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Study is first to map Earth's hidden groundwater

Geography's Scott Jasechko helps distinguish old versus modern groundwater

Scott Jasechko, an assistant professor with the geography department at the University of Calgary, was part of an international team looking at groundwater around the world. Photo courtesy of Scott Jasechko 

November 19, 2015

Groundwater is one of the planet’s most exploited, most precious natural resources. It ranges in age from months to millions of years old. Around the world, there’s increasing demand to know how much we have and how long before it’s tapped out.

For the first time since a back-of-the-envelope calculation of the global volume of groundwater was attempted in the 1970s, an international group of hydrologists has produced the first data-driven estimate of the Earth’s total supply of groundwater.

The study, led by University of Victoria professor Tom Gleeson, along with co-authors Scott Jasechko, hydrologist with the University of Calgary’s Department of Geography, and researchers from the University of Texas at Austin, McGill University and the University of Göttingen, was published this week in Nature Geoscience.

A key part of the study is the “modern” groundwater findings. The report shows that less than six per cent of groundwater in the upper two kilometres of the Earth’s landmass is renewable within a human lifetime.

“This has never been known before,” says Gleeson. “We already know that water levels in lots of aquifers are dropping. We’re using our groundwater resources too fast — faster than they’re being renewed.” 

With the growing global demand for water, especially in light of climate change, this study provides important information to water managers and policy developers as well as scientists to better manage groundwater resources in a sustainable way, he says.

Groundwater is essential to our modern civilization, notes Jasechko. About two billion people rely on it for drinking and it’s used in about 40 per cent of irrigation for food production.

Using multiple datasets (including data from close to a million watersheds), and more than 40,000 groundwater models, the study estimates a total volume of nearly 23 million cubic kilometres of total groundwater of which 0.35 million cubic kilometres is younger than 50 years old.

Jasechko puts those numbers into context. “If you pumped all the groundwater in the world into a pool on top of the land surface, that pool would be about 180 meters above the land — nearly as tall as the Calgary Tower.” And yet, while the volume of groundwater may be immense, Jasechko notes, “only a small, finite fraction of global groundwater is replenished within a human lifetime.” 

It is crucial to differentiate old from modern groundwater, says Gleeson. Young and old groundwater are fundamentally different in how they interact with the rest of the water and climate cycles. Old groundwater is found deeper and is often used as a water resource for agriculture and industry. Sometimes, it contains arsenic or uranium and is often more salty than ocean water. In some areas, the briny water is so old, isolated and stagnant, it should be thought of as non-renewable.

The volume of modern groundwater dwarfs all other components of the active water cycle and is a more renewable resource but, because it’s closer to surface water and is faster moving than old groundwater, it’s also more vulnerable to climate change and contamination by human activities.

Jasechko’s contribution to the study was crucial as he compiled thousands of groundwater tritium measurements. Tritium is a type of hydrogen that is found in groundwater samples that first flowed under the ground less than 50 years ago.

“By analyzing tritium in groundwater from around the world, I was able to distinguish modern versus older groundwater,” says Jasechko. “I created a 3D map of groundwater tritium which helped us identify where pools of young groundwater exist.”

The next step in painting a full picture of how quickly we’re depleting both old and modern groundwater is to analyze volumes of groundwater in relation to how much is being used and depleted.

“Since we now know how much groundwater is being depleted and how much there is, we will be able to estimate how long until we run out,” says Gleeson. To do this, he will be leading a further study using a global-scale model.