Isotope hydrology
Isotope hydrology is a field of hydrology that uses isotopic dating to estimate the age and origins of water and of movement within the hydrologic cycle. The techniques are used for water-use policy, mapping aquifers, conserving water supplies, and controlling pollution. It replaces or supplements past methods of measuring rain, river levels and other bodies of water over many decades.
Details
Water molecules carry unique fingerprints, based in part on differing proportions of the oxygen and hydrogen isotopes that constitute all water. Isotopes are forms of the same element that have variable numbers of neutrons in their nuclei.
Air, soil and water contain mostly oxygen 16 ( 16O). Oxygen 18 ( 18O) occurs in approximately one oxygen atom in every five hundred and is a bit heavier than oxygen 16, as it has two extra neutrons. From a simple energy standpoint this results in a preference for evaporating the lighter 16O containing water and leaving more of the 18O water behind in the liquid state (called fractionation). Thus seawater tends to be richer in 18O and rain and snow relatively depleted in 18O.
