exchange, D = resuspension, E = chemical or biochemical degradation.

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. 

Cssc Concentration on solids in surface mixed sediment layer (SMSL) (mol kg 1)... 

In the last step (Part 3), the sedimentary compartment (the surface mixed sediment layer , SMSL) was treated as an independent box (Table 23.7). The steady-state solution of the combined sediment/water system explained another characteristic of the observed concentrations, which, as mentioned above, could not be resolved by the one-box model. As shown in Table 23.8, for both congeners the concentration measured on particles suspended in the lake is larger than on sediment particles. The two-box model explained this difference in terms of the different relative organic carbon content of epilimnetic and sedimentary particles. This model also gave a more realistic value for the response time of the combined lake/sediment system with respect to changes in external loading of PCBs. However, major differences between modeled and observed concentrations remained unexplained. 

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... 

SMWL = surface mixed water layer, SMSL = surface mixed sediment layer, OP = open water (deep water). b Variable does not explicitly appear in Eq. 23-42 (see text). 

Atmosphere-ocean carbon exchange is much controlled by physical processes, including mixing of the surface and deep layers of the ocean across the thermo-cline. Biological processes favor the movement of carbon from the surface layer to deep layers and down to bottom sediments. The biological pump functions as a result of phytoplankton photosynthesis. 

Figure 1 Particle cycling in the surface mixed layer of marine sediments. The processing of the rain of particles to the seafloor results in exchanges of solutes across the sediment-water interface and alteration of the particulate reactants, so that the composition of accumulating sediment is signiflcantly different from that of the particulate rain...

Simulation modeling of the fate of DOC under different conditions emphasises these simple relationships in marine sediments. Figure 14 shows the rates at which DOC is stipulated to be produced by hydrolysis of POM at proximately 6, 36 and 60 mmol/m /day either at the surface of the sediment (TOP), or as a linear gradient from the surface down (LINEAR), or equally to all sediment layers down to 5 cm (MIX). The matrix in Figure 14 represents sediments receiving increasing quantities of POM (arrow down) at increasing frequencies (arrow across). 

In oceanic sediments, macro- (> 1 mm) and microorganisms adapt to the existing environment. They play an important role in the mixing of surface sediment layers. Burrowing by organisms in marine sediments is so common that it is the preservation of depositional structures that requires explanation, not their destruction (Arrhenius, 1952, p. 86). 

The Radon Deficiency Method. This is based on determining the radioactive disequilibrium between Rn and its precursor, Ra [63]. Away from the air-sea and sediment-water interfaces, the two radioisotopes are generally in decay equilibrium. Deficiency of Rn activity in the surface mixed layer is due to loss of the gas to the atmosphere, and from this disequilibrium between nuclides a transfer velocity can be calculated. One requirement of this method is that winds be steady for several days - a condition that often is not met at sea. In addition, the method measures evasion rates only. 

In particular, horizontal advection and horizontal diffusion in the Chesapeake Bay are comparable while vertical difiiision is a fast process that acts over short distances, and a model must account for all three. In this environment, atrazine that is discharged to the surface waters could be horizontally distributed over a distance of 1 km over a period of one week, since the time scale of horizontal advection-difiusion processes is 10 -10 s (approximately 3 hours). As atrazine is distributed horizontally, it also mixes vertically down the water coluitm. With the estimates of verticd diffiisivity for the Bay that are available in the literature, for a depth of 10-20 m the time scale for vertical diffusion processes is on the order of 15 minutes, and can be as short as 3 minutes. The sidfidic vraters are in the sediment porewaters and atrazine needs to be transported to the water-sediment inter ce in order to encounter and react with reduced sulfiir species. The characteristic horizontal and vertical scales that describe the flow in the Bay indicate that it is possible for atrazine to reach the depth of the water-sediment interface before it is horizontally transported out of the system. The subsequent exchange at the water-sediment interface depends on many factors, including half-life of atrazine, the hydraulic residence time of the bottom layer, turbulent processes, and other characteristics of the water column above the sediment layer. Simple box models cannot capture the dynamics necessary to describe these exchanges that ultimately govern the te of atrazine in the Bay. 

In a somewhat analogous fashion, the upper layers of aquatic sediments become contaminated by human activities and natural processes. Direct discharges from municipal, industrial, and agricultural sources deliver chemical contaminants to nearby water bodies where they deposit from the water column onto the sediment surface and eventually become mixed into the upper sediment layers. Once these contaminant sources are reduced or stopped the continued mixing of the sediments reverses the transport chemodynamics and reintroduces the contaminants into the water column. This chapter examines mixing processes on the sediment side of the interface that are capable of mobilizing chemicals in both directions across the sediment-water interface. 

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