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Submarine Groundwater Discharge Creates "Iron Curtain" |
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Matthew A. Charette, Assistant Scientist, WHOI
WHOI 2001 Annual Report
Nearly 97 percent of Earth’s
freshwater reservoir exists as
groundwater, yet groundwater
is often neglected in calculations of
freshwater and associated dissolved
substance input to the coastal zone.
Groundwater movement into salt-water
bodies, called submarine groundwater
discharge, occurs wherever the
water table is above sea level and hydraulically
connected to the ocean. Until
recently, however, submarine
groundwater discharge has received
little attention, mainly because it is difficult
to quantify. New applications of
geochemical tracers have led to great
advances in this emerging topic over
the past several years.
Recent estimates put global submarine
groundwater discharge at 2,200
square kilometers per year, roughly
equivalent to five percent of total river
input. Though this may not represent
a significant component of freshwater
flux to the oceans, some dissolved substances
carried by groundwater may be
orders of magnitude higher than they
are in rivers or receiving water bodies
and thus account for a significant
component of the geochemical budget
of certain elements.
On Cape Cod, the key biogeochemical
problem associated with coastal
groundwater flow is the introduction
of “new” nitrogen entrained by
groundwater plumes passing through
septic tank fields located along the
coastline. It is not unusual for Cape
Cod groundwater to contain dissolved
inorganic nitrogen concentrations
ranging from 100 to 1,000 times greater
than receiving water concentration.
This has caused eutrophication of
coastal embayments where much of
the nitrogen is stored as particulate nitrogen
(for example, as macroalgae).
A number of investigations have
considered the impact of submarine
groundwater discharge on a wide
range of spatial scales from global
fluxes to processes occurring at the
groundwater/ocean interface. This
zone was recently termed the “subterranean
estuary” by 1999 WHOI
Ketchum Award winner Willard
Moore (University of South Carolina).
He describes parallels with the surface
estuary, including that both are mixing
interfaces for freshwater and seawater
bodies and both are significantly
impacted by human activities.
Since early 2000, with support from
the National Science Foundation, Ed
Sholkovitz and I have been studying
the subterranean estuary of Waquoit
Bay, a large semi-enclosed surface estuary
located about 15 kilometers (9
miles) east of WHOI. On a routine
sampling trip, we observed relatively
high concentrations of dissolved iron
in the groundwater at the head of the
bay. From prior studies, we knew that
approximately 30,000 cubic meters of
submarine groundwater discharge per
day were flowing into the bay. Given
the iron content of the groundwater,
this meant that nearly 30 tons of iron
was being carried toward the bay each
year. But where was it going? There was
some surficial evidence of iron-stained
sands in the northeastern portion of
the bay, but not nearly enough to balance
the calculated input rate.
This led us to hypothesize that an
“Iron Curtain” was forming in the
subterranean estuary beneath the
head of the bay. Sediment cores we
collected from the intertidal zone on a
cold and rainy day in April 2001 validated
our hypothesis. They exhibited
iron oxide-rich sands colored dark
red, yellow, and orange and formed by
the oxidation of iron-rich groundwater
near the groundwater-seawater interface.
The iron oxide content of
these “Iron Curtain” sediments was
four to six times greater than that of
surface sands and 10 to15 times
higher than that of sands collected
from an off-site location.
Many elements readily attach to the
surface of iron oxides in the presence
of oxygen. Because of this immense
binding capacity, the Iron Curtain
sediments created a geochemical barrier
by retaining and accumulating certain
dissolved chemical species along
with the iron. Indeed, phosphorus concentrations in the iron oxide-rich sands
of Waquoit Bay were five to seven times
greater than in overlying surface sands.
The formation of an Iron Curtain
requires an aquifer that is rich in dissolved
iron and flowing into oxygenated
coastal sediments. Iron Curtains
may be common in coastal regions:
Conditions favorable to their formation
have been described for a sandy
German beach on the North Sea, New
York coastal areas, a freshwater flood
plain in Ontario, Canada, and in wetlands
of New Zealand’s North Island.
As we work toward a better understanding
of the geochemistry of subterranean
estuaries, we expect to find more
Iron Curtain type conditions. The next
step will be to examine their implications
for the health of coastal waters.
Originally published: May 1, 2001
Last updated: March 10, 2012 |
