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Isotope Hydrology: Fingerprinting Water

Students learn about water management and the IAEA's role during an exhibit on World Water Day in Vienna.

On Earth water moves in a complex dance between the sea, in the air, and land. This great pulse of water, of transpiration from the leaves of trees, evaporation from ocean and lakes, falling rain, rivers, or groundwater, is known as the hydrological cycle.

At each stage there is a small change registered by a difference in the concentration of oxygen and hydrogen isotopes in water that establishes a fingerprint. Isotopes of pollutants in the environment, such as trace metals, or chemical compounds that are dissolved in water, also provide clues.

Isotopes are atoms of an element that are chemically identical, but physically different. Nuclear science is able to distinguish them using mass spectometry. to "weigh" them. Both hydrogen and oxygen, the elements of water, possess mostly light isotopes. When there is evaporation of water vapour from the ocean their heavier isotopes condense first and fall as rain before the light water.

Most water vapour in the atmosphere is generated over oceans. The further rain falls from the coast, the fewer heavy isotopes it contains. But when water vapour condenses to form rain, heavier isotope laden water falls first, and the further it falls from the ocean over land the less heavy isotopes it will contain.

Mountains and winds make a slight difference, but the process can be measured and scientifically interpreted. By determining the composition of isotopes in a water sample, a record of its passage can be assembled.

This allows hydrologists to navigate groundwater sources and determine their history and the pathways they have followed: whether the reserve is replenished by rain, is at risk from pollution, whether it is young or old water.

For more information, see the Web pages of the IAEA Isotope Hydrology Section.

Last update: 26 Jul 2017

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