A hidden resource

When we think of fresh water we probably think of rivers, lakes and perhaps reservoirs. Yet even when we consider the huge lakes of North America or mighty rivers like the Amazon and the Ganges, over 97% of the planet's fresh water is to be found under the surface of the earth in the form of groundwater.

In many parts of the world groundwater is the main source of water for day to day use. Boreholes driven down into the saturated layer under the ground yield water for irrigation and urban use. At least 1,500 million urban dwellers are supplied with water from groundwater reserves. It is also extensively used as a low cost rural water supply.
Groundwater has many benefits. It is cheap to develop because of its naturally good quality and widespread occurrence. It is reliable even in times of drought due to the large amounts stored under the surface. It is also protected against catastrophic events - if natural disasters or war disrupt surface water distribution, then groundwater reserves can easily be developed.

Some areas near the surface may not be saturated with water and only have an intermittent supply of water. But, many areas deeper under the surface are saturated with water accumulated over hundreds or even thousands of years. These areas often supply permanent springs and can be tapped with boreholes to provide water. When we draw groundwater from deep aquifer supplies, we are tapping into water locked away under the surface and filtered through layer upon layer of rock. This is one of the reasons why groundwater is so reliable. The water in these areas may even be protected against recent surface pollution, providing a high quality supply where none was previously available.

A Fragile Resource

Groundwater is vulnerable to pollution. Some of the features of groundwater that make it so useful also make it a fragile resource. The way that groundwater moves under the surface of the earth means that even seemingly well separated waste and fresh water systems can come into contact.

Leaks from rusting storage tanks in a closed down petrol station might eventually enter public water supplies, even if extraction takes place many kilometres away from the original source of pollution. Without accurate hydrogeological information on groundwater movement it is impossible to plan development and extraction in a sustainable and safe way.

It is not just urban areas that contribute to possible pollution. With time, pesticides or fertilisers used in agriculture can enter groundwater systems, and even manure spreading and ploughing can have an impact.

(IAH)