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.
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.
One commonly cited application involves the use of stable isotopes to determine the age of ice or snow, which can help indicate the conditions of the climate in the past. Higher average global temperature would provide more energy and thus an increase in atmospheric 18O water, while lower than normal amounts of 18O in groundwater or an ice layer would imply that the water or ice represents an evaporation origin during cooler climatic eras or even ice ages.
Another application involves the separation of groundwater flow and baseflow from streamflow in the field of catchment hydrology (i.e. a method of hydrograph separation). Since precipitation in each rain or snowfall event has a specific isotopic signature, and the signatures of subsurface water can also be identified by well sampling, the composite signature in the stream is an indicator of, at any given time, what portion of the streamflow comes from overland flow and what portion comes from subsurface flow.
The isotope hydrology program at the International Atomic Energy Agency works to aid developing states (including 84 projects in more than 50 countries) and to create a detailed portrait of Earth's water resources.
- Masters, G. & P. Ela. 2008. Global Atmospheric Change. Chapter in: Introduction to Environmental Engineering and Science. 3rd ed. Prentice Hall.
- Kendall and McDonnell, 1998. Isotope Tracers in Catchment Hydrology. Elsevier