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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

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