In a new paper, hydrologists suggest at least part of the answer to the Earth's future water woes may be buried underneath the oceans.

Over the past several decades, massive aquifers beneath the seabed have been found off coastlines across the globe, filled with water that ranges from fresh to about two-thirds less saline than seawater.

In a review article recently published online in the journal Nature, researchers arrived at a new estimate for total usable global offshore groundwater: 500,000 cubic kilometers -- a quantity 100 times greater than the amount of water extracted from land aquifers since 1900, the study states.

Not all of this water is cheap or easy to access and turn into potable water, outside experts warn, and pumping this water may have onshore impacts.

But because more than 40 percent of the Earth's population lives about 60 miles from the coastline, the authors write that taking advantage of offshore groundwater could help dampen looming water scarcity problems likely to be compounded by sea-level rise and drought.

"It is a slightly unexpected scale and location for this water, and it's good news in many ways, especially because it's accessible to many of the world's growing coastal cities," said paper co-author Michael Edmunds, a professor at the University of Oxford's School of Geography and the Environment in the United Kingdom.

In the paper, the authors acknowledge that offshore groundwater "is not the answer to global water crises, but it has a strategic value that should be acknowledged so that it can be weighed against other options in long-term strategies."

"It's a kind of quirky resource," Edmunds added. "It's not straightforward, but it's there, and would have to be compared with the costs of desalinated water and building new dams on land."

Connections to land-based systems
Hydrologists think these submarine groundwater reserves started to form tens of thousands of years ago, during periods like the Last Glacial Maximum when much of the Earth's water was bound up in ice and sea levels were especially low. During this time, precipitation and meltwater seeped into the exposed shelf areas and filled water tables, which were then covered up by the ocean as sea levels rose again.

Today, scientists have located fresh offshore aquifers across the globe, from the continental shelf off New Jersey to the Japan Trench.

An earlier estimate of fresh offshore groundwater was reported back in 2010, at 300,000 cubic kilometers. But this paper takes into account new observations recorded from Greenland, Indonesia's Jakarta Bay and Gippsland Basin off Australia's southeastern coast to come up with the larger estimate.

However, this figure is still uncertain due to a paucity of observed data for large areas like the Sundra Shelf of Southeast Asia. "New discoveries in these areas could significantly alter the global volume concentrations," the paper states.

But however vast in supply and global in occurrence, offshore groundwater is not going to singlehandedly slake the globe's growing thirst, outside experts say.

Bill Cunningham, acting chief of the U.S. Geological Survey's Office of Groundwater, said that many of these offshore aquifers are connected to land-based systems. Pumping water offshore could therefore affect onshore groundwater supplies, he said.

"There's one special situation that's described in this review article: a pocket of fresh groundwater beneath the sea," Cunningham said. "That would be one situation where you could, in theory, put a well in there and pump that water out and use it without affecting a groundwater system."

However, he added, "we're really unsure of the nature and extent of those pockets," Cunningham said. "In my mind, it would be a pretty unique situation."

Purer than ocean water
Additionally, much of the water in these aquifers is not immediately usable.

"Almost none of the water that we're talking about would be drinkable right out of the seafloor -- there would have to be processing done," said Ward Sanford, a U.S. Geological Survey hydrologist who reviewed the paper. "It really comes down to an economic question."

Using more pure water from offshore aquifers rather than ocean water has the potential to reduce desalination costs, the authors argue -- with reverse osmosis, desalination costs for ocean water run between 53 cents and $1.50 per cubic meter, while the cost to process brackish water from offshore aquifers runs between 10 cents and $1.

But the additional cost of pumping the water onshore and, ultimately, to the tap, must also be factored in, Sanford said. These costs have so far discouraged exploration of offshore groundwater use.

"Even now, with coastal shortages, it's expensive to drill offshore. When you're drilling for oil, there's an economic driver for that -- so far, for water, there hasn't been," Cunningham said.

There are also environmental factors to contend with.

"With any desalination, your byproduct is either salt or very salty water," Sanford said. "The question with desalination is always what to do with the leftover salt."

Edmunds raised another potential environmental impact: If cities started to take advantage of offshore groundwater, the supply could be exhausted at some point.

"The caution is that these waters, by their definition, are nonrenewable," Edmunds said. "It's quite a fragile resource."

But lead author Vincent Post of the National Centre for Groundwater Research and Training and the School of the Environment at Flinders University in Adelaide, Australia, said in a statement that the potential use of this vast water store must not be ignored in the face of coming water challenges.

"Freshwater on our planet is increasingly under stress and strain so the discovery of significant new stores off the coast is very exciting," Post said. "More options can be considered to help reduce the impact of droughts and continental water shortages."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500