ADELAIDE, Australia, December 5, 2013 (ENS) – Huge reservoirs of low-salinity water have been discovered where they are least expected – buried under the seabed on continental shelves around the world.
A study published today in the scientific journal “Nature,” reveals that an estimated half a million cubic kilometres of low-salinity water has been located off Australia, China, North America and South Africa.
“The volume of this water resource is a hundred times greater than the amount we’ve extracted from the Earth’s sub-surface in the past century since 1900,” says lead author Dr. Vincent Post of the National Centre for Groundwater Research and Training and the School of the Environment at Australia’s Flinders University.
“Knowing about these reserves is great news because this volume of water could sustain some regions for decades,” said the groundwater hydrologist.
Groundwater scientists have known of freshwater under the seafloor, but thought it occurred only under rare and special conditions.
“Our research shows that fresh and brackish aquifers below the seabed are actually quite a common phenomenon,” Post said.
These reserves were formed over the past hundreds of thousands of years when on average the sea level was much lower than it is today, and when the coastline was further out, he explained.
“So when it rained, the water would infiltrate into the ground and fill up the water table in areas that are nowadays under the sea,” the scientist said.
“It happened all around the world, and when the sea level rose when ice caps started melting some 20,000 years ago, these areas were covered by the ocean,” he said. “Many aquifers were, and are still, protected from seawater by layers of clay and sediment that sit on top of them.”
The aquifers are similar to the ones below land, which much of the world relies on for drinking water, and their salinity is low enough for them to be turned into potable water, says Dr. Post.
There are two ways to access this water – build a platform out at sea and drill into the seabed, or drill from the mainland or islands close to the aquifers.
While offshore drilling can be costly, Dr. Post says this source of freshwater should be assessed and considered in terms of cost, sustainability and environmental impact against other water sources such as desalination, or building large new dams on land.
“Freshwater under the seabed is much less salty than seawater,” Dr. Post says. “This means it can be converted to drinking water with less energy than seawater desalination, and it would also leave us with a lot less hyper-saline water.
“Freshwater on our planet is increasingly under stress and strain so the discovery of significant new stores off the coast is very exciting,” he said. “It means that more options can be considered to help reduce the impact of droughts and continental water shortages.”
The flow of terrestrial groundwater to the sea is a natural component of the hydrological cycle, the authors say, adding that this process does not explain the large volumes of low-salinity groundwater found under continental shelves.
The authors say future research is needed to explore the potential use of these non-renewable reserves as a freshwater resource
The scope for continental shelf hydrogeology is even broader and the authors say it could contribute to the advancement of other scientific disciplines, such as sedimentology and marine geochemistry.
There is mounting evidence for the global occurrence of offshore fresh and brackish groundwater reserves. But while many nations may now have new reserves of freshwater offshore, Dr. Post says they must manage the seabed with care.
“Where low-salinity groundwater below the sea is likely to exist, we should take care to not contaminate it,” he warns.
“Sometimes boreholes are drilled into the aquifers for oil and gas exploration or production, or aquifers are targeted for carbon dioxide disposal. These activities can threaten the quality of the water,” cautions Post.
Post also warns that these water reserves are not renewable. “We should use them carefully,” he said. “Once gone, they won’t be replenished until the sea level drops again, which is not likely to happen for a very long time.”
The study “Offshore fresh groundwater reserves as a global phenomenon” by Vincent E.A. Post, Jacobus Groen, Henk Kooi, Mark Person, Shemin Ge and W. Mike Edmunds is published in the latest issue of “Nature.”