It ranges in age from months to millions of years old. Using data and computer models, an international team of researchers estimated that less than six percent and perhaps as little as one percent of water found close to the Earth's surface is renewable in a human lifetime. "This has never been known before," Tom Gleeson of Canada's University of Victoria and the lead author of the study, said in a statement. "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." 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. 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. 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. According to the study, most groundwater is found in tropical and mountainous regions, with some of the largest deposits in the Amazon Basin, the Congo, Indonesia and along the western borders of North and South America. The least amount of modern groundwater is not surprisingly in more arid regions such as the Sahara. says Dr. Kevin Befus, who conducted the groundwater simulations as part of his doctoral research at the University of Texas and is now a post-doctoral fellow at the United States Geological Survey. "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.
– "We're using our groundwater resources too fast—faster than they're being renewed," Dr. Tom Gleeson says in a University of Victoria press release. Gleeson, along with fellow researchers, published the most accurate map of Earth's groundwater supply to date on Monday in Nature Geoscience. The study found there is about 14.3 million cubic miles of groundwater left on Earth. But that's not as encouraging as it sounds, according to Reuters, because no more than 6% of it is renewable within a human lifetime. "That's never been known before," says Gleeson, who adds that people are going to need to manage that resource better as water demands grow amid ongoing drought. "Groundwater can and should be thought of as a very useful buffer to climate extremes," he says. "It's a valuable and strategic resource for mitigating the extreme impacts of climate." The amount of groundwater less than 50 years old is enough to cover every continent with approximately 10 feet of water, according to the study. But while that young groundwater is more quickly replenished, it's also more easily polluted by people and impacted by climate change, Reuters reports. The rest of Earth's groundwater—which can be millions of years old—is much deeper underground and can be extremely salty and contain harmful elements, according to the press release. The next step is to figure out how dire the situation is. "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," Gleeson says. (But at least we might have more helium left that we thought.)