Chris Elder of Dunedin asks :-

What is old water? I have heard of, or read of, water being thousands of years old. How does it differ from fresh water and how do scientists determine its age?

Uwe Morgenstern, a physicist with the Institute of Geological and Nuclear Sciences in Lower Hutt, responded.

Old water is water that has been underground for a long period of time. This can be years, hundreds of years, or thousands of years.

Deep aquifers often contain water that left the surface thousands of years ago. That can be due to the aquifer being disconnected from its previous discharge source, or because it takes the water thousands of years to reach that point in the deep aquifer. Old water has normally a more evolved chemistry (high ion concentrations), sometimes so high that it is unsuitable for drinking purpose. This come about because water flowing through geological material dissolves some salts from the rocks, which is where the salty ocean comes from, and slowly dissolves some geological material, for example limestone, which is where some underground caves come from.

Tritium dating is the standard dating tool for groundwater in the age range recent to 100 years. Tritium, very heavy hydrogen whose nucleus contains one proton and two neutrons, was discovered by Ernest Rutherford. It is formed in nuclear reactions between cosmic rays and atoms in the upper atmosphere. Being chemically similar to hydrogen, it combines with oxygen to form water molecules and comes to earth in rainwater.

Tritium is radioactive with a half-life of 12.32 years and decays by beta decay to He3, also discovered by Rutherford. As a component of the water molecule, it is the most conservative tracer for groundwater.

When the water infiltrates the ground and becomes isolated from the atmospheric tritium source, its tritium concentration decreases over time due to radioactive decay. The tritium concentration of groundwater therefore reflects the time the water has been underground.

Such measurements are vital to checking healthy water supplies drawn from underground. Our lab is the most accurate in the world at using tritium to date water in aquifers.

When water has been underground for over a year the pathogens die. Hence it is a safety requirement that the fraction of water less than a year old in an aquifer from which drinking water is obtained, must be less than 0.005% of the water present in the aquifer. Tritium dating checks that.