As most of you know, global demand for water is rising rapidly. In 1975, global demand for water was about 3,000 billion cubic meters per year. By 2025, the UN predicts that global consumption will have doubled to about 6,000 billion cubic meters per year (see the report here). As developing countries industrialize, the amount of water devoted to agriculture will not change, while demand for industrial uses will rise. In developed countries, up to 60% of water is used for industrial purposes.
As development in dry regions continues, water in those areas will become increasingly expensive. Fortunately, there are alternative sources of water right beneath our feet. These “hidden” water sources were once considered uneconomical – they were too deep or too inaccessible to be considered worth extracting. But just as unconventional oil and gas resources have become available as hydrocarbon prices have risen, alternative water sources are becoming increasingly worthwhile.
Let’s take a quick look at two of these water resources.
Most groundwater currently extracted for human use resides in the first few hundred feet of sediment or bedrock beneath our feet. However, geologists have long known that fresh and brackish waters can be found at much greater depths. This “deep groundwater” typically doesn’t interact with surface water because it is separated from the surface by thick layers of impermeable rock.
It’s expensive to drill down to deep groundwater. Since water has much less economic value than hydrocarbons extracted from similar depths, there have been few attempts to extract it, except in extremely arid regions where desalinization is too expensive. In the future, as demand for fresh water rises, we may see arid regions making greater use of these fossil (non-renewable) groundwater resources.
Continental shelf water
In contrast to deep groundwater, which has been well understood for a very long time, continental shelf water is a newly discovered resource. In a paper published in Nature, a team of scientists at the National Centre for Groundwater Research and Training in Australia describe their attempts to quantify the amount of fresh and brackish water trapped in sediments beneath the seafloor.
The results were startling – the team concluded that sediment layers on the continental shelves contain about 100 times the total amount of groundwater extracted by humans since 1900.
Where did this water come from? Students of sequence stratigraphy know that the continental shelves are alternatively exposed and submerged by water. When the shelves are submerged, sediments build up. When they are exposed, rainwater percolates into these sediments. If the next submersion results in the deposition of an impermeable layer, the fresh or brackish water is trapped, similar to the way in which hydrocarbons become trapped between impermeable layers.
While it’s reassuring to know that these resources exist, don’t expect a rush on the continental shelves anytime soon. Continental shelf aquifers can be buried beneath thousands of feet of marine sediments, requiring offshore wells that may cost more than ten million USD to drill. In addition, the water, while less salty than seawater, is still often brackish, requiring desalinization.
As a result of these costs, continental shelf aquifers will probably only be developed if water becomes quite scarce indeed, or if the energy required for desalinization becomes expensive enough to make offshore drilling for water worthwhile.