Tritium/Helium (3H/3He)
Most of the tritium in the world today has been produced by atmospheric nuclear weapons testing, and the
tritium (as part of the water molecule) has mostly entered the ocean . Water in contact with the atmosphere
will have some tritium in it, and this tritium will be decaying to a stable, inert isotope, helium-3.
Being near the atmosphere, this He-3 will escape, so that no excess He-3 can accumulate.
Once this water leaves the surface, the helium-3 can no longer escape and begins to accumulate. By measuring the
accumulated He-3 (referred to as excess He-3 ) and the amount of tritium remaining in the water, we can
determine the elapsed time since the water last was at the surface.
Determination of the 3H/3He ratio can be used to calculate the 3H/3He apparent age of ground water from a single water sample.
As with CFCs, the 3H/3He age is defined as the time elapsed since the parcel of water was isolated from the atmosphere following recharge.
Much of this information was derived from the USGS Information .
