"It's another wrinkle that needs to be looked at," Sherwell said. "We are pretty sensitized to the fact that you can get [methylmercury production] in marshes."
Mitchell is not the first to document the role of swamp bacteria in producing methylmercury. The link was discovered about two decades ago, in part by Cindy Gilmour, his supervisor at the Smithsonian research center.
Gilmour said Mitchell's new work is important for calculating precisely how much mercury is oozing from the marshes. That's necessary so researchers can judge the health risk, she said.
In one four-month period, from August 2007 through November 2007, Mitchell measured 27 mg of methylmercury flowing out of one tiny creek in Kirkpatrick Marsh. That's enough to contaminate more than 100 fish above Environmental Protection Agency health advisory limits, he said.
"It's not a trivial matter. The EPA estimates that up to 10 percent of women of child-bearing age eat enough mercury in fish that they are at risk," said Gilmour, a biogeochemist. "Mercury has developmental impacts on babies' brains, especially declines in IQ."
Because of mercury contamination, the federal government advises women of childbearing age and children not to eat more than six ounces of albacore tuna per week, and no shark, swordfish, king mackerel or tilefish.
On a recent morning, Mitchell plunged into the marsh wearing tall rubber boots, a pair of laptop computers slung over his shoulders. He sloshed between curtains of tall reeds.
Then he teetered across a pond on an eight-inch-wide wooden bridge that's rotten and wobbly and lacks hand railings. "If you're going to fall, fall to your left - not your right. It's deeper there," he warned, balancing his computers.
From a machine stationed on a platform in the swamp, he retrieved 12 bottles of water. Each was a sample the device had drawn from the creek over the previous 12 hours. He would tote them back to his lab to analyze their mercury content.
As he linked his laptop to another device measuring water speed and volume, an otter paddled up, as if curious about his findings. Mitchell delivered this sobering report: "There is certainly more methylmercury coming out of this marsh than going in."
tom.pelton@baltsun.com
