Chasing freshened groundwater beneath the seafloor
PhD student Sophie ten Hietbrink just came back from an expedition outside the United States east coast searching for freshwater beneath the seafloor.
You might not expect to find freshened groundwater beneath the seafloor, hidden under a salty ocean. Nevertheless, researchers have discovered offshore freshened groundwater reserves in many places around the world. One of those regions lies off the coast of New England in the northeastern United States, where a large volume of freshened groundwater is stored beneath the seafloor. This summer, the scientific drilling expedition 501, supported by the International Ocean Drilling Programme (IODP3) and the U.S. National Science Foundation (NSF).
Bringing together expertise from several areas
The expedition, called IODP³-NSF Expedition 501 New England Shelf Hydrogeology, is unique in many ways. It brings together ocean drilling specialists from IODP3 with hydrogeologists, experts who study groundwater movement and composition. Until now, no dedicated expedition has applied hydrogeological techniques on such a large scale to offshore groundwater systems. Many of the methods commonly used to study groundwater on land are, for the first time, being deployed in the marine environment.
In May the expedition left the port in Bridgeport (Connecticut) in the US on the vessel L/B Robert and went out to the drilling area. 41 scientists are part of the science team on the expedition, that will last until August 9.
Onboard the lift-boat platform
One of the researchers that has been on board the lift-boat platform used to drill into the seafloor is Sophie ten Hietbrink, PhD student at the Department of Geological Sciences, Stockholm University. (She was onboard until June 21 when she left the platform on the rotation scheme for the researchers.) Sophie ten Hietbrink is writing her PhD thesis about offshore groundwater freshening driven by ice sheets, glaciers, and permafrost.
“There is thus considerable overlap between my personal research interest and the goals of the expedition, so I was very excited when I was given the opportunity to contribute to the scientific endeavor of this expedition,” she says. One of the tools she uses is the radiocarbon dating of dissolved carbon in the groundwater, from which groundwater ages can be derived. This will help us to figure out when the fresh and saline groundwater was emplaced, providing clues about the processes that drive the offshore groundwater flows.
Preparing water samples and running measurements
Photo: Antonia Witzleb/ECORD:IODP3_NSF
Onboard she has been working as a geochemist. This means that she split and prepared the water samples from the sediment cores and from the groundwater that is pumped up for all the different analyses that will be done later by many different scientists. In total there are well over 20 analyses planned for the water samples. Furthermore, she ran some measurements already on the ship, such as salinity, which provides crucial information for the operational planning. For example, salinity data can help the researchers to decide from which depth they want to pump up groundwater to sample.
Operations running 24 hours a day
The researchers are working on a liftboat, a special type of vessel that can elevate itself above sea level by standing on three legs. This creates a stable platform from which they can safely drill. Operations run 24 hours a day, every day of the week. To ensure plenty of hands are always available, the researchers work in rotating day and night shifts. Sophie ten Hietbrink worked on the day shift, which means that she was on duty from 12:00 to 00:00.
“Although the hours are long, it has been incredibly rewarding to be right in the middle of the action and to witness science happening in real time. I also really enjoyed meeting new researchers from all around the world, with each bringing their own unique expertise to the table. Expeditions like this really are a fantastic opportunity to learn directly from a variety of experts.”
Stockholm University will host the ESSAC office
Sophie ten Heitbrink is the only scientist from Stockholm University that participates in this summer´s expedition. But there are other researchers at the university involved in the IODP3 activities. Through the years, Swedish researchers have been involved with a lot of IODP expeditions. Professor Matt O´Regan at the Department of Geological Sciences, Stockholm University is currently the Swedish ESSAC representative in this activity.
Matt O´Regan will also host the office for Science Support and Advisory Committee (ESSAC) for the consortium of European countries that contribute to the International Ocean Drilling Program in Stockholm from next year (2026). Professor Helen Coxall at the same department has sailed on several previous IODP expeditions. The involvement of Swedish scientists in IODP³ is supported by the Swedish Research Council (Vetenskapsrådet), since IODP³ is a regarded as a national Swedish infrastructure.
Read more about Sophie ten Hietbrink
More about the expedition
The IODP³-NSF Expedition 501 New England Shelf Hydrogeology aims to understand how groundwater systems under land are connected to sub-seafloor groundwater systems discovered in the past 50 years. The expedition also aims to understand the role that sea-level change and glaciation may have played over tens-to-hundreds of thousands of years in charging these offshore aquifers with freshened/slightly saline water.
As part of this three-month effort (ending in August), the expedition will use a lift-boat platform to drill into the seafloor in three locations south of Martha’s Vineyard on the American east coast to collect water and sediment samples to better understand these aquifers. Upon completion of the offshore operations, the wells that are drilled will close naturally and reseal the aquifer. No additional water will be collected and no infrastructure for additional water sampling or pumping will be left on the seafloor or in the wells.
Onshore operations will take place in January and February 2026 at MARUM, University of Bremen.
The ECORD Science Operator will manage the mission-specific platform expedition on behalf of IODP³ and NSF. The co-chief scientists are professor Brandon Dugan (Colorado School of Mines), professor Karen Johannesson (University of Massachusetts Boston) and professor Rebecca Robinson (University of Rhode Island).
Read more on the expedition website
Follow the expedition blog
Last updated: June 26, 2025
Source: Communications Office