Not all the water in the sea is seawater.
Scientists think there are vast reserves of fresh groundwater buried under the oceans — a potentially valuable resource for coastal cities that need freshwater.
A recent report in Nature estimates the amount of fresh groundwater around the world at about 120,000 cubic miles — that's 100 times more than all the groundwater that has been pumped up from wells since the 1900s. The reserves are scattered across coastal regions around the world.
Researchers drilled down at various spots and used modeling techniques to calculate how much water there is altogether. The water isn't immediately drinkable, but it's much less salty than seawater and therefore cheaper to desalinate.
The study's lead author, Vincent Post of the National Centre for Groundwater Research and Training in Australia, says scientists knew such freshwater reserves existed but thought they formed only under rare conditions, according to ScienceDaily.
"Our research shows that fresh and brackish aquifers below the seabed are actually quite a common phenomenon," Post tells the science news site. He adds, "Knowing about these reserves is great news because this volume of water could sustain some regions for decades."
Two-thirds of the world's population will be living under water stress conditions by 2025, according to estimates by the U.N. Food and Agriculture Organization. In particular, coastal regions of the U.S., South America, the southern parts of Africa, Europe and Australia could see their water supply drop by 20 percent or more by 2050, according to the United Nations Environmental Program.
This isn't the first time scientists have found fresh groundwater buried in the seafloor, but the study is the first global survey of all the known undersea reserves, says Mark Person, professor of hydrology at the New Mexico Institute of Mining and Technology. He says scientists have made such discoveries around the world — including in coastal regions off Massachusetts, New Jersey, Florida, Indonesia and Tanzania.
"There's just been an explosion of interest in documenting all these instances of freshwater," says Person, one of the study's authors.
So how did all this water get there? Several million years ago, the sea level was a lot lower, so rainwater and runoff from glaciers filled up the water tables in these areas. Over time, sea levels rose and covered up the aquifers, which are sealed in by layers of sediment.
And why is this just coming to light? The depth of these reserves ranges from 650 to 3 miles. Person points out oil companies have to drill much deeper than that to find oil, so their instruments are not turned on at the level of these freshwater reserves.
Post tells ScienceDaily that there are two ways to get to the water: "Build a platform out at sea and drill into the seabed, or drill from the mainland or islands close to the aquifers."
That's not likely to come cheap.
While places such as Cape May, N.J., are already drilling and desalinating freshwater underground for use, getting to freshwater reserves under the oceans will probably be more expensive, says Kenneth Miller, professor of earth and planetary sciences at Rutgers University.
Miller's research has involved drilling into freshwater reserves offshore, and he says drilling three holes about 2,500 feet down cost around $13 million. And some reserves will be saltier – and need more processing — than others, depending on what kinds of sediment surrounds them. Finer grains seal in fresher water while coarser grains hold saltier water, Miller says.
"[Tapping the freshwater reserves] represents a potential alternative that may be economic," says Person, the study co-author. He notes, however, that the scientists have not yet tapped into one of these reserves and that this is a non-renewable resource.
And the study points out that water is relatively cheap now, but these reserves could be important if coastal areas have less water in the future.