Scientists have found a giant aquifer under the Northeastern U.S., estimated to hold at least 670 cubic miles of freshwater. If it was on the surface, they say, it would create a lake spanning 15,000 square miles, which is twice the size of Lake Ontario.

Finding that much groundwater would be a big deal anywhere, especially given the growing threats of droughts and water shortages around the world. But this aquifer isn't just underground — it's also under the ocean, buried hundreds of feet below the seabed. It's the largest deposit of its kind known to science, and it also hints at an even bigger prospect: Based on the way it seems to have formed, similar freshwater caches may be hiding under salty coastal seas worldwide.

There were clues about this aquifer as early as the 1970s, when companies drilling for oil off the U.S. East Coast would sometimes find freshwater instead. These were just isolated reports, though, offering little evidence they might all be one big aquifer. Then, in 2015, a team of scientists took a research vessel out to investigate more closely, using electromagnetic imaging to peek below the sea floor.

Their findings, published June 18 in the journal Scientific Reports, point to a vast reservoir of low-salinity water trapped in porous sediments under the salty ocean. Rather than scattered deposits, they describe a continuous aquifer spanning more than 200 miles of coastline, from New Jersey to Massachusetts and possibly beyond. It starts at the shoreline and stretches out across the continental shelf, generally for about 50 miles but in some places up to 75. The top of the aquifer is about 600 feet below the ocean floor, they report, and it extends down to about 1,200 feet.

"We knew there was fresh water down there in isolated places, but we did not know the extent or geometry," says lead author Chloe Gustafson, a Ph.D. candidate at Columbia University's Lamont-Doherty Earth Observatory, in a press release. And since its formation suggests this kind of thing might not be uncommon, she adds, it "could turn out to be an important resource in other parts of the world."

Shore enough

map of newly discovered aquifer off U.S. East Coast The aquifer is marked by the hatched yellow area in this map, while the solid lines with triangles mark the research vessel's tracks. The dotted white line near shore shows the edge of the glacial ice sheet that melted about 15,000 years ago. (Map: Gustafson et al./Columbia University)

The researchers found the aquifer by dropping receivers to the sea floor, which let them measure electromagnetic fields in the sediments below. They examined the effects of natural disruptions like solar wind and lighting strikes, as well as from a device towed behind the ship that emitted electromagnetic pulses. Saltwater conducts electromagnetic waves better than freshwater does, so any freshwater would stand out in the data as a region of lower conductivity.

The surveys were conducted off southern New Jersey and Martha's Vineyard, and based on the consistency of data from those study areas, the researchers were able to "infer with a high degree of confidence" that a continuous aquifer hugs the coasts of Massachusetts, Rhode Island, Connecticut, New York and New Jersey. More research will be needed to clarify the boundaries, and if they extend much farther north and south, this water deposit could rival the Ogallala Aquifer, the largest groundwater system in North America and one of the largest aquifers on Earth.

Water flowing underground

offshore groundwater illustration An illustration of how the offshore aquifer might recharge, with low-salinity water flowing from land and high-salinity groundwater rising from deeper deposits. (Image: Gustafson et al./Scientific Reports)

There are two ways all this freshwater might have ended up under the ocean, the researchers explain. One scenario starts around 15,000 years ago, near the end of the last glacial period, when much of the world's water was frozen in massive ice sheets, including one that covered northern North America. Sea levels were also lower, exposing many parts of the U.S. continental shelf that are now underwater.

As the ice sheets melted, sediments formed large river deltas on the shelf, where freshwater was trapped in isolated deposits before sea levels eventually rose. This preserved pockets of "fossil water" in the seabed, and until now it was the standard explanation for any freshwater aquifer found under the ocean.

This aquifer might have begun as fossil water, but it also seems to still be replenished by modern underground runoff from land, the study suggests. This is similar to the way groundwater feeds terrestrial aquifers, as water from rainfall and water bodies percolates down and accumulates underground. Near the ocean, though, groundwater in coastal sediments may be pumped toward the sea by the rising and falling pressure of tides overhead, explains study co-author and Columbia geophysicist Kerry Key, who compares the process to soaking up water through the sides of a sponge by pressing up and down on it.

The water in the newfound aquifer tends to be freshest near shore, the study found, growing slightly saltier the farther out you go. That suggests it's still being supplied by fresh groundwater from land, which gradually mixes with saltwater seeping in. Its fresher near-shore water has about the same salinity as terrestrial freshwater — less than 1 part per thousand (ppt) of salt — while its outer edges have about 15 ppt. For comparison, the typical salinity of seawater is 35 ppt.

With a grain of salt

dusk at beach in Cape May, New Jersey A sunset off the coast of Cape May, New Jersey, near the southern side of the aquifer. (Photo: Jorge Moro/Shutterstock)

Some of this water might already be useable, but saltier water from the outer aquifer would probably need to be desalinated for most uses, the researchers note. On top of extracting the water, that introduces the costs, energy demand and pollution often associated with desalination, although the drawbacks should be milder than usual, since this is about 57% less salty than typical ocean water.

Even without desalination, however, it might not make much sense to pump water from this aquifer anytime soon. Most of the U.S. East Coast isn't particularly prone to severe water shortages, at least for now, so there's little incentive to spend money or risk environmental problems by tapping into it. This might still be a valuable discovery, though, both for what it can tell us about the way coastal environments work and how it might help us deal with water scarcity in the future.

"We probably don't need to do that in this region," Key says, "but if we can show there are large aquifers in other regions, that might potentially represent a resource."

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

Huge freshwater aquifer found under the ocean
The aquifer is hundreds of feet under the seabed, spans 200 miles along the U.S. East Coast, and hints at similar deposits elsewhere.