Some of the farmland in California's San Joaquin Valley gets only about 5 inches of rain a year. Credit: Richard Thornton/Shutterstock

First, the good news: There is no shortage of groundwater.

Now, the bad: Most of it was deposited so long ago that pumping it is like mining a nonrenewable resource.

Those are the conclusions of a new article in the journal Nature Geoscience that estimates the Earth has 5.4 cubic miles of groundwater – enough to cover the planet’s entire land surface to a depth of nearly 600 feet.

But only a small portion has accumulated over the past 50 to 100 years, meaning most of it is so old – from centuries to millennia – that it isn’t sustainable to keep pulling it out at this rate.

“We’re using our groundwater resources too fast – faster than they’re being renewed,” said Tom Gleeson, a Canadian hydrogeologist who led the study, in the Daily Mail, a British newspaper.

As Reveal reported earlier this year, drought-stricken California is now pumping groundwater to the surface that has been found to be 10,000 to more than 30,000 years old.

Those underground aquifers are already being tapped out in other places in the world. Saudi Arabia didn’t manage its groundwater supply, and one day water just stopped coming up from the ground.

Now, Saudi Arabian companies have set up shop in the Arizona desert, where they use groundwater to grow alfalfa, which is then shipped back home to feed cows for the country’s dairy industry.

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Tom Knudson is a reporter for Reveal, covering the environment. He is the recipient of two Pulitzer Prizes and a 2004 award for global environmental reporting from the International Union for the Conservation of Nature and Reuters. Over the years, he has reported on a wide range of subjects, including the abuse of migrant forest workers in the American West, overfishing in Mexico's Sea of Cortez and the environmental degradation of California's Sierra Nevada mountain range. Knudson is based in Reveal's Emeryville, California, office.