Olhos de Aqua – Eyes of Water – is the name of a popular holiday destination in Portugal’s Algarve. The name refers to an unusual natural phenomenon – during low tide fresh water flows from round springs to the surface. Such springs are also present near the North Sea coast. At both locations, coastal groundwater, having found its way through layers of rock or sand, flows along the gradient towards the sea. The water appears again near the water line or, in some cases, at up to 50 metres sea depth, as is the case for example near the Croatian island of Braç. Such springs often exist in karst coastal regions made up mainly of cavernous limestone.
Submarine groundwater springs are widespread, but easy to overlook. “Science hasn’t given them much thought so far,”,says Nils Moosdorf, a geologist at the Leibniz Centre for Tropical Marine Research (ZMT) in Bremen/Germany. “It is estimated that submarine groundwater discharge accounts for up to ten per cent of the water carried into the sea through rivers.” Moosdorf and his scientific fellow Till Oehler are studying the importance of such springs for humans and coastal ecosystems.
The scientists went on a search for submarine fresh water springs and got lucky finding evidence across the globe in ancient scripts, travel guides, diving magazines, press articles and through folk memory.
Often springs were used as a source for drinking water as for example in Bahrain in the Persian Gulf, where reports dating back to the 15th century talk of “a man being able to dive into the sea with a fur and filling it with fresh drinking water”.
In island states with a shortage of water, such as Tahiti or Fiji, the population still rely on freshwater springs if the water supply breaks down. Valuable rainwater is often collected in cisterns and will only be used for drinking. For washing, people then use the freshwater springs at the beach.
Submarine groundwater springs rich in nutrients …
“The springs are often extremely rich in nutrients,” explains Nils Moosdorf. “They attract a lot of fish. The so-called ‘Wonky Holes’ of the Australian coast are an example. They are hotspots for fishers and divers alike thanks to their fish abundance.” The geologist adds: ”In Japan, oysters from aquaculture thrive particularly well in the vicinity of such springs.”
In Peru, water from submarine sources is pumped into lorries to be distributed across the country, while in Greece it is used to water the fields. In the 1990s, Florida even planned to tap submarine groundwater on a large scale, as drinking water is running short in the city of Miami. One of the springs in this US state provides roughly up to 40 cubic metres of fresh water per second.
…and are culturally important
“Fresh water flowing out of the ocean has something magical and inspires people’s imagination,” reckons Moosdorf. “In many places, submarine groundwater springs are meaningful and culturally important to people. In ancient Greece, for example, horses were sacrificed to the God Poseidon at such a spring near Argos.” The geologist can also report on freshwater springs from Bali, where the Hindu sea temple Tanah Lot was built on one of them. Each year, two million people go on pilgrimage to the location.
Despite their significance submarine springs are threatened. Well-drilling on land can lead to submarine groundwater springs drying up. Raw sewage and residues from farming can pollute the spring water. This is not just a problem for humans who use the springs for drinking but also for coastal ecosystems such as coral reefs growing nearby. Sea-level rise will have a further impact on the hydraulic conditions for submarine groundwater discharge.
“It is only when we take a closer look that the particular value of these invisible water resources becomes apparent. Scientists and managers of coastal regions should keep an eye on them,” says Moosdorf, summing up the implications of his work.
Moosdorf, N., and T. Oehler: "Societal use of fresh submarine groundwater discharge: An overlooked water resource." Earth-Science Reviews (2017).