How can tritium in water be used to study water circulation?

Tritium is a radioactive isotope of hydrogen that is produced naturally in the upper atmosphere through cosmic-ray-induced nuclear reactions. This tritium eventually enters the water cycle and reaches Earth's surface via rain.1

Because tritium is radioactive and has a half-life of approximately 12.5 years, it undergoes continuous decay when stored or isolated from its source. By analyzing the specific tritium content within a water sample, scientists can determine how long that water has been removed from the atmosphere. This process allows researchers to elucidate the details of water circulation among oceans, the atmosphere, rivers, and lakes.1