Australasian Science Magazine

By Stephen Luntz

Vast quantities of fresh water have been found underneath the oceans, possibly providing a cheaper and more environmentally friendly alternative to desalination for thirsty cities.

Freshwater has been detected when drilling for oil and gas on continental shelves, but Dr Vincent Post of the Flinders University School of the Environment says these were thought to be rare exceptions. “Our research shows that fresh and brackish aquifers below the sea bed are actually quite a common phenomenon,” says Post.

So common, in fact, that a Nature paper on which Post was lead author estimates that 500,000 km3 of low-salt water is available worldwide. By comparison, Melbourne’s Thomson Dam has a capacity of only 1 km3 while the Warragamba Dam in Sydney holds about 2 km3.

Post says that the source of the water is not fully understood but it is likely that the vast majority formed during sea-level lows during glacial periods. When continental shelves were exposed, fresh water could infiltrate into the soil. The fresh water reserves that formed remained protected by layers of clay from the salt water above after sea levels rose.

“One thing that could play a role is if clay layers were deposited after the fresh waters were formed,” he says. “We know of a case in South America where this happened and then sea level rose. If this happens fast enough it prevents the water becoming saline.”

The estimates for freshwater under continental shelves are only 10% of the shallow (to a depth of 750 metres) groundwater beneath the continents themselves. However, Post points out that “most of the population is concentrated around coastlines, so in some places it might be easier to get water from under the ocean than from a long way inland”.

A paper published last year in Water Resources Management estimated that the cost of extracting fresh water from beneath shelves close to cities was lower than desalinating sea water, and had less negative environmental effects.

Nevertheless obstacles remain. Over-extraction of groundwater can cause land subsidence, and Post says similar effects could be problematic in shallow waters.

“Where low-salinity groundwater below the sea is likely to exist, we should take care not to contaminate it,” says Post. Oil and gas drilling already “break the integrity of the geological layers, creating point sources of salination,” which Post says needs to be addressed if the water is to be regarded as a resource.