For millennia, weary travelers have uttered, "Water!" with relief and joy as they encounter fresh, drinkable water to quench their thirst. The need for freshwater is especially urgent when out at sea, surrounded by a literal ocean of undrinkable saltwater. Sophisticated sensing technology is now revealing vast deposits of freshwater under the ocean floor, which could one day provide a reliable source of water for communities on land.

Sound fishy? It's not a mirage — just an extension of geological concepts to the seafloor, which remains poorly explored even today. Discoveries made deep below the waters may hold promise for the resource-constrained future.

The First Signs of Water Underneath Water

Back in the 1800s, fishers off the coast of Florida reported "boils" of water that rose to the ocean surface and did not taste salty, as described in Scientific American. How is this possible? Freshwater is less dense than saltwater because the salt fits in the space between water molecules and adds weight. Therefore, freshwater will naturally rise to the surface of the ocean unless it is constrained by a waterproof barrier, such as a layer of impermeable rock.

To begin to understand the source of this freshwater in the ocean, it's important to know that geological formations do not end at the coastline. Continents extend offshore to form a rocky underwater shelf. Some of these rocks have cracks that allow seawater to penetrate. In many regions, the continental shelf is covered by layers of sedimentary rock, which contains interconnected pores that fill with water. As on land, these layers of rock may be permeable or impermeable.

In the 1960s, the U.S. Geological Survey drilled boreholes off the New Jersey coast and unexpectedly struck freshwater. Later studies revealed that the water was a mixture of rainwater and seawater — located 65 kilometers (40 miles) out to sea. In the 1990s, researchers started to map these freshwater deposits with controlled source electromagnetic sensing, a technology used by the petroleum industry to detect offshore oil deposits. Electromagnetic sensing essentially measures how well the seafloor conducts electrical current.

Mapping Freshwater Deposits

The sodium and chloride ions in saltwater can conduct electricity. Therefore, portions of the seafloor that contain saltwater in rock cracks and pores will conduct electricity better than portions of the seafloor that contain freshwater under a layer of impermeable rock.

To map the freshwater deposits off the coast of New Jersey and Martha's Vineyard in Massachusetts, a research ship transmitted electromagnetic fields hundreds of meters down to the seabed, as reported in Scientific Reports. Return signals were detected by floating receivers attached to a line trailing the ship and by sensors on the sea floor. The results clearly showed that there are freshwater deposits — or submarine aquifers — off the coasts of these areas.

The amount of water in these deposits is still unclear. This is because electromagnetic sensing will give the same reading for a highly porous rock filled with fresher water as for a less porous rock filled with slightly salty water.

Aquifers as a Source of Freshwater

On land, modern-day humans often get water by drilling into aquifers that may or may not be capped by a layer of impermeable rock, as outlined by the U.S. Geological Survey. Aquifers are layers of porous rock or sediment that hold water. Some are shallow and can be replenished by rainfall. Other aquifers are much deeper and hold water that could be thousands of years old.

The simplest explanation for the deposits of freshwater under the ocean is that they are — or were — extensions of aquifers on land. Some of these submarine aquifers appear to be fed by modern-day rain that falls on land, while others may be ancient — filled by glaciers or rain when sea levels were lower. The "boils" of freshwater detected by fishers in the 1800s must have come from a break in the waterproof rock layer, much like freshwater bubbles out of natural springs on land.

Researchers estimate that there may be one million cubic kilometers of freshwater trapped in submarine aquifers around the world. For reference, New York City uses 1.4 cubic kilometers of water per year. Therefore, these submarine aquifers could be an invaluable resource for drought-prone regions that need to irrigate crops and care for people. Studies are underway to identify additional submarine aquifers around the world.

Extracting the Water Underneath Water

All aquifers have different characteristics, which are important to understand before considering any large-scale operation to extract water. The water in any submarine aquifer contains at least some salt, so some desalination would be required before the water could enter the water supply.

According to the National Oceanic and Atmospheric Administration, the salinity — or salt content — of seawater is about 35 parts per thousand (ppt). Freshwater has a salinity of less than 1 ppt. Samples taken from the submarine aquifers off the coast of New Jersey and Martha's Vineyard had salinity between 0.2 and 9.0 ppt. The higher the salinity, the more energy-intensive the desalination process.

For a submarine aquifer that's no longer connected to a source of water on land, pumping out freshwater could create a void that would be filled by saltwater. This would steadily increase the salt content of any remaining water in the aquifer. For a submarine aquifer that's connected to an aquifer on land, extracting from the submarine aquifer could cause mixing that increases the salinity of the land aquifer, negatively impacting natural habitats or people. Pumping from the submarine aquifer could also drain the onshore aquifer and even cause the land to sink.

In some situations, these risks may be offset by the enormous potential of submarine aquifers, so research groups may dive further into the problem. As the human population continues to grow, we must find ways to get the resources we need while protecting the natural environment, and if we can find safe ways to access and extract freshwater from these aquifers, then it may be a promising step forward.

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