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Off Martha's Vineyard, a mysterious pool of freshwater beneath the sea floor

Photo credit: Woods Hole Oceanographic Institution
A jackup rig is elevated above the sea surface.

Local scientists believe there’s a mysterious pool of freshwater that sits hundreds of meters beneath the sea floor off the coast of Martha’s Vineyard, and they want to know how it got there.

A team from the Woods Hole Oceanographic Institution (WHOI) plans to embark on a scientific drilling expedition early next summer to better understand so-called offshore freshened groundwater deposits.

According to Rob Evans, a senior scientist at WHOI, researchers are relatively confident that the deposits are between 6 and 30 miles off the coast of the island, but it’s unclear where the water came from and whether it’s being refilled. 

Evans said there are three primary theories about the origin of the deposits.

“The easiest to understand is that the rocks that are below the [continental] shelf connect back onto shore. It’s just an extension,” he said, “so any kind of rainfall, any kind of groundwater that's falling on land [is] getting down into an aquifer beneath the land, and is flowing out … under the continental shelf.”

In a study that looked at a deposit off the coast of New Jersey, the water in the deposit appeared “fairly young,” Evans said, suggesting that it was, in fact, water that had flown through an aquifer system and extended offshore. 

The other two theories involve the formation of ice sheets, and the idea that the freshwater has been trapped in place for 10,000 years. 

Now, after 20 years of planning, scientists could begin to get clarity.

“We’ll get an idea on the age [of the water in the deposits]. How long has it been in the sub bottom? What's are its chemical properties?“ Evans said. “That'll hopefully resolve that question: is it just leakage from our current water supply, or is it a deposit that's been trapped there?” 

Locals may see the drilling work underway, Evans said, as his team expects to have access to either a large drill ship or a jackup rig to study deposits as close as 20 nautical miles from Martha’s Vineyard’s shores. The work should run for 70 days, likely beginning in June 2024.

“I really want the community to understand that this is a scientific expedition. We're looking for for water, not oil,” Evans said. “I mean, you can imagine how the rumors could start.” 

Eve Zuckoff covers the environment and human impacts of climate change for CAI.