Isotope hydrology

Isotope hydrology uses variations in water’s stable and radioactive isotopes to trace water sources, movement, mixing, and age.

Isotope fractionation: The separation of isotopes caused by differences in their physical or chemical behavior. Evaporation and condensation fractionate water isotopes, creating traceable signatures.
Stable isotope: An isotope that does not undergo radioactive decay. Stable isotopes of hydrogen and oxygen trace water without serving as clocks.
Tritium: A radioactive isotope of hydrogen with a half-life of about 12.3 years. Tritium identifies relatively recent recharge and helps assess groundwater renewal.
Global Meteoric Water Line: An empirical relationship between hydrogen and oxygen isotope ratios in global precipitation. It provides a baseline for recognizing evaporation and other departures in water samples.
Groundwater residence time: The time water spends within an aquifer or groundwater system before discharge or extraction. Tracer ages often represent distributions of residence times rather than one simple age.
Craig–Gordon model: A model describing the isotopic composition of water vapor produced by evaporation from a water surface. It predicts how evaporation alters the isotope signature of surface water.
Hydrogen isotopes: The isotopes of hydrogen, principally protium, deuterium, and radioactive tritium. Their differing abundance in water helps identify sources and transport histories.
Carbon-14 dating: A method estimating age from the radioactive decay of carbon-14 in formerly living material or dissolved carbon. Carbon-14 in groundwater can constrain residence times over millennia, with geochemical corrections.
Local Meteoric Water Line: A regional relationship between hydrogen and oxygen isotope ratios in precipitation. Local precipitation patterns help identify whether sampled water matches regional recharge.
Groundwater age dating: The estimation of the time since groundwater was isolated from the atmosphere or entered an aquifer. Age estimates depend on assumptions about mixing, recharge history, and tracer input.

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