Fresh water hidden beneath the sea:
A new frontier for water security
Sara Polanco on IODP3-NSF Expedition 501: New England Shelf Hydrogeology
Off the coast of New England lies a hidden reservoir of ancient, fresh water, locked in sediments beneath the seafloor.
ANZIC expeditioner, Dr Sara Polanco, is part of a team of scientists heading out to study it.
Their discoveries could reshape our understanding of offshore groundwater systems, past climates, and water security in a warming world.
Sara’s childhood dream was to sail out into the ocean and make scientific discoveries, just as Darwin and Humboldt had in the 19th century. Now, that dream is close to becoming a reality.
Sara is ANZIC’s expeditioner on Expedition 501 and it’s obvious that the hunt for knowledge, the need to understand, and the adventure of the sea are at the heart of her motivation.
“When I was a child, learning about the great voyages of discovery in the 19th century— sailing the oceans and uncovering new science — I remember thinking, ‘How amazing would that be?’”
“So it’s a dream come true to be part of this important expedition,” Sara says. “And it’s exciting to be studying something as surprising and vital as fresh water beneath the seafloor!”
Dr Sara Polanco is a geoscientist and Honorary Senior Lecturer at the University of Sydney and a Lecturer at the University of Newcastle. She has been selected as a member of the science party for Expedition 501, which is run jointly by IODP3 and the US National Science Foundation. The expedition will explore groundwater systems beneath the seafloor off the east coast of North America.
There’s fresh water under the sea?
It’s relatively recently that we learned that – yes – there’s fresh water under the seafloor. Not in cavernous lakes or bubbling springs, but stored within the pores of sediments and rocks beneath the ocean. This offshore groundwater can be much fresher than seawater and may stretch for hundreds of kilometres along continental margins.
This sub-seafloor ground water is ancient – it is thought to come from rain and/or meltwater from past ice ages, trapped when sea levels were lower and the coastline stretched further out. As the oceans rose, these freshwater systems became buried beneath layers of sediment, quietly sealed beneath the sea.
One of the best understood examples of an offshore freshwater system is found on the Atlantic continental shelf off New England where groundwater within aquifers over 100 km offshore has low salinity.
How old? How fresh? Time to drill in.
Expedition 501: New England Shelf Hydrogeology will drill deep into these reserves to discover how vast they are, how the salinity changes as you move further from the coast, and to solve the mystery of where the water came from and how long it is been stored offshore.
The science team will be drilling, coring, logging, and analysing the water stored between the grains of sediments and rocks so that we can obtain a complete characterisation of these sub-seafloor aquifers.
“We will be doing isotopic analysis of the water we collect from sediment pores,” explains Sara. “The ranges of different various isotopes are like a fingerprint. Melt water from a glacier is different to that which falls as rain. So we will be able to determine the source of this trapped water.”
“Did this freshwater seep into the rock from the base of melting ice sheets, infiltrate from glacial lakes, directly from rain water or a combination?”
The age of the water is unknown too. Was it being slurped up by woolly mammoths and mastodons — or was it trapped beneath the seafloor long after they disappeared? Expedition 501 aims to find out.
“It’s a remarkable thing about water – it’s the same water that has always been on Earth. We drink the same water the dinosaurs drank. Water is neither created nor destroyed, the only thing that changes is the form that water takes as it travels through the water cycle,” Sara reminds us.
It’s a thought that puts into context the importance of the research being undertaken.
The shifting shape of coastlines
Characterising the age and chemical signature of the groundwater is only part of the picture. Understanding how the sediments beneath the sea floor are organised – their layering, composition, and structure – is just as important. These features shape how water moves, where it can be stored, and how it might have been recharged during past periods of low sea level.
As well as exploring offshore groundwater, Sara is working to better understand how the shape of the continental shelf has changed through time.
“My research looks at how and why landscapes change in time and space,” Sara explains. “I work to decode the dynamic evolution of landforms like the continental shelf – landscapes that shift as sea level rises and falls. One of the key questions I ask is: what is the interplay between sea-level change, sediment deposition, and the reshaping of these coastal environments?”
Sara will take home the data collected on the expedition and use it to build 3D numerical models of the region. These models will help reconstruct how the New England shelf has formed and evolved over thousands of years – and how processes like sea-level fall, sediment loading, and the slow bending of the Earth’s crust (flexural isostasy) have influenced both the shape of the shelf and the flow of groundwater beneath it.
“The relationship between sea-level change and the exposure of the shelf plays a big role in how and where freshwater can accumulate,” she says. “When the shelf is exposed during periods of low sea level, rainwater can seep into the sediments and rocks – recharging offshore aquifers.”
Her research aims to reveal how aquifers beneath the sea floor are connected to the landscapes onshore, offering new insights into the hidden links between changing coastlines, groundwater systems, and sea-level history.
Tapping into offshore water
The knowledge built by scientists on Expedition 501 won’t only illuminate ancient and fascinating processes. There is also a pressing need to understand these under-sea freshwater systems today.
“If we want to make use of this water, we need a full understanding and characterisation of these systems,” Sara explains. “If we don’t quantify it and understand how aquifers beneath the sea are connected to those we can access from the shore, we can’t know how to sustainably use them.”
“We need good science to underpin how we tap into this water in the future.”
Beyond their potential as a resource, these offshore freshwater systems could also play an important role in helping to mitigate salinity intrusion into coastal aquifers – a growing concern as sea levels rise and put pressure on freshwater supplies.
And what the expedition uncovers off the coast of the New England can be applied back home.
“This is important for Australia. We have similar systems of offshore freshened groundwater here,” she says. “They are potentially immense.”
“The workflows, methods, skills and knowledge from this expedition can be applied to Australia. And being the driest inhabited continent on earth, with a mostly coastal population, that’s going to be crucial for water security in coastal areas in the future, especially as sea level rises.”
Like a true adventurer, Sara’s not daunted by any of the potential drawbacks of life at sea – like seasickness, close quarters, or long shifts.
“I’m really looking forward to the 12-hour shifts!” she admits. “Being completely immersed in one thing – just the science – I’m so stoked to be involved!”
Learn more
Follow along as the expedition sails via the Expedition Blog or check out the FAQ.
Details of the research in the Scientific Prospectus and the expedition page.