Basics of Environmental Science

Physical Resources / 105

If more water is abstracted from an aquifer than flows in to replenish it, the amount of water available

will gradually diminish. In coastal regions, such over-exploitation of the resource brings an additional

hazard. Beneath the sea bed, the sediment is permanently saturated with salt water. The salt water

moves inland, beneath the freshwater aquifer, with a boundary of brackish water separating the two

water bodies. As the freshwater aquifer is depleted, this boundary moves further inland and closer to

the surface, allowing salt water to penetrate the soil. As Figure 3.8 shows, a point can be reached at

which water abstracted for irrigation starts to become brackish and the more that is abstracted the

saltier it is. Since most crop plants are very intolerant of salt, the effect can be to sterilize the affected

land. It is a problem in many coastal areas, but especially serious in low-lying islands, such as coral

atolls (TOLBA AND EL-KHOLY, 1992, p. 117).

This form of contamination is known as ‘salinization’ (or in the USA as ‘salination’). Salt water

intrusion can occur only in coastal areas, but salinization quite unconnected to the proximity of sea

water affects regions far inland. According to the UN Environmental Programme (UNEP), 7 million

ha is affected in China, 20 million ha in India, 3.2 million ha in Pakistan and the Near East, and 5.2

million ha in the United States. Parts of southern Europe also suffer from this problem (TOLBA

AND EL-KHOLY, 1992, p. 290). It arises because of the way water moves through soil.

Figure 3.7 Mole drainage. Left, mole plough; right, cross-sectional view of

mole drain

Figure 3.8 Salt water intrusion into a freshwater aquifier