East Coast has a giant offshore freshwater aquifer—how did it get there?

One-quarter of the world’s population is currently water-stressed, using up almost their entire fresh water supply each year. The UN predicts that by 2030, this will climb to two-thirds of the population.

Freshwater is perhaps the world’s most essential resource, but climate change is enhancing its scarcity. An unexpected source may have the potential to provide some relief: offshore aquifers, giant undersea bodies of rock or sediment that hold and transport freshwater. But researchers don’t know how the water gets there, a question that needs to be resolved if we want to understand how to manage the water stored in them.

For decades, scientists have known about an aquifer off the US East Coast. It stretches from Martha’s Vineyard to New Jersey and holds almost as much water as two Lake Ontarios. Research presented at the American Geophysical Union conference in December attempted to explain where the water came from—a key step in finding out where other undersea aquifers lie hidden around the world.

As we discover and study more of them, offshore aquifers might become an unlikely resource for drinking water. Learning the water’s source can tell us if these freshwater reserves rebuild slowly over time or are a one-time-only emergency supply.

Reconstructing history

When ice sheets sat along the East Coast and the sea level was significantly lower than it is today, the coastline was around 100 kilometers further out to sea. Over time, freshwater filled small pockets in the open, sandy ground. Then, 10,000 years ago, the planet warmed, and sea levels rose, trapping the freshwater in the giant Continental Shelf Aquifer. But how that water came to be on the continental shelf in the first place is a mystery.

New Mexico Institute of Mining and Technology paleo-hydrogeologist Mark Person has been studying the aquifer since 1991. In the past three decades, he said, scientists’ understanding of the aquifer’s size, volume, and age has massively expanded. But they haven’t yet nailed down the water’s source, which could reveal where other submerged aquifers are hiding—if we learn the conditions that filled this one, we could look for other locations that had similar conditions.

“We can’t reenact Earth history,” Person said. Without the ability to conduct controlled experiments, scientists often resort to modeling to determine how geological structures formed millions of years ago. “It’s sort of like forensic workers looking at a crime scene,” he said.

Person developed three two-dimensional models of the offshore aquifer using seismic data and sediment and water samples from boreholes drilled onshore. Two models involved ice sheets melting; one did not.

Then, to corroborate the models, Person turned to isotopes—atoms with the same number of protons but different numbers of neutrons. Water mostly contains Oxygen-16, a lighter form of oxygen with two fewer neutrons than Oxygen-18.

Throughout the last million years, a cycle of planetary warming and cooling occurred every 100,000 years. During warming, the lighter ¹⁶O in the oceans evaporated into the atmosphere at a higher rate than the heavier ¹⁸O. During cooling, that lighter oxygen came down as snow, forming ice sheets with lower levels of ¹⁸O and leaving behind oceans with higher levels of ¹⁸O.

To determine if ice sheets played a role in forming the Continental Shelf Aquifer, Person explained, you have to look for water that is depleted in ¹⁸O—a sure sign that it came from ice sheets melting at their base. Person’s team used existing global isotope records from the shells of deep-ocean-dwelling animals near the aquifer. (The shells contain carbonate, an ion that includes oxygen pulled from the water).

Person then incorporated methods developed by a Columbia graduate student in 2019 that involve using electromagnetic imaging to finely map undersea aquifers. Since saltwater is more electrically conductive than freshwater, the boundaries between the two kinds of water are clear when electromagnetic pulses are sent through the seafloor: saltwater conducts the signal well, and freshwater doesn’t. What results looks sort of like a heat map, showing regions where fresh and saltwater are concentrated.

Person compared the electromagnetic and isotope data with his models to see which historical scenarios (ice or no ice) were statistically likely to form an aquifer that matched all the data. His results, which are in the review stage with the Geological Society of America Bulletin, suggest it's very likely that ice sheets played a role in forming the aquifer.

“There’s a lot of uncertainty,” Person said, but “it’s the best thing we have going.”

Searching for water

Person’s results don’t mean that ice sheets were the only contributor to filling up the Continental Shelf Aquifer. It could have been connected to onshore groundwater through past underwater channels through sediments or even filled by rain. There’s also a chance that the aquifer is still connected to onshore groundwater, meaning that it is constantly being supplied with new freshwater.

The only way to know for sure is to sample the aquifer. After years of submitting proposals, Person’s team secured a $21 million grant from the Integrated Ocean Drilling Program—a marine research collaboration focused on seafloor sediments—to drill offshore in summer 2025. It will be the first-ever hydrogeologically oriented offshore drilling campaign.

“I think it’s fabulous that they’re going to do the drilling,” said Deborah Hutchinson, a geologist emeritus at the United States Geological Survey who was not involved in the research. “If it’s not going to provide the answers, I don’t know what will.”

Confirming that ice sheets played a role in forming the aquifer would tell scientists to search for other offshore aquifers near historically glaciated locations. But in the meantime, researchers have been finding them in some areas that weren’t recently covered in ice, highlighting the importance of studying other hypotheses about filling the aquifers.

Since the electromagnetic technique was developed, researchers have discovered five more aquifers off the coasts of New Zealand, Malta, Israel, Hawaii, and California. There is also a sixth near New England, which may be connected to the Continental Shelf Aquifer, and others are being studied near Hong Kong.

With so many offshore aquifers discovered in such a short amount of time, scientists think the total number is much higher.

“It’s going to be likely impossible to map all of the [offshore] groundwater systems in the world,” said Aaron Micallef, a geomorphologist at the Monterey Bay Aquarium Research Institute who has collaborated with Person and studied the New Zealand and Malta aquifers. To better understand all offshore aquifers around the world, he explained, scientists need to “look at case studies that are representative of other places.”

Understanding offshore aquifers more generally is crucial to determining whether nations could tap into them as climate change and drought threaten traditional water sources. If the aquifers are actively recharging, however slowly, they could represent a potential water resource. If not, they are an emergency fallback—still useful in a drought-stricken world.

Micallef, who is from Malta—currently one of the top 10 most water-scarce countries—said that finding offshore water there was largely “based on hope.”

It’s not just water-scarce nations that could benefit but also water-intensive coastal metropolises. Person published a paper in the journal Groundwater in 2010 suggesting that the volume of freshwater in the East Coast aquifer is more than two orders of magnitude higher than all the groundwater pumped annually in the US, making it an immense potential resource.

Tapping offshore aquifers wouldn’t be cheap. But neither is a world where we truck drinking water into a city like New York, Person explained.

While the science is progressing, we’re still far from drawing water from under the sea. There might be unforeseen environmental threats, technology constraints, or legal difficulties from operating in international waters. Also, undersea aquifers don’t keep out 100 percent of saltwater, meaning they would require some desalination. Compared to pure seawater, however, the energy required to get the salt out of this water would be much lower.

Difficulties aside, researchers are united in believing that offshore aquifers could become important. Person likes to borrow a quote from Benjamin Franklin: “The worth of water is realized when the well goes dry,” he paraphrased, “which is basically saying you have to spend whatever money it takes to have enough water, because we can’t live without it.”

Hannah Richter is a freelance science journalist and student in MIT's Graduate Program in Science Writing. She primarily covers environmental science and astronomy.