100 / Basics of Environmental Science
concentrations equalize. When cells are exposed to sea water, its salt concentration is higher than the
concentration inside the cell, and water moves out of the cell. Salt water thus has a dehydrating
effect and its salts must be removed before land-dwelling plants or animals can use it.
This is expensive, and there is another source of fresh water: the polar icecaps. The idea may sound
absurd, but probably it would be technologically and economically feasible to tow large icebergs into
low latitudes, moor them close to the shore, and ‘mine’ them for fresh water. An iceberg would begin
to melt as it entered warm water, but the rate of melting would be low enough to ensure the survival of
the great bulk of the ice and the loss would be acceptable. Clearly, the resource is vast and possibly self-
renewing. There is a major disadvantage, however. Because the communities needing the water are
located far inland, but the iceberg is at the coast, water must still be transported over a long distance.
Combined with the cost of towing, this would probably make the operation prohibitively expensive.
‘Iceberg mining’ has not yet been attempted and neither has a rival scheme, suggested by Walter
Rickel, Governor of Alaska, to construct a submarine pipeline to carry water 3220 km to California
from the headwaters of Alaskan rivers. The scheme was considered, but rejected because of its
estimated $100 billion cost (REINHOLD, 1992).
Desalination (www.ce.vt.edu/enviro2/wtprimer/desalt/desalt/html), on the other hand, is used widely
in the Near and Middle East. It is also used in the United States. For some years the Office of Saline
Water, of the Department of the Interior, has maintained a demonstration desalination plant at Freeport,
Texas, and there is a large plant in Arizona. More recently, water shortages in California led to the
construction of a plant at Catalina yielding 580280 litres of fresh water a day, and plants are also to
be built at Santa Barbara and Morro Bay (REINHOLD, 1992).
The purpose of desalination is the removal of salts from sea water, but not all sea water is equally
saline. Together, temperature and salinity determine the relative densities of different water bodies,
which form water masses analogous to air masses. Plotted on a graph, seawater masses can be identified
by their position along a temperature-salinity (T-S) curve. Salinity is conven-tionally reported in parts
per thousand (per mille). In the centre of the North Atlantic, for example, the T-S curve ranges from
8°C and 35.1 per mille to 19°C and 36.7 per mille; around Antarctica the seawater temperature is 2–
7°C and salinity 34.1–34.6 per mille (HARVEY, 1976, pp. 61–63). Elsewhere, salinity may be markedly
higher or lower. The Mediterranean loses more water by evaporation than it receives from inflowing
rivers and precipitation; it also loses water at depth and gains inflowing water near the surface through
the Straits of Gibraltar. This regime results in a salinity higher than that of the Atlantic, ranging from
about 37.0 per mille near Gibraltar to about 39 per mille at the eastern end. The Black Sea, in contrast,
has an average salinity of about 19.0 per mille, the Caspian 12.86 per mille, and the Red Sea 41.0 per
mille (DAJOZ, 1975, pp. 126–128). Variable though the salinity of sea water is, it remains true that all
sea water is too salty to drink: fresh water has a salinity of less than 0.3 per mille.
Water that is neither fresh nor sea water is known as ‘brackish’ and its salinity is even more variable.
Oligohaline water is only slightly more saline than fresh water, with a salinity of 0.5–5.0 per mille;
mesohaline water has 5.0–16.0 per mille; polyhaline water has 16.0–40.0 per mille; and saline water
has more than 40.0 per mille. Water in the Great Salt Lake has a salinity of 170 per mille and that in
the Dead Sea 230 per mille. Yet all these are ‘brackish’ waters.
Salinity is measured by titrating a sample of water with silver nitrate until all the chloride ions have
been precipitated, and adding potassium chromate, which reacts with silver nitrate when all the
chloride has been precipitated, forming potassium chromate, which is red. The reaction is:
Cl- + AgNO 3 ↔ AgCl↓ + NO 3 -