Forest Fire Fallout: mercury

[Guest guest]

January EHP has lots of environ articles, including this summary:

Forest Fire Fallout

http://www.ehponline.org/docs/2007/115-1/forum.html#fore

In the summer of 2005, wildfires raged over 3.4 million hectares of Alaska

and Canada's northern boreal forests, according to combined figures from

the Canadian Large Fire Database and the Alaska Large Fires Database. It

was the region's second worst fire season on record. The worst was the

year before, when 5.7 million hectares burned. The number of very large

" megafires " in the circumpolar region is increasing, says Merritt

Turetsky, an assistant professor in the Department of Plant Biology at

Michigan State University. But alongside the obvious hazards posed by

smoke and flames is one perhaps unexpected risk: emissions of mercury (Hg)

released from the peat that is relatively common in these northernmost

forests.

For peat's sake. Peatlands in boreal forests act as natural sponges of

atmospheric mercury. Significant amounts of mercury are thus released when

such forests burn, a more frequent occurrence due to climate change.

image: AP Photo/A.J. Chapman

About 80% of the world's peatland is located at high latitudes. Peat soils

absorb more Hg than other soils because the Hg is buried by the

accumulating peat after it falls to the soil surface. This Hg can then be

transformed to methyl Hg (MeHg), which accumulates as it goes up the food

chain. The EPA has deemed MeHg a possible carcinogen and set a limit of 2

ppb Hg for drinking water; the FDA put the limit for MeHg in seafood at 1

ppb.

Because they are far from human populations, boreal forest fires can burn

for weeks before they are reported, and are allowed to burn longer than

their southerly counterparts. " We've seen more than a doubling of burn

area per year since the 1950s, " says Turetsky. Dry seasons being up to a

month longer than in the past—partly a result of climate change—are a big

factor, Turetsky and coauthors argue in volume 33, number 16 (2006) of

Geophysical Research Letters. The authors estimate that as the scale of

the fires increases, exacerbated by projected global warming, the impact

of Hg on the food chain also will increase.

" Any changes in carbon cycling will influence the release of this mercury

to the environment, " notes Bindler, an assistant professor of

ecology and environmental science at Umeå University, Sweden. " The real

health concern lies in the aquatic food chain and the concentrations of

mercury in the fish we eat. "

Turetsky began to notice high rates of MeHg in her work on carbon storage.

She and coauthor Harden have quantified organic matter and Hg

concentrations in samples of frozen peat cored from sites across western

Canada. Their samples, taken from sites that varied in forest canopy, soil

drainage, and forest age, were augmented with comparable samples recorded

in a database of organic matter storage across the boreal region. They

used a 20-year record of the extent and timing of fires and a simple fire

emission model to estimate how much of this stored Hg could potentially be

released into the atmosphere as a result of wildfire activity across

western Canada. They found Hg emissions 15 times higher than previous

estimates, which had not accounted for peat's ability to store Hg.

" Many peatlands in interior North America are forested and have very dense

soils, " says Turetsky. " This forested peat can store a lot of carbon from

the atmosphere, which is good, and it can store a lot of mercury from the

atmosphere as well, which is also good—until these fire emissions occur. "

The emissions affect both the atmosphere and runoff into northern lakes

and streams.

Turetsky and colleagues have gone a step further than previous emissions

models, says Bindler, who calls their model and ideas " conceptually very

sound. " For him, a question remains, though: to what extent do the study's

estimates of soil Hg represent other boreal forests?

Turetsky hopes the study will call attention to the startling increase of

wildfires across northern North America. " We really should be paying

attention to growing toxicities in the north as well, " she says. " In the

Great Lakes, where I am, we catch lots of salmon. Increasingly, those

catches carry mercury warnings. "

A.