Groundwater, the globe's most dependable water insurance system, is not as renewable as researchers once thought and its availability varies greatly around the world.

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A new study published in the science journal Nature GeoScience found that just six per cent of the groundwater in the upper two kilometres of the Earth's crust is actually renewed over a human lifetime.

As a consequence, the vast majority of groundwater now being consumed at a rapid rate by agriculture, human communities and the oil and gas industry took hundreds, thousands or even millions of years to collect in the earth. (Some groundwater in Canada is more than a billion years old.)

''It begs the question of what is renewable in terms of groundwater,'' said hydrologist Tom Gleeson at the University of Victoria and one of the paper's authors. ''When we talk about groundwater, it can be 100, 1,000 or 10,000 years old and it was all at some time precipitation. But it all comes down to timescale.''

Scientists classify groundwater, the water that supplies aquifers and wells, as ''young'' or ''modern'' if it has seeped and pooled in the earth for only 25 to 100 years. It is generally more readily available and of better quality than old or ancient groundwater and is more vulnerable to contamination.

The study, which used extensive computer modelling, mapped the extent of young groundwater around the world by tracking tritium, a radioactive tracer, in thousands of groundwater samples from around the world.

Tritium is a byproduct of atmospheric nuclear testing during the 1950s and 1960s. The element fell to the ground in rainwater and is now a standard measurement for mapping young groundwater.

According to Gleeson and his collaborators, all the world's young groundwater, if pumped and poured over the planet, would make a three-metre-deep pool, roughly the height of a basketball net.

In contrast, the remaining 94 per cent of the world's groundwater, which is largely brackish, if pumped from depths of two kilometres would create a 180-metre flood on Earth, about the height of the Calgary Tower.

The report concluded that ''groundwater replenished over a human lifetime of 25 to 100 years is a finite, limited resource with a spatially heterogeneous distribution dependent on geographic, geologic and hydrologic conditions.''

''The results are a call to better manage and protect the resource,'' said Gleeson. ''They show where groundwater is renewable and where it is most vulnerable to contamination and climate change.'' Not surprisingly, young or modern groundwater is the most susceptible to both.

Canadian monitoring lacks: expert

Groundwater supports a critical part of the Canadian economy.

Approximately 80 per cent of the rural population and 43 per cent of the nation's agricultural productivity depend on groundwater. Groundwater also provides industry, including the water-intensive oil and gas sector, with 14 per cent of its water needs.

Yet federal researchers admit that they know relatively little about groundwater availability, quality and behaviour in Canada.

A 2011 report by the National Round Table on the Environment and the Economy, a federal agency closed by the Harper government, noted that information ''on groundwater supplies is largely absent and is needed because of its link to many of Canada's surface water sources.''

To date, Natural Resources Canada has mapped 19 of the nation's so-called ''key'' 30 aquifers. It doesn't expect to complete its mapping until 2025.

Meanwhile, mining projects such as hydraulic fracturing in northern British Columbia and bitumen mining in northern Albertan could contaminate extensive groundwater supplies with stray gas, salt water, bitumen or other hydrocarbons.

John Cherry, the nation's leading expert on groundwater contamination, has repeatedly warned that provincial governments have failed to set up rigorous groundwater monitoring programs in regions being fracked by the oil and gas industry in Saskatchewan, Alberta and British Columbia.

''From my hydrogeological perspective, I view shale gas development as a big experiment for which we have minimal scientific basis for predicting the outcome or the impacts of stray gas on groundwater quality,'' he said.

Recent studies have shown that the fracking industry can violate aquifers, cause minor earthquakes and aggravate the leakage of stray methane from aging oil and gas wells by rattling existing oil and gas infrastructure.

A recent Stanford study, for example, found that the fracking of oil and gas less than a mile from aquifers or the Earth's surface now takes place across North America with few restrictions, posing increased risk for drinking water supplies.

''We Canadians are leaders in many areas of groundwater science, but we are at the bottom of the ranking of countries that use modern science in or for groundwater mapping and protection,'' Cherry said.

World aquifers stressed: NASA

Earlier this year, NASA scientists reported that the human economy was rapidly mining nearly one-third of the world's largest aquifers even though researchers know little about how much water remains in them.

The NASA study found that 13 of the planet's 37 largest aquifers studied between 2003 and 2013 were being depleted while receiving little to no young water. These aquifers support two billion human beings with drinking water.

The scientists identified the world's most stressed groundwater supply as the Arabian Aquifer System. It provides drinking water for places like Saudi Arabia and more than 60 million people.

Other dangerously stressed aquifers included the Indus Basin aquifer of northwestern India and Pakistan and the Murzuk-Djado Basin in northern Africa.

California's Central Valley, which has been mined heavily by industrial agriculture and is suffering rapid depletion, was labeled ''highly stressed'' as well.

Researchers now say the aquifer that supports much of North America's food production ''lacks sufficient natural recharge to balance current use rates,'' which has been exacerbated by an increased dependence on groundwater during the California drought. (California is the first state to pass sustainable groundwater management legislation, but it doesn't go into effect until 2040.)

''The degree to which a society understands and protects its groundwater is a measure of the society's commitment to taking care of the 'public commons,' or in other words, the commitment to environmental sustainability,'' explained Cherry.

''This is because the problems that show up in groundwater take much longer than an electoral cycle or two. Therefore, if our present society behaves responsibly on this, the primary benefits are for our children and mostly our grandchildren. The Europeans sort of understand this, but we cowboys on this continent find the concept beyond our imagination.''