You may not realize this, but nearly a third of the world's freshwater is stored inside the Earth in the form of groundwater -- the H2O that falls to Earth as rain and then soaks into the soil. Some of it clings to soil particles or roots of plants, but the rest of it moves downward through cracks in soil, sand and rocks, until it reaches solid rock that it can't penetrate. Then it fills the space above that barrier, forming an aquifer.
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That water in the Earth, which ranges from months to millions of years in age, is easy for humans to tap with wells, which is why groundwater is a major source of irrigation water for agriculture, and also supplies drinking water to roughly half the U.S. population.
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And unfortunately, we're using it up more rapidly than it can be replaced naturally, a new study published in Nature Geoscience indicates.
The study found that that less than 6 percent of groundwater in the upper 1.2 miles (2 kilometers) of the Earth's landmass is renewable within a human lifetime.
"We already know that water levels in lots of aquifers are dropping," Tom Gleeson of Canada's University of Victoria, one of the study's co-authors, explained in a press release. We're using our groundwater resources too fast - faster than they're being renewed."
The researchers studied data sets and models of groundwater around the world to estimate the total volume of the planet's supply at 23 million cubic kilometers. (A cubic kilometer of water equals about 264 billion gallons.) Most of that water is older water, with only 0.35 million kilometers composed of "modern" water that's younger than 50 years. The modern water tends to be closer to the surface.
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The study found that most modern groundwater is in tropical and mountain regions. Some of the largest deposits are in the Amazon Basin, the Congo, Indonesia and in North and Central America running along the Rockies and the western cordillera to the tip of South America. The least amount of modern groundwater is not surprisingly in more arid regions such as the Sahara.
A 2012 study by Gleeson and other researchers found that 1.7 billion people across the planet live in places where groundwater resources and groundwater-dependent ecosystems are threatened.