The ability of wetlands to sequester carbon and nurture wildlife is the reason a construction project of about 12,000 acres is being considered at the Blackwater National Wildlife Refuge on Maryland's Eastern Shore. A similar wetlands construction proposal in the San Francisco Bay area has been delayed for study into whether the new marshes would release significant mercury.
The issue is whether the amount of mercury flowing out of wetlands is small enough to be a minor health risk compared with the marshes' many positive qualities.
"It's a double-edged sword," Brian Branfireun, a hydrologist at the University of Toronto, said of wetlands construction. "There is a benefit to creating more wetlands habitat. But then there is a question of if you're creating a more serious mercury problem."
Mitchell is not suggesting paving over wetlands to improve public health. Rather, he said, society needs to stop spewing mercury into the air through the burning of coal. "The best away to get mercury out of fish is to stop putting mercury into the environment," he said.
But if the country is going to keep burning coal - the source of half of U.S. electricity - steps could be taken in marshes to reduce the production of methylmercury, Mitchell said. Adding activated charcoal to absorb the mercury in newly constructed wetlands might reduce the release of the contaminant, he said.
Other scientists have suggested building gravel beds under artificial wetlands to increase the flow of oxygen through the sediment, which could discourage the bacteria that create methylmercury. Or reeds and cattails could be planted instead of leafy plants like lily pads, which produce more bacteria when they die.
Mitchell said natural wetlands - such as Kirkpatrick Marsh, owned by the Smithsonian research center - should be left alone so the wildlife is not disturbed.
But to the east, on the other side of the Bay Bridge, the federal and state governments are studying a vast wetlands-building proposal at the Blackwater refuge. Funding for this effort could come in part from power companies that face new state greenhouse gas limits and want to build wetlands as a way of "offsetting" their pollution.
The Maryland Department of Natural Resources will look carefully at the mercury-producing potential of the proposed Blackwater wetlands as they are designed, said John Sherwell, manager of DNR's power plant research program.
