Enormous freshwater reservoir discovered off the East Coast may be 20,000 years old and big enough to supply NYC for 800 years

A giant reservoir of "secret" fresh water off the East Coast that could potentially supply a city the size of New York City for 800 years may have formed during the last ice age, when the region was covered in glaciers, researchers say.

Preliminary analyses suggest the reservoir, which sits beneath the seafloor and appears to stretch from offshore New Jersey as far north as Maine, was locked in place under frigid conditions around 20,000 years ago, hinting that it formed in the last glacial period due, partly, to thick ice sheets.

Last summer, researchers went on an expedition to follow up on reports from the late 1960s and early 1970s of fresh water beneath the seafloor off the East Coast. "It was quite the project and sort of a lifelong dream," Brandon Dugan, the expedition's co-chief scientist and a professor of geophysics at the Colorado School of Mines, told Live Science.

The research voyage, known as Expedition 501, lasted three months and dredged up 13,200 gallons (50,000 liters) of water from beneath the seafloor in three locations off the islands of Nantucket and Martha's Vineyard. The results aren't finalized yet, but so far it looks as if the reservoir might stretch farther underground than early reports suggested, meaning it might be even bigger than previously thought.

Dugan and his colleagues also think they know what created the reservoir thanks to preliminary radiocarbon, noble gas and isotope analyses, he said.

Fresh water in the region was first reported 60 years ago by the U.S. Geological Survey (USGS), during offshore mineral and energy resource assessments between Florida and Maine. "In a very peculiar way, they found fresh water in the sediment beneath the ocean," Dugan said. "In the 1980s, some of the USGS people came up with ideas of how that fresh water could get there. Then it went quiet for a while — no one was talking about it."

In 2003, Dugan and Mark Person, a professor of hydrology at the New Mexico Institute of Mining and Technology, rediscovered these records and came up with three ideas of how fresh water can end up beneath the ocean. One way that a submarine freshwater reservoir can form is if sea levels are very low for a long time and rainfall seeps into the ground. Then, when sea levels rise again over hundreds of thousands of years, that fresh water gets trapped in the underlying sediment, Dugan said.

A second possibility is that tall mountains close to the ocean funnel rainwater directly down into the seabed from their high elevation point, he said. And thirdly — related to the first hypothesis — a freshwater reservoir can form under the ocean if ice sheets expand, causing sea levels to drop. Meltwater collects at the bottom of ice sheets because they grind against the bedrock, producing heat. The huge weight of the ice sheet then pushes that water into the ground, trapping it beneath layers of sediment.

More than two decades later, the researchers are finally close to getting an answer, with preliminary data indicating that most of the fresh water came from glaciers some time during the last ice age (2.6 million to 11,700 years ago). "We kind of ruled out the large topography for New England, because we don't have big mountains next to the coast," Dugan said. However, "there might be a rainfall component" blended in the glacier water, he said. "You can imagine that in front of a glacier you have rainfall, so it's probably a mixed system."

Expedition 501 extracted water samples from sites 20 to 30 miles (30 to 50 kilometers) off the coast of Massachusetts. The researchers drilled down to 1,300 feet (400 meters) below the seafloor, which was deep enough to reveal a thick layer of sediment engorged with fresh water sitting beneath a layer of salty sediment and an impermeable "seal" of clay and silt.

"We have a seal at the top [of the fresh water] that keeps the seawater above from the fresh water below," Dugan said. This seal is strong enough to separate the two layers now, but it wasn't robust enough to stop a glacier from forcing water down through it — if that is what happened. "Whatever emplaced that water didn't care if there was a seal. There was enough energy to flush it with fresh water," he said.

Salinity measurements showed that water freshness in the reservoir drops with distance from the shore, but it stays well below ocean salinity in the areas studied last summer. The drill site closest to Nantucket and Martha's Vineyard had a salt content of 1 part per 1,000, which is the maximum safe limit for drinking water. Farther offshore, salt content was 4 to 5 parts per 1,000, and at the farthest site, the researchers recorded 17 to 18 parts per 1,000 — or about half of the ocean's average salt content.

"The important part was we collected all the samples we need to address our primary questions," Dugan said. "When we're done drilling and we pull our equipment out, the holes collapse back in and seal themselves up."

Now, scientists are studying the reservoir in finer detail, including any microbes, rare earth elements, pore space — which can help researchers better estimate the reservoir's size — and the age of the sediments, which will help narrow down when it formed. More definitive results about how and when the reservoir formed are expected in about one month's time, Dugan said.

"Our goal is to provide an understanding of the system so if and when somebody needs to use it, they have information to start from, rather than recreating information or making an ill-informed choice," he said.