The East Coast has a huge offshore freshwater aquifer – how did it get there?

Image of a large boat with a tall tower in the center and a crane at the back.  It floats on a dark blue ocean and lies in front of a white cloud.
Enlarge / It may take a scientific drillship to figure out how vast undersea aquifers formed.

A quarter of the world’s population currently suffers from water shortages, which means that almost the entire freshwater supply is used up every year. The UN predicts that this will rise to two-thirds of the population by 2030.

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

Scientists have known for decades that an aquifer exists off the east coast of the US. It stretches from Martha’s Vineyard to New Jersey and contains almost as much water as two Lake Ontarios. Research presented in December at the American Geophysical Union conference sought to explain where the water came from — an important step in discovering where other undersea aquifers are hidden around the world.

As we discover and study more of them, offshore aquifers may become an unlikely source of drinking water. Learning the source of the water can tell us whether these freshwater reserves are built up slowly over time or if they are a one-time emergency supply.

Reconstructing history

When there were ice caps along the east coast and sea levels were considerably lower than now, the coastline was about 100 kilometers further out to sea. Over time, fresh water filled small pockets in the open sandy soil. Then, 10,000 years ago, the planet warmed and sea levels rose, trapping fresh water in the giant aquifer of the continental shelf. But how that water ended up on the continental shelf in the first place is a mystery.

Paleohydrogeologist Mark Person of the New Mexico Institute of Mining and Technology has been studying the aquifer since 1991. Over the past three decades, he said, scientists’ understanding of the aquifer’s size, volume and age has increased dramatically . But they haven’t yet discovered the source of the water, which could reveal where other submerged aquifers lurk. If we knew the conditions this aquifer was in, we could look for other locations with similar conditions.

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

Persoon developed three two-dimensional models of the offshore aquifer using seismic data and sediment and water samples from boreholes drilled on land. Two models involved the melting of ice caps; one not.

To confirm the models, Person then turned to isotopes: atoms with the same number of protons but different numbers of neutrons. Water usually contains oxygen-16, a lighter form of oxygen with two fewer neutrons than oxygen-18.

Over the past million years, a cycle of warming and cooling of the planet occurred every 100,000 years. When warming up, the lighter 16O in the oceans evaporated into the atmosphere faster than the heavier 18O. As it cooled, that lighter oxygen came down as snow, creating ice caps with lower levels of 18Oh and leaving behind oceans with higher levels of 18O.

To determine whether ice sheets played a role in the formation of the aquifer on the continental shelf, Person explains, you need to look for water that has been depleted in 18O – a clear sign that it came from ice sheets melting at the base. Person’s team used existing global isotope records from the shells of animals living in the deep ocean near the aquifer. (The garnets contain carbonate, an ion that contains oxygen from the water).

Persoon then incorporated methods developed in 2019 by a Columbia graduate student that use electromagnetic imaging to accurately map submarine aquifers. Because salt water is more electrically conductive than fresh water, the boundaries between the two types of water are clear when electromagnetic pulses are sent through the seabed: salt water conducts the signal well, and fresh water does not. The results look a bit like a heat map, showing areas where fresh and salt water are concentrated.

Persoon compared the electromagnetic and isotope data to his models to see which historical scenarios (ice or no ice) would be statistically likely to form an aquifer that matched all the data. His results, which are under review at the Geological Society of America Bulletin, suggest it is highly likely that ice sheets played a role in the aquifer’s formation.

“There’s a lot of uncertainty,” Person said, but “it’s the best we can do.”

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