Freshwater reservoir off Jersey Shore

EM survey location on the US Mid-Atlantic Margin. Blue squares show the location of seafloor EM/MT receiver stations and red lines show the locations of the controlled-source EM transmitter and towed-EM receiver tows. Green circles along the New Jersey profile show the locations of drill holes from IODP expedition 313. Yellow triangles show older AMCOR drill holes.

Everyone knows the Atlantic Ocean is salty.

But if you swim 55 miles off the Jersey Shore and dig hundreds of feet below the seafloor, you’d hit freshwater.

Researchers have begun mapping out the vast reservoir they believe spans at least from New Jersey’s coast to Martha’s Vineyard in Massachusetts.

In a study published last week in the journal Scientific Reports, scientists from Columbia University and Woods Hole Oceanographic Institution say the aquifer system contains about 739 trillion gallons of low-salinity groundwater that is trapped in porous sediments. That’s the same size as a lake covering 15,000 miles.

“We knew there was freshwater down there in isolated places, but we did not know the extent or geometry,” lead author Chloe Gustafson, a Ph.D. candidate at Columbia’s Lamont-Doherty Earth Observatory, said in a statement. “It could turn out to be an important resource in other parts of the world.”

Hints of the aquifer came decades ago. In the 1970s, oil companies drilling holes in the ocean’s floor found small pockets of freshwater.

Then, four years ago, two scientists spent 10 days on a research vessel off South Jersey and Massachusetts and dropped receivers to the seafloor to measure electromagnetic fields on the bottom. An apparatus towed behind the ship also emitted artificial electromagnetic pulses.

The way it works? Salt water is a better conductor of electromagnetic waves than freshwater, so areas of freshwater could be spotted.

They found that the freshwater wasn’t in pockets but was continuous and extended in some places 75 miles off the shoreline. It begins about 600 feet below the ocean floor and ends about 1,200 feet deep, the researchers believe.

The researchers say the reservoir likely formed 15,000 to 20,000 years ago, toward the end of the last Ice Age, when water along Long Island, New York and New Jersey was trapped in miles-deep ice and sea levels were lower. The ice melted, river deltas were created on top of the continental shelf and pockets of freshwater were confined.

The aquifer, a huge natural resource, could become important in the future as the population steadily rises and the world’s water needs grow.

“If water from the outer parts of the aquifer were to be withdrawn, it would have to be desalinated for most uses,” study coauthor and Columbia geophysicist Kerry Key said in a statement. “But the cost would be much less than processing seawater.”

Contact: 609-272-7258 Twitter @AvalonZoppo

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