Multi-National Effort to Study Fresh Water Found Beneath Ocean off New England
By Bonnie Phillips / ecoRI News staff
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| British Geological Survey hydrogeologist Rachel Bell, left, discusses groundwater sampling logistics in front of the sampling manifold with Expedition 501 co-chief scientist Rebecca Robinson of URI. (Maryalice Yakutchik/ECORD IODP³) |
On May 19, the L/B Robert chugged out of Bridgeport, Conn.,
heading to an area of the Atlantic Ocean off the island of Nantucket.
The Robert isn’t a fishing boat, or at least the kind that
brings back fish, crabs or lobster.
This expedition was fishing for water. Fresh water. Under
the ocean.
How can that be, you ask? How can fresh water be underneath
the ocean floor? And how did it get there?
Rebecca Robinson, a professor of oceanography at the University of Rhode Island’s Graduate School of Oceanography, is attempting to answer those questions.
Robinson was one of three chief scientists leading the
multi-national team of 41 researchers on the first-of-its-kind New England Shelf Hydrogeology expedition aboard the
Robert to study water and sediment samples taken from up to three
locations beneath the ocean on the New England Shelf.
After 74 days offshore, the team returned in August with 718
core samples to be analyzed in the expedition’s researchers’ respective labs.
“Sampling of this offshore freshened groundwater to the extent that we can make comprehensive geochemical assessments of its history, including its age, is unprecedented in scientific ocean drilling,” said Robinson, who is also an associate director of URI’s Coastal Institute.
‘We could see fresh water’
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| Sunrise on the Robert. (LEBER@ECORD_IODP3_NSF) |
The interest in undersea fresh water off the New England
coast was sparked in 1976, when the U.S. Geological Survey drilled a test well in the middle of Nantucket to see
how far down the island’s groundwater went. The “water came from so deep that
it seemed like it in was in the seafloor and not in the glacial till that makes
up the island,” Robinson said of the results.
Underground, water is contained in geologic reservoirs
called aquifers. These water-bearing rock layers store the groundwater that
makes up about 90% of the total available fresh water in the United States.
The age and composition of aquifers are variable. Some are
shallow and easily refilled by rain, but others are deep and contain water that
has remained there for millennia, perhaps since the last glacial period. The
type of rock also differs regionally, from the limestone strata below Florida
to the sedimentary layers in the Northeast.
“We drilled through stuff that looked like beach sand, and
through really tight clays near shore,” Robinson said. “Once we got to a
certain depth, it didn’t matter if it was clay or sand, we could see fresh
water.”
Fresh water reaches aquifers beneath the seabed through two main processes.
First, rainwater that falls on coastal land can percolate
down into an onshore aquifer and flow laterally through highly permeable rock,
extending under the shoreline and out to the seabed. For this fresh water to
travel long distances, it must be contained by a hard cap layer, often made of
highly compacted clay. While loose clay holds water, compacted clay is nearly
impervious, trapping the less-dense fresh water below the seafloor.
Robinson explained it this way: “The clay is hard to move water through. Think of sand as being like a pool filter and clay like a concrete pool wall.”
The second mechanism relates to past ice ages. When global
ice sheets grew, they dramatically lowered the sea level, exposing large
portions of the continental shelf as dry land. During the last major ice age
(roughly 12,000 to 20,000 years ago), rain falling on this exposed shelf could
have percolated into the subsurface, becoming trapped beneath a cap layer. When
the ice melted and sea levels rose again, this water remained in place.
Researchers on the expedition will use various techniques in
their labs to determine the age of the fresh water off Nantucket, whether it’s
from glacial melt or coming from sources on land. What the researchers learn
will be an important part of determining whether the water is a renewable
resource.
“If it’s on the young side, hundreds or thousands of years
old, it’s likely being replenished by rain,” Robinson said. “If it’s really
old, that suggests that it’s a confined aquifer, which is really finite. If we
took [the water] out, it wouldn’t refill.”
Knowing how the water got there “will help,” Robinson said.
