Tritium is the only radioactive isotope of hydrogen. While ordinary hydrogen comprises over 99.9% of all naturally occurring hydrogen, tritium only comprises about a billionth of a billionth of natural hydrogen. Most tritium is produced as a result of gases in the upper atmosphere interacting with cosmic radiation. The amount of natural tritium approximately 7.3 kilograms.
About five times this amount remains from past atmospheric nuclear weapons tests.

Tritium is also produced as a fission product in nuclear weapons tests and in nuclear power reactors. Each year about 20,000 curies (2 grams) of tritium is created through nuclear reactions. Tritium is used as a component in nuclear weapons to boost the yield of both fission
and thermonuclear (or fusion) warheads.

Tritium can be taken into the body by drinking water, eating food, or breathing air. It can also be taken in through the skin. Ingested tritium oxide is also almost completely absorbed, moving quickly from the gastrointestinal tract to the bloodstream. Within minutes it is found in varying concentrations in body fluids, organs, and other tissues. Tritium is eliminated from the body with a biological half-life of 10 days. The health hazard of tritium is associated with cell damage caused by the ionizing radiation that results from radioactive decay, with the potential for subsequent cancer induction.

Although Hanford did not produce tritium for defense purposes, it did research on tritium production. Some tritium-producing targets that were bombarded with radiation in Hanford reactors are buried at the site. Tritium was also a byproduct from numerous Hanford plutonium-making processes.

In January 1999 samples were taken from a huge tritium plume stretching from the 200 Area to the Columbia. The results showed a concentration of 1.8 million picocuries per liter. In December of the year, lab results showed the amount had tripled and now were 400 times the federal standard.

Read about tritium at the Department of Energy

Read about Hanford's tritium contamination at the Seattle Times