Water is one of the mot commonplace compounds on Earth. Yet, thousands of people die every day because they do not have enough, or because their supply is contaminated. How can this happen on a planet that has an estimated 1400 million cubic kilometers of water? That is 250 000 million liters for every man, woman and child (1 cubic kilometer is 10-12 liters. The problems arise not from shortage of water, but from its unequal distribution.
Rivers, lakes soils and the atmosphere, the obvious sources of freshwater, contain about 200 000 cubic kilometers of water-less than one-fifth of 1 per cent of the world’s total water supply.
There is another important store of water, much of it fresh that is easy to overlook. The rocks of the upper part of the Earth’s crust contain many holes. Some are caverns, but most of them are tiny pores-such as the spaces between grains of sand in sandstone or networks of equally fine cracks. Most pores are filled with water. After the oceans, porous rocks contain the Earth’s largest store of water; one calculation puts the total at more than 50 million cubic kilometers, of which at least 4 million cubic kilometers is fresh water.
Some rocks are more porous than others. More important, the pores in some rocks are either large or join up so that water can flow through them easily. Such rocks are said to be permeable; sandstones and gravels are good examples of permeable rocks. In other rocks, water can hardly flow at all; clay has very small pores, whereas pumice is full of good-sized holes but they rarely link up. These and similar rocks are impermeable. Layers of rock that are porous and permeable enough to store water and let it flow through them easily are called aquifers.
In temperate countries the absence of water in the landscape is often a good clue to the presence of one of these great underground stores. Where there are rocks of low permeability at the surface, such as clays or granites, or altered rocks such as slates, only a little rain soaks into the ground. Most of the rainwater flows straight to streams or rivers. These impermeable rocks from landscapes with rivers that rise quickly, even flooding their banks after heavy rain, but diminish or even dry up after a spell of dry weather.
In areas of permeable rocks, most rain soaks into the ground. It reaches rivers only after passing slowly through an aquifer. Permeable areas usually have only a few streams, but they flow with little variation throughout the year and rarely flood.