Estuarine processes

There are many differences between the chemistry of continental surface waters and seawater. In particular, seawater has a much higher ionic strength than most continental water (see Fig. 5.3) and seawater has a huge concentration of sodium and chloride ions (Na+ and Cl-) (Table 6.1), in contrast with calcium bicarbonate-dominated continental waters (see Section 5.3). Seawater is a high-concentration chemical solution, such that mixing only 1% (volume) of seawater with average riverwater produces a solution in which the ratio of most ions, one to another, is almost the same as in seawater. Thus, the chemical gradients in estuaries are very steep and localized to the earliest stages of mixing. In addition to the steep gradient in ionic strength, in some estuaries there is also a gradient in pH.

Unidirectional flow in rivers is replaced by tidal (reversing) flows in estuaries. At high and low tide, water velocity drops to zero, allowing up to 95% of finegrained suspended sediment (mainly clay minerals and organic matter) to sink and deposit. The efficiency of estuaries as sediment traps has probably varied over quite short geological timescales. For example, over the last 11 000 years as sealevel has risen following the last glaciation, estuaries seem to have been filling with sediment reworked from continental shelves. We might regard estuaries as

Table 6.1 Major ion composition of freshwater and seawater in mmoll Global average riverwater data from Berner and Berner (1987); seawater data from Broecker and Peng (1982).



























temporary features on a geological timescale, but this does not reduce their importance as traps for riverine particulate matter today.

6.2.1 Aggregation of colloidal material in estuaries

In estuarine water the steep gradient in ionic strength destabilizes colloidal material (i.e. a suspension of very fine-grained (1 nm to 1 mm) material), causing it to stick together (flocculate) and sink to the bed. We can better understand this by considering clay minerals, the most abundant inorganic colloids in estuarine waters. Clay minerals have a surface negative charge (see Section 4.5) that is partly balanced by adsorbed cations. If surface charges are not neutralized by ion adsorption, clay minerals tend to remain in suspension, since like charges repel. These forces of repulsion are strong relative to the van der Waals' attractive forces (see Box 4.7) and prevent particles from aggregating and sinking. It follows that anything which neutralizes surface charges will allow particles to flocculate. Many colloids flocculate in an electrolyte, and seawater—a much stronger electrolyte than riverwater—fulfils this role in estuaries. The cations in seawater are attracted to the negative charges on clay surfaces. The cations form a mobile layer in solution adjacent to the clay surface (Fig. 6.2) and the combined 'electrical double layer' is close to being electrically neutral. Adjacent particles can then approach each other and aggregate. In nature, this simple explanation is vastly complicated by the presence of organic and oxyhydroxide coatings on particles.

Sedimentation in estuaries is localized to the low-salinity region by the physical and chemical effects discussed above. The sediment is, however, continuously resuspended by tidal currents, moving upstream on incoming tides and downstream on the ebb. The net effect is to produce a region of high concentration of suspended particulate matter, known as the turbidity maximum. The turbidity maximum is an important region because many reactions in environmental chemistry involve exchange of species between dissolved and particulate phases. Clearly, these reactions occur most where particle concentrations are high and decrease as particle concentrations decline away from the turbidity maximum.

Solid Electrolyte solution

Clay mineral particle

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