How do you determine the age of >60 year old water? 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?
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.