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Researchers Find Substantial Amount of Mercury Entering the Ocean through Groundwater

March 22, 2007

Researchers from the Woods Hole Oceanographic Institution (WHOI) have found a new and substantial pathway for mercury pollution flowing into coastal waters. Marine chemists have detected much more dissolved mercury entering the ocean through groundwater than from atmospheric and river sources.

Mercury is toxic to animals and humans in large concentrations, particularly in the form known as methyl mercury, which accumulates in fish. To date, WHOI researchers examined total mercury, not the more biologically dangerous form, though that is a logical next step. These initial findings of mercury moving through the coastal groundwater system are significant for researchers trying to quantify the impact of mercury in the marine environment.

The lead author of the study is Sharon Bone, a former undergraduate summer student fellow and research assistant in the laboratory of WHOI marine chemist Matt Charette. Bone is now a first-year graduate student at the University of California at Berkeley.

The findings were published online on March 21 by the journal Environmental Science and Technology and will appear in a printed issue later this spring.

Mercury pollution comes mostly from industrial emissions to the atmosphere, especially from coal burning. After getting into the air, mercury particles eventually precipitate with rain or snow onto the land or directly into the oceans. Inland deposits of mercury are also weathered and carried to the coast in runoff from streams and rivers, where they accumulate in the sediments that build up along the shoreline.

At the same time, wherever aquifers are connected to the ocean, fresh groundwater can be discharged and salty sea water can penetrate landward into groundwater—both passing through and picking up this mercury embedded in the sediments. This phenomenon of "submarine groundwater discharge" has been receiving more attention in recent years because scientists have shown that the flow of groundwater into the ocean carries a substantial amount of dissolved nutrients, metals, and trace elements.

"This pathway for delivering nutrients and contaminants into the ocean has long been overlooked and ignored because it was difficult to quantify," said Charette, whose lab has advanced such methods in recent years. "This study is a first of its kind for quantifying the amount of mercury flowing out of the system."

Working in Waquoit Bay in Falmouth, Mass., Bone and colleagues started by analyzing cores of coastal sediments, observations from shoreline wells, and measurements of submarine groundwater flow to determine the amount of mercury flowing out of the subterranean estuary. Then, while sampling surface waters, Bone detected concentrations of dissolved mercury in the bay that were much higher than would be expected from simple atmospheric deposition and runoff.

Charette and fellow chemist Carl Lamborg of the WHOI Department of Marine Chemistry and Geochemistry couldn't believe what they were finding. "We were surprised by how much mercury we detected in Waquoit Bay," said Lamborg. "We thought, 'this can't be right,' and went back to the lab to check the results several times. We realized that if these numbers are right, then something unusual must be going on."

After checking and rechecking their methods and data, the research team found total mercury concentrations that were an order of magnitude (at least 10 times) higher than what should be deposited by simple outfall from the atmosphere, and substantially more mercury than could flow in from local streams. The source had to be submarine groundwater pushing mercury out from the sediments.

Once deposited in water, mercury is often changed by microbes into methyl mercury, a known nerve toxin that can accumulate in the tissues of marine animals that consume the microorganisms and smaller fish. Methyl mercury is particularly dangerous to the developing nervous systems of fetuses, young children, and animals. In recent years, researchers have cautioned pregnant women to limit the consumption of certain types of fish due to concerns about mercury contamination.

Charette and colleagues hope to return to Waquoit Bay and other sites to examine which forms of mercury—including toxic methyl mercury—are present, in what concentrations, and how they are moving through the environment. They would also like to see studies of the effect of the mercury on the biological systems in the area.

Funding for this research was provided by the Chemical Oceanography division of the National Science Foundation, the WHOI Postdoctoral Scholarship program, and the WHOI Summer Student Fellowship program. WHOI would also like to thank the Waquoit Bay National Estuarine Research Reserve for its support as the host site for this research.

Woods Hole Oceanographic Institution