Researchers have uncovered previously hidden
sources of ocean pollution along more than 20 percent of America's coastlines,
including the Pacific Northwest.
The study, published online Aug. 4 in the
journal Science, offers the first-ever map of underground drainage systems that
connect fresh groundwater and seawater, and also pinpoints sites where drinking
water is most vulnerable to saltwater intrusion now and in the future.
Audrey Sawyer, assistant professor of earth
sciences at The Ohio State University and leader of the study, said that while
scientists have long known that freshwater and seawater mix unseen below
ground, until now they hadn't been able to pinpoint exactly where it was
happening, or how much.
Together with partners at NASA's Jet
Propulsion Laboratory, Sawyer was able to learn more about the previously hidden
water exchanges via computer analysis, and without extensive and costly field
"We're all pretty familiar with the idea
that rain falls on land and flows out to the ocean in rivers, but there's
another, hidden component of rainfall that infiltrates the ground near the
coast and spills into the ocean below sea level," Sawyer said. "If
you've ever been swimming in the ocean and felt a cold spot, there's a good
chance that the cold spot is due to groundwater seeping out from underfoot."
"We call it 'submarine groundwater
discharge,'" she continued. "Freshwater flows out to sea, and vice
versa. Urbanization, agricultural development, climate, and topography all
affect how much water flows in either direction, and the exchange has a big
impact on both onshore groundwater that we drink and offshore seawater where we
swim and fish."
The study identified 12 percent of the
continental U.S. coastline -- including the northern Gulf Coast from
Mississippi to the Florida Panhandle, northern Atlantic Coast and Pacific
Northwest -- where the once-hidden drainage systems make the ocean most
susceptible to freshwater contamination from septic tanks and fertilizer
runoff. There, excess nutrients in the water can cause harmful algal blooms to
form and remove vital oxygen from the water. This contamination from land to
sea endangers fisheries and coral reefs as well as water recreation and
In contrast, another 9 percent of coastline --
including Southeastern Florida, Southern California, and Long Island--are
especially susceptible to the opposite threat: contamination from sea to land,
the study found. In these areas, saltwater intrudes inland and infiltrates the
fresh groundwater supply.
"It takes only a small amount of
saltwater to render drinking water non-potable, so saltwater invasion is a big
concern for water resource management in coastal areas," Sawyer said.
Among the sites on the map with the worst
impacts are Los Angeles and San Francisco, which the study found to be
vulnerable to both ocean contamination and saltwater intrusion simultaneously.
Overall, more than 15 billion tons of
freshwater flows through invisible underground networks into the ocean along
the continental U.S. coastline every year, the researchers found.
That sounds like a lot of water, but it's less
than 1 percent of the total amount that flows from the continental United
States into the ocean, pointed out study co-author Cédric David of JPL. The
other 99 percent comes from rivers and surface runoff.
Still, David explained, the study is
significant because it provides the first continental-scale high-resolution
estimate of that 1 percent--a portion which, when compared to the other 99
percent, can be particularly rich in nutrients and other contaminants.
"This Ohio State-JPL collaboration has
removed the cloak from hidden groundwater transfers between land and sea,"
Sawyer, David and James Famiglietti, also of
JPL, combined U.S. topography and climate models to identify key inland regions
that contribute groundwater and contaminants to the coast. They examined
rainfall, evaporation rates and the amount of known surface runoff to calculate
the missing portion of water that was running out below ground, and melded
those results with terrain and land-use data to identify where the water ended
For example, precipitation is similar in the
Pacific Northwest and the mid-Atlantic regions, but the study found that
underground drainage rates into the ocean were approximately 50 percent higher
in the Pacific Northwest because the steep terrain there carries more
groundwater to the coastline.
Land use was critical to discharge in Florida,
the study found, and Sawyer said that she was surprised by the big effect that
canals had there.
Since the early 20th century, Floridians have
constructed thousands of miles of canals along the state's coasts for
transportation, irrigation and recreation. The study found that the canals
capture water that would otherwise flow underground and out to sea.
As an example, Sawyer cited four adjacent
counties along the Gulf Coast that have dramatically different amounts of
discharge depending on the number of canals: Pinellas and Hillsborough
counties, which have many canals, had about half as much below-ground drainage
as Pasco and Hernando counties, which don't.
The researchers commented that increased
urbanization -- and the extensive pavement that goes along with it -- will also
decrease submarine groundwater drainage in coastal regions where the population
is growing, which increases the likelihood of saltwater intrusion.
"That's why we hope others will use our
analysis to better plan strategies for coastal land development and groundwater
management that help preserve water quality," Sawyer said. "Right
now, we've created a map of American coastlines, but we hope to be able to do
it for the world shortly, as data become available."