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 ma-
terial (i.e. a suspension of very fine-grained (1 nm to 1mm) 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 adsorp-
tion, 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 any-
thing 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 solu-
tion 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 physi-
cal and chemical effects discussed above. The sediment is, however, continuously
resuspended by tidal currents, moving upstream on incoming tides and down-
stream on the ebb. The net effect is to produce a region of high concentration
of suspended particulate matter, known as the turbidity maximum. The turbid-
ity 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.
The Oceans 183Table 6.1Major ion composition of freshwater and seawater in mmol l-^1. Global average
riverwater data from Berner and Berner (1987); seawater data from Broecker and Peng (1982).
Riverwater Seawater
Na+ 0.23 470
Mg^2 + 0.14 53
K+ 0.03 10
Ca^2 + 0.33 10
HCO 3 - 0.85 2
SO 42 - 0.09 28
Cl- 0.16 550
Si 0.16 0.1