Contamination or not fossil water?
It had been assumed that "fossil" reserves found hundreds of metres underground would be largely untouched by modern water sources ... The new study reveals that about half of the deep groundwater has had contact with rains and snows that fell in the past 60 years.
Scientists will use the term fossil in this context to mean water that last touched the atmosphere more than about 10,000 years ago.
The second radioisotope to be checked was tritium, a heavy form of hydrogen which, in contrast, decays very rapidly. It was put in the atmosphere by A-bomb tests in the 1950s/1960s, so its presence is a sign of water's youth.
'Fossil' groundwater's modern secret
However, half of the wells in our study that are dominated by fossil groundwater also contain detectable levels of tritium, indicating the presence of much younger, decadal-age waters and suggesting that contemporary contaminants may be able to reach deep wells that tap fossil aquifers.
Global aquifers dominated by fossil groundwaters but wells vulnerable to modern contamination
Still fossil water?