The global water cycle: what can isotopes tell us?

The radioactive isotope tritium (3H) and the stable isotopes deuterium (2H) and oxygen-18 (18O) are rare components of the water molecule H2O.They offer a broad range of possibilities for studying processes within the water cycle. Tritium was released to the atmosphere during the test phase for hydrogen bombs. The very low natural levels were overwhelmed by concentrations several orders of magnitude higher. Since then, tritium levels have been progressively decreasing due to washout processes and the admixture of moisture from the oceans. Due to the long residence time of ocean water as compared to the half-life of tritium (up to 2,000 years and 12.4 years respectively) the oceans act as a sink for tritium. Tritium in precipitation has been monitored since the beginning of the bomb tests in late 1952. These data enabled the establishment of timescales for the transport of water through different compartments of the hydrological cycle. For example, groundwater, which does not contain tritium, must have infiltrated as rain before 1952 because the natural concentrations have already decayed below the detection limit. If one measures 1,000 tritium units (TU) in glacier ice, this layer must originate from snowfalls in 1963. It was the time, when tritium injections to the atmosphere reached a maximum.

The isotopes of the water molecule, tritium, deuterium and oxygen-18, label the global water cycle
Origin, age, temperature, precipitation, evaporation infiltration... 
an outline of what isotopes can tell us about.

The stable isotopes label in a different way: since a water molecule containing deuterium (2H) or oxygen-18 (18O) is heavier than a normal 1H1H16O molecule, water vapour forming precipitation will be depleted in heavy isotopes relative to ocean water. Condensation forming raindrops from a cloud reverses this process. The heavier molecule condenses first, i.e. the rain is isotopically enriched, and the cloud moisture is subsequently depleted as the rain out continues.
The isotopic fractionation during phase transitions (vapour-liquid-solid) is temperature dependent. A water sample is thus labelled depending on the environmental conditions it has experienced. It is therefore obvious that the illustrated examples (per mil values relate to oxygen-18) vary on a broad scale in space and time. Heavy rains are more depleted than light rains; summer precipitation is less depleted than winter precipitation. Precipitation in Polar Regions is isotopically lighter than in low latitudes, groundwaters originating from infiltration at high altitudes are lighter than those formed in lowlands. Surface waters preferentially loose the lighter water molecules due to evaporation. They are often enriched in heavy isotopes as compared to the isotopic composition of rainwater from which they were formed.

A global network of isotopic input data is necessary to decipher this ongoing tracer experiment in the laboratory of nature.