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.

The researchers figured that groundwater with high levels of tritium was renewed since the 1960s. Groundwater with negligible levels was older.

By looking at 3,500 measurements of tritium in groundwater from 55 countries and using computer models to trace the flow of groundwater around the world, they were able to estimate how much groundwater was young and renewable and how much was older.

They also confirmed the total quantity of groundwater around the world using a variety of data like the permeability of rock to the flow of water and how much water could be stored in different places, based on how porous the rock there was.

A look at previous estimates of total groundwater showed the crude calculations were not far off.

"When we actually went back and traced what the actual calculation, it was literally two lines of text that someone could do at a bar," Gleeson said. "But the amazing thing was that they were right."

His team came up with almost exactly the same number.

Plentiful but finite

They estimated that the total amount of groundwater in the world was 22.6 million cubic kilometres — enough to cover all the land on Earth to a depth of 180 metres. The amount that was renewable was no more than 1.3 million cubic kilometres or less than six per cent. But the researchers said that was likely an overestimate due to the types of rock in the areas where most of the measurements were taken. Correcting for that suggested that the actual amount of groundwater renewable within 50 years was likely only 0.35 million cubic kilometres, or enough to cover all the land on Earth to a depth of three metres.

The good news is that the amount of renewable groundwater on Earth is quite large —- three times larger than all other fresh water contained in lakes and rivers on Earth, the researchers reported.

But it isn't evenly distributed. There was less groundwater, especially younger groundwater, in more arid regions.

Gleeson said in places like California and the U.S. Midwest, people are already using "non-renewable" water that is thousands of years old and in places such as Egypt, they're tapping into water that may have last been renewed a million years ago. Such old water isn't just non-renewable on human timescales — it tends to be saltier and more contaminated than younger groundwater.

In addition, overusing groundwater, either old or young, can lower subsurface water levels and dry up streams, which could have a huge effect on ecosystems on the surface, Gleeson added.

He hopes the study will help remind and motivate people to manage their groundwater resources better. "And realize that it's finite and a limited resource that we need to respect and manage properly."