Expeditions off the coast: one million cubic kilometers of fresh water

Es was on the last leg off the coast of Malta, while the final measurement was still in progress, when the small ship Hercules suddenly trembled. In the galley, the chief scientist Marion Jegen dropped her bread and hurried to the stern where the “Pig” was hanging on the winch. The researchers on board the Hercules, this makeshift research vessel, were looking for groundwater that was suspected to be under the seabed here in autumn 2018. And the heart of this search was the so-called Pig, a robust, 400 kilogram metal housing full of transmission electronics. Only the front of this bin is reminiscent of a pig’s snout, the name also stands for a coarse metal casting. Usually the pig plows a path on the seabed for the measuring instruments, for more than 300 meters of unrolled rope with transmitters and electrodes. The ship pulls the pig, the pig the electrode tail. But now the Pig seemed to be stuck.

On the stern, chief scientist Marion Jegen checked the tensiometer of the winch: Normally the tensile force on the cable is around 800 Newtons, now the value was already 8000 Newtons, and it continued to rise rapidly. The experienced geophysicist knew this would be the end of the line at 15,000 Newtons, and the optoelectronic cable would then reach its maximum load. If the thick wire tears, it could whip over the stern on which she and her fellow researchers were at great speed. Despite the tense situation, the researchers stood out picturesquely in front of the blue sea, with their sunglasses, yellow helmets and red life jackets. Among them were Jegen’s colleague Amir Haroon from the Geomar Helmholtz Center for Ocean Research, the researcher Katrin Schwalenberg from the Federal Institute for Geosciences and Natural Resources (BGR) and the Maltese geology professor Aaron Micallef from the University of Malta. All of you would later write several scientific publications on this drinking water search. But now they feared the fate of the stuck pig.

Water scarcity on land (red) and reservoirs in front of the arts: The green areas were above water during the last Ice Age and could now contain significant amounts of fresh water under the sea.

Image: C. Berndt, GEOMAR, A. Micallef

The captain stopped the ship immediately. Then he had to back up carefully while slowly hauling in the winch cable and pulling the ship backwards over the Pig. After an hour of tension, everyone breathed a sigh of relief when they were able to safely retrieve their Pig and the last records. At this point in time it was still unclear whether drinking water was slumbering under the seabed off Malta, because the measurement voyage at that time was one of the first ever for the targeted geoscientific search and measurement of drinking water resources under the seabed. The results were only published this summer.

In recent years, offshore aquifers have only been specifically explored off the coasts of New Zealand, North America, Hawaii and Israel. Other drinking water finds always happened randomly and selectively and offered hardly any information regarding their extent. In your Study from 2021 in the journal Reviews of Geophysics Micallef and his colleagues estimate that a total of one million cubic kilometers of drinking water could lie dormant beneath the ocean floor worldwide. With this amount the Black Sea could be replenished twice and the North Sea even twenty times. This arouses desires, because population growth, climate change and environmental pollution contribute to the growing international scarcity of drinking water in coastal regions. Malta is Europe’s most arid country; for years it has been dependent on cost-intensive desalination plants with high electricity consumption, which have to cover more than half of the water demand. Freshwater supplies off the coast would be of great use to many regions, but researchers so far know little about their exact location, size and properties.


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