“If it’s connected to land actively, then it probably started as an aquifer,
maybe when sea level was lower, and has been recharging. If it’s totally
isolated, it will help to know if it happened during the last glacial maxim or
prior to that, or if it was multiple sea level changes that caused it.”
Drilling for water
The $25 million Expedition 501 was
organized and funded by the European Consortium for Ocean Research Drilling as
part of the International
Ocean Drilling Programme (IODP³) and the U.S. National Science
Foundation.
The location of the fresh water was pinpointed using electromagnetic transmitters towed by a boat. The transmitters send electromagnetic signals into the ocean which, once they reach the sea floor, become stronger or weaker depending on the material through which they are passing.
“Salt water and fresh water have different ions and
different electrical resistivity,” Robinson said. Freshwater is a poor
conductor of electricity, so the transmitters can distinguish it from
saltwater.
Once the fresh water was mapped, drilling could begin.
The 185-foot liftboat Robert, once on the site,
lowered three massive pillars to the seafloor to provide a stable base above
the waves for the drilling operation. The ship was equipped with a small
drilling rig, and 50,000 liters of water was pumped up from several different
depths at three locations.
“It was a challenge to pump significant amounts of
groundwater out of the wells without destabilizing them,” Robinson said. “To
prevent a column of sediment from collapsing, we had to be strategic about
where we were pumping, the flow rate through the equipment, and where we placed
our equipment.”
The water samples were tested for, among other things, salinity. Some samples registered salinity of just 4 parts per thousand. The ocean’s average salt content is 35 parts per thousand, and the drinking standard for fresh water in the U.S. is less than 1 part per thousand.
“We were sampling the water that was in between the grains
[of sand] while we were out there,” Robinson said. “We could measure the
salinity of the water while we were at sea.”
Robinson, who said she was thrilled by the expedition’s
“success in sampling such difficult formations and with the astonishing amount
of water we were able to recover for science,” will study the origins and
history of the nitrogen in the groundwater by examining the composition of the
samples in her lab at the University of Rhode Island.
“We will study the nitrogen cycling of the water and how it
is impacted by the freshened water,” said Robinson, whose research focuses on
the chemical cycles of nitrogen and carbon in the ocean. “All organisms need
nitrogen for life, so its cycling marks different types of microbial
processing. Learning what happens along its flow path can tell us something
about its history,” including what types of microbes are there, what they used
for nutrients, and what byproducts they might generate, which could affect the
drinkability and usability of the water.
Robinson will also determine the source of the nitrogen. “Is
it organic matter that sank to the sea floor or is it part of the fresh water
that came in?” she said.
What’s next
The discovery of possibly drinkable water under the ocean off the coast of New England could have vast implications for a world that is becoming increasingly dry.
According to a story by ProPublica, much of the Earth is suffering a
pandemic of “continental drying,” affecting 75% of the world’s population.
Nearly 6 billion people in 101 countries are facing a decline in water supply.
“There are places that have offshore aquifers that are
really water hungry, islands for example,” Robinson said. “The idea that
drinkable water is just right there is really appealing. Our population is not
shrinking, and we do lots of water-hungry activities. As we move forward, this
might be potential fresh water we could access, that’s easier to clean up than
seawater and pristine enough to use for other activities.”
But she added there’s lots of work to do first, key among
that work learning where the water comes from and what will happen if the fresh
water is withdrawn. “Will it pull seawater in,” for instance, she said.
“We need to look for every possibility we have to find more
water for society,” professor Brandon Dugan of the Colorado School of Mines,
another chief scientist on Expedition 501, told The Associated Press.
Robinson said the success of Expedition 501 will trigger
more exploration. Are there other subsea aquifers? “We need to go to multiple
places,” she said. “Is there a way to tap this resource, and if so, is it a
one-size-fits-all type of thing or what factors do we need to consider? It’s
very energy-intensive to pull water from the seafloor. We have to figure out
how to do that in a way that doesn’t cause harm.”
The third chief scientist on the expedition was professor Karen Johannesson of the University of Massachusetts Boston. After six months of lab analysis by team members, the full science team will convene at the University of Bremen in Germany in January and February to further examine the cores, collect more data, and write preliminary reports about the initial findings.
