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Tritium (symbol T or 3H) is the radioactive isotope of hydrogen with half-life of 12.32 years and a decay rate of 5.626% per year. It can replace hydrogen in H2-gas, forming HT, and in water, forming HTO (partially tritiated water) or T2O (tritiated water).

Tritium is produced naturally in the upper atmosphere by cosmic radiation. Industrially, tritium is produced in nuclear reactors. The emitted electrons from small amounts of tritium cause phosphors to glow so tritium is used to make self-powered lighting devices that are used in watches and exit signs.

The release of excess tritium into the atmosphere from nuclear weapons tests conducted between 1952 and 1963 ‘tagged’ rainwater, and thereby all surface waters with HTO. This tracer perfectly follows the water in atmospheric, oceanic and hydrological transport and mixing processes. Atmospheric tritium concentrations peaked between 1962 and 1965 and most of this excess (i.e. bomb produced) tritium was precipitated during the same time period and a few years afterward. Since then the deposition rate has tapered off sharply.

Thus the presence of excess tritium in the water of an aquifer unequivocally proves that recharge occurs on a time scale of years to decades. The actual tritium level, combined with approximate local tritium history of precipitation may give more specific information about the make-up of the aquifer.