A new study, published December 5 in the international scientific journal Nature, reveals that vast underwater offshore reserves of fresh and brackish water are a global phenomenon.
The discovery is providing new opportunities to stave off a looming global water crisis but scientists emphasize that too little is known about the environmental impacts should we decide to use it for drinking water. Where exploitation is considered, the development of the technology to abstract and purify the water with minimal environmental impacts should have priority.
“The estimated 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 (NCGRT) and the School of the Environment at Flinders University, Australia. “Knowing about these reserves is great news because this volume of water could sustain some regions for decades.”
To put it in numbers – an estimated 500 000 cubic kilometers of low-salinity water are buried beneath the seabed on continental shelves around the world.
"Our study is the first comprehensive review of all the studies that have reported on submarine groundwater (that is, water in the pores of the rocks and sediments below the ocean floor) with a salinity lower than ocean water. While it was already known that such water exists, our review has now demonstrated that this is not an anomalous phenomenon that only occurs under special conditions, but that instead the presence of fresh and brackish water under the seafloor is rather common."
In a way, Dr Post said, this shouldn’t be a surprise because the sea-level changes over the past hundreds of thousands of years tell us that the continental shelves were exposed for much longer periods of time than they were covered by the ocean. Fresh groundwater had an opportunity to infiltrate in the sediments and recharge the now-submerged aquifers.
The water, 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.
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, Dr Post explains. “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.”
Coastal groundwater systems – long-term perspectives. Presented by: Dr Vincent Post
The significance of this finding is that knowing about the potential presence of these reserves means that in coastal areas where the demand exceeds the available resources on land, the exploitation of offshore aquifers may come into view.
"We are not necessarily advocating this solution, as we know too little about the environmental impacts, but it is an option to add to the range of available solutions."
In particular where seawater desalination is considered, the desalination of brackish groundwater from offshore aquifers, where available, could present a viable alternative.
"While our review shows that offshore fresh and brackish groundwater is common, it doesn’t mean it can be found everywhere. Future research should focus on better mapping their occurrence. Where exploitation is considered, the development of the technology to abstract and purify the water with minimal environmental impacts should have priority." – Dr Vincent Post
“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. It means that more options can be considered to help reduce the impact of droughts and continental water shortages.”
But while nations may now have new reserves of freshwater offshore, Dr Post says they will need to take care in how they manage the seabed: “For example, where low-salinity groundwater below the sea is likely to exist, we should take care to not contaminate it.
“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.”
Dr Post also warns that these water reserves are non-renewable: “We should use them carefully – once gone, they won’t be replenished until the sea level drops again, which is not likely to happen for a very long time.”
- "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, was published in the December 5 issue of Nature.
- National Centre for Groundwater Research and Training – www.groundwater.com.au
Featured image: Cape Point – HDR by http://www.flickr.com/photos/82955120@N05/8299022203
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