Groundwater

The rocks immediately below the Earth's surface hold an estimated 9 million km³ of water - about 40 times as much as in all the rivers, lakes and marshes in the world.

In the ground, water seeps downwards through the so-called zone of aera­tion, in which the pores in the rock are filled with a mixture of water and air.

Below this region groundwater accumulates in the zone of satura­tion, in which the pores are full of water. The boundary between the two zones is the water table; it can be seen as the level at which water stands in wells and boreholes.

The height of the water table varies with the season and with the weather, and as expected is highest following heavy rain. Overall topography is reflected in the conformation of the water table; it rises under hills and slopes down into valleys, where it may reach the surface to form springs.

Porous and Permeable Rocks

The movement of water underground is con­trolled mainly by the type of rock through which it passes.

Rocks with a large amount of space between their grains are described as porous, and porosity is expressed as the percentage of free space (or voids) in the total volume of rock. Sand­stones and limestones, for example, have porosi­ties of up to 15 per cent, but they also allow water to pass through them and are therefore termed permeable.

Some rocks, such as clays, have high porosity but low permeability because their close-packed grains impede the flow of water. A crys­talline rock such as granite, on the other hand, can have low porosity because of its closely interlock­ing grains but have a high permeability because the joints and fissures in the rock are intercon­nected and allow the passage of water.

Aquifers

Any rock that is both porous and permeable con­tains groundwater and is known as an aquifer. Where an aquifer is sandwiched between two impermeable strata, water may be trapped under high hydraulic pressure. If a hole is bored into the aquifer, this pressure forces the water to the sur­face as an artesian well (so-called after Artois in north-eastern France).

Such conditions occur in the Paris Basin, where a chalk aquifer is trapped between two layers of clay. Some of the early fountains in Paris made use of natural hydraulic pressure. But today the enormous demands for water by the population have lowered the water table 100m or more and although the aquifer is re­charged by rain falling on the exposed rim of the basin, it is insufficient to maintain the original groundwater level.

Artesian basins occur on a much larger scale in Australia, where Jurassic sandstones act as an aquifer fed with rainwater from the mountains of the Great Divide. This basin covers an area of 1.5 million km² in Queensland and adjoining states. Tube wells, drilled to a depth of 300m, are lined with a metal casing to prevent collapse and con­tamination of the water. As water is removed the surrounding water table is lowered and grades down towards the well in a zone of depression, which may extend for 10km or more around a deep well.

Springs

Where the surface of the ground intersects the water table, water is discharged as a spring. There are various types.

Valley springs occur along the lower slopes of valleys where the water table reaches the surface. A line of such springs called the fontanili range along the foot of the Italian Alps, where they emerge from beneath the glacial outwash gravels and feed the rivers that drain the fertile Plain of Lombardy.

Stratum springs develop at the junction of an aquifer and an impermeable layer. For example, there is a line of stratum springs at the foot of the Chiltern Hills in southern England where chalk lies on top of a layer of clay. The formation of dry valleys in such chalk country has been accounted for by a lowering of the water table because of the recession of the hills and consequent lowering of the spring line.

Fault-line springs form where water-bearing rocks are brought next to impermeable strata by movement along a fault.

Artesian springs (also known as Artesian aquifers ) occur when the upper surface of an aquifer is breached by faulting or where a synclinal fold traps water in its deep axis. The oases of the Sahara owe their existence to the presence of artesian springs.

Vauclusian springs develop in limestone regions where underground waters emerge from a cave system because of the presence of impermeable rocks beneath the limestone. The classic example of this type of spring occurs in the Fontaine de Vaucluse in Savoy, south-eastern France, where the River Sorgue flows out from below a sheer limestone cliff. The feature has given its name to this kind of spring.

Many areas of recent volcanic activity have thermal springs. Groundwater becomes heated as it passes through hot rocks, as in Iceland and in the Lake Taupo district of New Zealand. In some places the water boils underground and jets of steam are forced high in the air as geysers.