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An expedition in 2020 will look for groundwater resources under the seabed around Malta
Photo: Thore Sager / GEOMAR
Fresh water is perhaps the most valuable raw material on earth.
But it is also becoming scarcer in regions that previously always had enough water - for example due to climate change, environmental pollution, population growth and intensive agriculture.
The Mediterranean area, the west coast of North and South America, but also large parts of Australia, Africa and Southwest Asia are affected.
Although 70 percent of the earth's surface is covered by water, about 97 percent of it is salty and therefore inedible.
Geoscientists have known for a long time that there are huge reserves of freshwater under the ocean floor.
An international team of researchers under German management estimates the total volume of these fresh water reserves in the journal Reviews of Geophysics to be roughly one million cubic kilometers.
That is roughly twice the volume of the Black Sea.
Fresh water under the sea floor was first detected in the early 1960s off the coast of Florida.
In the decades that followed, researchers came across such reservoirs again and again when searching for oil and gas deposits.
For the current overview, scientists from six countries created the first global inventory of offshore groundwater resources from around 300 geological records.
Researchers from the Geomar Helmholtz Center for Ocean Research Kiel and the University of Malta were in charge.
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The map gives an overview of offshore groundwater resources around the world, where the thickness of the water-bearing layer is known
Photo: Micallef / Christoph Kersten / GEOMAR
The deposits are mainly located in areas up to 55 kilometers from the respective coasts and up to a water depth of 100 meters.
They were mainly formed in the last 2.5 million years in phases with particularly low sea levels - i.e. during the ice ages.
Back then, after rainfall, the water seeped into the ground and formed groundwater deposits in many places over the millennia.
These were flooded after the sea level rose and were partially preserved.
Such reserves are not a local phenomenon, says first author Aaron Micallef of Geomar: "It has been documented off most continental margins around the globe."
“Many of these reservoirs are connected to groundwater layers on land and are refilled from there,” says co-author Marion Jegen from Geomar.
"Others are cut off from the land and are gradually becoming too salty."
Together with other researchers, the geophysicist probed the seabed off the coast of Malta for layers of rock bearing the groundwater last year.
The team not only examined the permeability of the subsoil, but also the salt content with the help of electromagnetic waves.
On the basis of boreholes, conclusions can also be drawn about the age of the enclosed water.
"Ultimately, we try to understand the dynamics," says Jegen.
In the current study, the authors emphasize that 60 percent of humanity live in regions with strained water resources.
"Access to clean water is the basis of sustainable socio-economic development and has been included in the sustainability goals of the United Nations," it continues.
Such supplies are currently of particular interest for the coastal metropolises Cape Town in South Africa and Melbourne and Perth in Australia, writes the team.
But interest is also growing in other regions around the world.
"Offshore groundwater can be used if the costs of researching, pumping out and treating the water are lower than the costs of seawater desalination," the authors write.
So far, however, the technological requirements for this have hardly been defined.
These depend, among other things, on the depth of the reservoirs and their distance from the coast, as well as on the quality of the water they contain.
However, one has to assess the environmental impact before exploiting such deposits, the authors write.
Geophysicist Jegen also emphasizes this: "Whether you can and should use these reservoirs is an open question."
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joe / dpa