From http://www.cbc.ca/news/technology/groundwater-study-1.3318137
The water that supplies aquifers and wells that billions of people rely on around the world is, from a practical perspective, mostly a non-renewable resource that could run out in many places, a new Canadian-led study has found.
While many people may think groundwater is replenished by rain and melting snow the way lakes and rivers are, underground water is actually renewed much more slowly.
In fact, just six per cent of the groundwater around the world is replenished and renewed within a "human lifetime" of 50 years, reports University of Victoria hydrogeologist Tom Gleeson and his collaborators in a new study published in the journal Nature Geoscience today.
That water tends to be mainly found within a few hundred metres of the surface, where it is most vulnerable to being contaminated by pollution or depleted by higher temperatures and reduced rainfall as a result of climate change, the researchers found.
"Groundwater is a super-important resource," Gleeson said in an interview with CBC News. "It's used by more than a third of the world's population every day for their drinking water and it's used by agriculture and industry."
More than a third of the Canadian population relies on groundwater, including the entire population of P.E.I. and some fairly large urban centres such as Kitchener-Waterloo, Cambridge and Guelph in Ontario, Gleeson added.
Because groundwater is so important to billions of people around the world, Gleeson and colleagues at the University of Texas at Austin, the University of Calgary, and the University Gottingen were interested in finding out how much groundwater there is in the world and to get an idea of when it will run out.
Nuclear clues
Scientists had previously made a rough estimate of the amount of groundwater in the world, but no one knew how much is renewable and how quickly it's replenished.
Gleeson and his colleagues came up with a way to figure out what groundwater was less than 50 years old. In the 1960s, during the Cold War, a number of countries were doing above-ground nuclear testing. This introduced a radioactive form of hydrogen, called tritium, into the world's water supply.