When the Mercury Falls: Autumn leaves taint river with poison

Fall leaves that drop into stagnant waterways could release significant doses of a highly toxic form of mercury, new research suggests.

LEAFY. Methylmercury concentrations spiked in Minnesota’s Little Cobb River after leaves fell in autumn. Balogh/Metropolitan Council Environmental Services
LEAFLESS. County Ditch 86 saw only a small rise in methylmercury. Balogh/Metropolitan Council Environmental Services

Mercury-tainted fish pose a considerable health risk to people (SN: 3/9/91, p. 152). Before the metal can enter the aquatic food chain, however, it must be converted from its common, inorganic form into the organic compound methylmercury. That process, promoted by bacteria, occurs readily in wetlands, which are rich in organic matter but oxygen-deprived, and landfills (SN: 7/7/01, p. 4: Landfills Make Mercury More Toxic).

Leaf-littered waterways represent another source of methylmercury, according to the new study. For 9 months during 2000, Steven J. Balogh and his colleagues at Metropolitan Council Environmental Services in St. Paul, Minn., monitored concentrations of mercury and methylmercury at sites on two southern Minnesota waterways, the Little Cobb River and County Ditch 86. These waters feed into a tributary of the Minnesota River and eventually join the Mississippi.

Overall concentrations of mercury were highest in May and June, when spring melt and rains washed sediments into the rising waters. Methylmercury concentrations, however, remained low at that time, suggesting that most of the mercury flushed into the waters was in the inorganic form.

The largest spike in methylmercury along the Little Cobb River occurred late in the fall, during a dry period when the river was usually stagnant. There, in early November, the researchers found as much as 4.9 nanograms of methylmercury per liter of water. However, they didn’t find more than one-tenth that much methylmercury in the stagnant County Ditch 86. Earlier in the year, the methylmercury concentration in both waterways was typically below 0.5 ng/l.

The explanation for the autumn difference between the waterways lies in the falling leaves, Balogh suggests. The Little Cobb River’s course is heavily wooded with deciduous trees, which drop leaves into the river. County Ditch 86, on the other hand, has almost treeless banks.

Stagnant conditions reduce the amount of oxygen present in accumulations of dead leaves, so bacteria thrive, the researchers say. Those bacteria could have methylated the mercury that leaves, while alive, absorbed from the atmosphere, they add.

In experiments, methylmercury concentrations rose more in leaf litter decaying in water collected from the river than in litter in the laboratory’s distilled water. That suggests methylmercury production occurs in the stagnant river, the team reports in an upcoming issue of Environmental Science and Technology.

Judging from this “very original study,” substantial amounts of methylmercury could be accumulating in fish living downstream from leafy areas like those along the Little Cobb River, says Drew Bodaly of Fisheries and Oceans Canada in Winnipeg, Manitoba. He notes that the concentrations of methylmercury found in the river in November were as high as those in rivers near industrial pollution sources.

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