Physical Resources / 99It is easy to over-dramatize the problems of eutrophication. They are confined to still or slow-moving
waters, which limits their extent. Nevertheless, remediation is often necessary, because the affected
water body represents a valuable resource, and it is always complicated and expensive. Prevention
being better than cure, control of discharges into surface waters, introduced primarily to improve the
quality of river water that is not liable to eutrophication, will nevertheless reduce eutrophication in
lakes fed by the improved rivers. The principal cause of river pollution is identical to that which
produces artificial eutrophication.
24. Salt water, brackish water, and desalination
Water is a scarce resource in many parts of the world. Even in regions where rainfall is usually
adequate, periodic droughts can bring shortages, and restrictions on water use are fairly common in
Britain, despite its generally moist, maritime climate. These restrictions have never been so severe as
to direct serious attention to alternative sources of supply, however, except on some offshore islands,
such as the Isles of Scilly, in the Western Approaches off Land’s End, where a desalination plant has
been proposed.
Since almost all the water on Earth is in the oceans, sea water is the most obvious place to seek
supplies and, after all, nowhere on the Isles of Scilly is more than a mile or so from the sea. The
disadvantage of sea water, of course, is its salt content. Industrial plants located in coastal areas can
use sea water directly for cooling, which is why many British nuclear power plants are located at the
coast, but sea water is useless for agricultural or domestic purposes. The cells of living organisms
are contained within membranes that are partially permeable, allowing water molecules to pass, but
blocking the passage of larger molecules, in a process known as ‘osmosis’. If a partially permeable
membrane separates two solutions of different concentrations, an osmotic pressure will act across
the membrane, forcing water molecules to pass from the weaker to the stronger solution until the
Figure 3.5 Evolution of a lake into dry land, marsh, or bog