According to a group of researchers led by Dr Vincent Post from the Australia’s National Center for Groundwater Research and Training and Flinders University, about 500,000 cubic km of freshwater are buried beneath the seabed on continental shelves around the world.

The groundwater, which could perhaps be used to eke out supplies to the world’s burgeoning coastal cities, 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. Knowing about these reserves is great news because this volume of water could sustain some regions for decades,” said Dr Post, who is the first author of the paper published in the journal Nature.

“Groundwater scientists knew of freshwater under the seafloor, but thought it only occurred under rare and special conditions. Our research shows that fresh and brackish aquifers below the seabed are actually quite a common phenomenon.”

“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. 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.”

“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. 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,” the scientist explained.

“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 very costly, 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 even building large new dams on land.

“Freshwater under the seabed is much less salty than seawater. 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,” Dr Post said.

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Vincent E.A. Post et al. 2013. Offshore fresh groundwater reserves as a global phenomenon. Nature 504, 71–78; doi: 10.1038/nature12858