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I have been looking around at this topic recently, and it seems that it is relatively easy to determine the age of young water, by testing the concentration of anthropogenic contaminants. However, I've also heard about scientists determining the age of water as being >60 years old.

How exactly is this done, and how accurate is it?

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    $\begingroup$ 1. Where did you hear about this? 2. What do you mean by "age" of water? Age of what? Formation of H2O molecules? Existence of certain water body on the surface of the Earth? $\endgroup$ – Gimelist Nov 26 '16 at 11:49
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According to this pdf: Physikalische Grundlagen der Klimaarchive Eis und Grundwasser (mostly German - some English) all methods to determine the age of groundwater bodies older than ~60 years are radiometric:

39Ar: 30 yr - 2000 yr
14C: 1,000 yr - 30,000 yr
36Cl: 100,000 yr - 1,000,000 yr
81Kr: 100,000 yr - 1,000,000 yr
40Ar: 100,000 yr - >10,000,000 yr
4He: 10 yr - >10,000,000 yr

Another source (german only) also mentions 32Si with a similar range as 39Ar.

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Klanomath is technically correct regarding natural radionuclides, although in practical terms it is only radiocarbon and 36-Cl that are used to date water. Tritium, from atom bomb tests in the 1950s and 1960s used to be used, but the concentration has now decayed to levels indistinguishable from background. In some parts of the world other radionuclides such as 137-Cs from Chernobyl, can still be traced in the water. In about 40 years from now it will be possible to distinguish between pre- and post-60 year old water on the basis of the dates from which certain pesticides were banned. Similarly, in 50 years time we will be able to distinguish pre- and post- ban of CFCs. Most of the non-radiogenic indicators will be in vanishingly small quantities, typically in parts per trillion, or less, so will require pre-ultra-concentration of large water samples, and will be horribly expensive to analyze.

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The 'age' of groundwater is typically taken to mean the time since the water became isolated from the atmosphere. Tritium is still be used since it is produced in the atmosphere in small amounts and it can be measured by low-level counting or by the ingrowth of its decay product He-3. The half life of tritium is ~ 12.3 years so it is good for young groundwater. I don't know why the article states a large range for He-4. I expect it is a mash-up of He-3 ingrowth from tritium decay and the subsurface production of He-4 from alpha decay of heavy radioactive elements (an alpha particle is a He-4 nucleus). Thus it can be used to estimate age of very old water.

Argon-39 would be very useful in age dating groundwater but there are currently only about 3 labs in the world who can measure it and radioactive counting needs a large volume of water. The atom-trapping method is still in early stages of development. That method will be a big breakthrough for krypton, too.

Chlorine-36 is produced in the atmosphere and at low levels in the subsurface, which complicates its use. It is also produced in nuclear reactors and by atomic bombs.

The methods can be very accurate but you have to understand and account for the different sources and reactions that can affect the measurements.

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