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.

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.

Carbon 14 dating is also used as part of isotope hydrology as all natural water contains dissolved carbon dioxide.

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.

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