First map of Earth’s hidden groundwater reserves shows we’re using them too quickly!

An international team of hydrologists has come up with the best estimate yet for Earth’s total supply of groundwater, saying that nearly 23 million cubic kilometres of groundwater is contained in hidden reserves under the surface of the planet. And while that might sound like a lot, it’s not enough to sustain us if we keep consuming it as fast as we are right now.

The study suggests that less than 6 percent of groundwater in the upper 2 kilometres of the Earth’s landmass is renewable within a human lifetime. That statistic is concerning, not only because the uppermost water is what we can access for drinking, but also because the lengthy renewal cycle is slower than our consumption habits.

“This has never been known before,” said lead researcher Tom Gleeson of the University of Victoria in Canada. “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.”

To come up with their global groundwater map, the researchers compiled multiple datasets, including data from almost a million watersheds and more than 40,000 groundwater models. Of the nearly 23 million cubic kilometres of total groundwater on the planet, approximately 0.35 million cubic kilometres is younger than 50 years old.

The distinction between young and old groundwater is important. Young (or modern) groundwater lies closer to the surface and is more likely to be drinkable. In comparison, older groundwater – which can date as far back as millions of years – lies deeper in Earth’s landmass, and may contain arsenic or uranium. It’s often stagnant and saltier than seawater, and as such, is only usually suitable for agricultural or industrial purposes.

Young groundwater’s proximity to the surface means it’s easier for us to access it and also easier to renew with fresh rainwater – but it’s also more readily exposed to human contamination and more vulnerable to environmental risks like climate change.

The researchers’ map reveals that most of Earth’s groundwater reserves are stored in tropical and mountain regions, including the Amazon Basin, the Congo, Indonesia, and in North and Central America. Arid regions, as one might presume to be the case, don’t have as much water underground.

“Intuitively, we expect drier areas to have less modern groundwater and more humid areas to have more, but before this study, all we had was intuition,“ said one of the team, Kevin Befus, who is now with the United States Geological Survey. ”Now, we have a quantitative estimate that we compared to geochemical observations.”

The researchers hope their findings, published in Nature Geoscience, will help water managers, policy developers, and scientists to better manage Earth’s remaining groundwater in more sustainable ways. In the meantime, Gleeson will be leading a new study, designed to track depletion rates on a global scale.

“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,” he said.

This was originally published by 'Science Alert'