Ground water, or underground water, has always carried an aura of mystery: springs emerging from the ground, water standing in a well, windpumps constantly topping up farm dams, water gushing from irrigation boreholes. Where does it come from? Are there underground lakes? Yet this enigmatic source supplies one fifth of South Africa's fresh water needs and is often the only supply of water in small towns and rural areas. Some European countries obtain practically all their water from underground. Ground water has traditionally been regarded as always fit to drink and inexhaustible.

In recent times these two assumptions are being challenged. Boreholes are drying up due to over-exploitation; ground water is being contaminated by pollution. Our understanding of the behaviour of ground water often falls short of our need to manage this valuable resource. The study of ground water is called geohydrology. It has been described as the study of a system which nature devised, then buried the equipment and threw away the plans. The various techniques employed by geohydrologists in this study are effective but often do not tell the whole story. A more complete story is told by the isotopes of the elements that make up water molecules - hydrogen and oxygen - and some of the substances dissolved in ground water - such as carbon, nitrogen and sulphur - which assist us in understanding the behaviour of ground water.

When rain water falls on earth, processes such as evaporation and condensation slightly alter the concentration of the rarer, and heavier, isotopes of these elements. These minute changes can be measured by sensitive mass spectrometers. Some of this water infiltrates into the soil and eventually percolates down to where all the cracks and pores in the rock are filled with water - the water table. This is the process through which ground water is recharged, or replenished. The slightly altered isotopic composition "labels" the recharge and allows this label to be followed along the ground water flow paths providing information on the sources of the ground water.

Some of the isotopes which label infiltrating water are radioactive - they decay to form other elements by the emission of radiation. The minute traces of radioactivity can be measured by very sensitive nuclear detectors. Once the water has entered the ground, it becomes isolated from the source of the radioactivity. The decay of radioactive isotopes such as of hydrogen and dissolved carbon over decades to thousands of years gives a measure of how long the water has been underground, how fast it is flowing, or how rapidly is it being recharged.

Isotope hydrology, the study of so-called environmental isotopes in water, has in many ways revolutionised our ability to understand the behaviour of ground water. We are now better able to pinpoint areas where replenishment occurs, actual and potential sources of pollution, and can take steps to protect these. We can estimate rates of movement and recharge and thus determine realistic rates at which ground water can be exploited without exhausting the supply. With relatively few isotope measurements - an isotope "snapshot" - we can visualise the behaviour of a ground water body and create a conceptual model which might have taken years of conventional water level and other measurements to achieve. Equally importantly, we can test existing numerical models and provide quantitative data for them. A well-proven model is a powerful tool in the hands of a water resources manager.

Isotope hydrology, once seen as too academic and expensive, is now widely accepted by geohydrologists as appropriate science and technology to enable us to wisely exploit and protect our precious ground water resources.