Tritium (3H)

Tritium is a naturally and anthropogenically occuring isotope of hydrogen with a half life of 12.32yr (Lucas and Unterweger, 2000). Naturally, it occurs in low concentrations (1 3H for every 1018 1H atoms) by cosmic rays striking air molecules. The cosmogenic isotope then combines with oxygen to form water. It is then deposited as rain or snow.
Tritium is also produced as a by-product of nuclear explosions, as it is essential in the construction of boosted-fission nuclear weapons. During the 1950's, atmospheric testing of nuclear weapons produced a large spike of tritium concentrations (TU= tritium unit) throughout the world. The diagram is from Ottawa, Canada, and is the longest recorded record of monthly tritium values.

The banning of above-ground nuclear testing in 1963 caused a steady decline in atmospheric tritium, thus showing a "bomb spike" that corresponds with 1963/1964. Precipitation while testing was occuring has inflated tritium values, and still may be an order of magnitude greater than present values. The presence of this high-tritium water indicates that there is a source of water that is of this time period. Tritium is useful for dating waters within the last 50 years, and is one of the most important isotopes being used to study active recharge. On the other hand, water with little to no tritium would indicate that it had been precipitated before nuclear testing commenced, and is quite old, at least older than the 1950s.