An Introduction to Environmental Chemistry

These reactions can significantly affect fluxes of riverine material to the oceans

and therefore must be quantified in order to understand global element cycling.

6.2.2 Mixing processes in estuaries

Water flow in estuaries is not unidirectional; it is subject to the reversing flows

of tides. There is therefore no constant relationship between a fixed geographic

point and water properties (e.g. calcium ion (Ca^2 +) concentration). For this

reason, data collected in estuaries are usually compared with salinity (Box 6.1)

rather than location. The underlying assumption is that salinity in an estuary is

simply the result of physical mixing and not of chemical changes. If the estuary

has just one river entering it and no other inputs, the behaviour of any compo-

nent can be assessed by plotting its concentration against salinity.

184 Chapter Six

Electrical double layer

Fixed net

negative charge

(particle surface)

Mobile layer of

cations in solution

Solid Electrolyte solution

Clay

mineral

particle

Fig. 6.2Electrical double layer, comprising a fixed layer of negative charge on the particle

and a mobile ionic layer in solution. The latter is caused because positive ions are attracted to

the particle surface. Note that with increasing distance from the particle surface the solution

approaches electrical neutrality. After Raiswell et al.(1980).