An Introduction to Environmental Chemistry

6.2 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 bicar-

bonate-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 fine-

grained 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

182 Chapter Six

60 °N

30 °

142

376

131

30

17

232 1311

110

31

70

28

113

260

286

2390

945

1798

100

25

59

3000

62

17

150

18

154

66

210

0 °

30 °S

150 ° 120 ° 90 ° 60 ° 30 ° 0 ° 30 ° 60 ° 90 ° 120 ° 150 ° 180 °E

< 50

Sediment yield

(tonnes km–2 yr–1)

50 – 500

> 500

133

67

Fig. 6.1Annual river sediment flux from large drainage basins to the oceans. Numbers in 10^6 tonnes yr-^1 and

arrow size proportional to the numbers. After Milliman and Meade (1983), with permission from the University

of Chicago Press.