Study: Most mercury in Lake Superior comes from atmosphere
on Jan 13, 2016
A new study by the U.S. Geological Survey confirms that most of the mercury in Lake Superior is coming from airborne deposition, mercury that floated around in the atmosphere before falling into the lake in rain and snow.
The study results, released Wednesday by Wisconsin Sea Grant, show that the more remote Great Lakes — Superior and Huron — get most of their mercury from the atmosphere while the lower lakes in more developed areas — Erie and Ontario — get most of their mercury from point sources such as industry or runoff from on shore.
Lake Michigan’s mercury comes about half from each source, runoff and air, the research concluded.
The study used a new kind of “chemical fingerprinting,’’ comparing isotopes to determine the source of the mercury. And researchers involved say it shows that, for Lake Superior, continued emphasis needs to be placed on reducing mercury that is going into the air and later falling into the water.
Researchers collected sediment samples from 58 locations across the Great Lakes for the project, analyzing them for stable isotopes of mercury and used those chemical “fingerprints” to determine sources. They compared the mercury signatures in the lakes against those previously found in lake trout and burbot (eelpout) from the same lakes.
The mercury in the fish in Huron and Superior more closely resembled mercury from the atmosphere rather than mercury from lake sediment.
“It tells us that you can’t just solve the legacy mercury issues, but we have to shut off the mercury that’s going into the atmosphere if we are going to reduce mercury in Lake Superior and Lake Huron,” Dave Krabbenhoft, a Wisconsin-based mercury researcher for the USGS who took part in the study, told the News Tribune. “I think what really surprised us was how different it (the source of most mercury) was from one end of the Great Lakes to the other.”
James Hurley, Wisconsin Sea Grant director and part of the research team, said “determining where mercury comes from is important because it helps us figure out the best way’’ to stop it from going into the environment.
While the study used new methods, the results bolster what others have found in past years — that even lakes in remote areas like northern Minnesota can contain high levels of mercury that falls from the sky. That mercury can be transformed into a toxic state, called methylation, in plants and then moves up the food chain, contaminating fish and animals that eat fish, including people.
Mercury is a potent, bioaccumulating neurotoxin that can cause developmental and neurological problems, even death in high doses. It is especially dangerous for fetuses and young children.
Some mercury that goes into the atmosphere occurs naturally, such as from volcanoes. But much of it comes from burning coal and other industrial processing, such as making taconite iron ore pellets. Mercury that falls into Lake Superior is likely coming from as far away as China and as close as Minnesota.
Minnesota officials for years have been working to remove mercury from land-based sources such as batteries, switches, thermostats and dental fillings. The state also has moved to reduce coal-burning power plant contributions and is helping to develop new ways to reduce mercury from taconite processing.
Still, many Northland lakes and rivers hold fish that are unsafe for some people to eat because of high mercury levels. A recent study found that nearly 1 in 10 newborn children in the Lake Superior region has levels of mercury in their blood considered unsafe by federal guidelines.
President Obama also has proposed power plant guidelines that will result in less mercury going into the atmosphere, and a recent global treaty sets limits for all nations to reduce mercury levels.
Global mercury emissions declining
A related study released Wednesday shows some good news on the mercury front, finding efforts to cut mercury emissions in North America and Europe have more than offset increases in Asia and other parts of the globe.
Between 1990 and 2010, global mercury emissions from manmade sources declined 30 percent, according to the joint effort by Harvard University, Peking University, the U.S. Geological Survey, the Max Planck Institute for Chemistry and the University of Alberta.
The reduction in atmospheric mercury was most pronounced over North America and Europe, where several factors helped, including phasing mercury out of commercial products, controls on coal-burning power plants and a move to more natural gas to generate electricity. Researchers also noted that efforts to combat acid rain pollution, requiring pollution control devices to scrub nitrous oxide and sulfur dioxide out of smokestacks, had the unintended benefit of also capturing mercury.
Reductions in U.S., Canadian and European mercury emissions have helped reduce the overall atmospheric load during the past 30 years even as more mercury was coming from Asia.
The data “shows that local and regional efforts to reduce mercury emissions matter significantly. This is great news for focused efforts on reducing exposure of fish, wildlife and humans to toxic mercury.”
The findings are important for government officials and regulators who may be debating additional mercury restrictions because it shows their regional efforts can make a “tangible’’ effect on global mercury totals, said Vincent St. Louis of the University of Alberta, a co-author of the study with Krabbenhoft and others.
The study entitled “Observed decrease in atmospheric mercury explained by global decline in anthropogenic emissions” was published in the Proceedings of the National Academy of Sciences.
Krabbenhoft also is involved in new studies that will help fingerprint exactly where mercury isotopes are coming from — local, regional or global sources — and another study in Wisconsin and Minnesota that will determine if mercury in St. Louis River and Fox River fish is coming from old mercury in the sediment or new mercury in the atmosphere.
“Federal employees are taxpayers, too. And we really want to know if paying hundreds of millions of dollars to dredge up mercury (in the sediment) is really going to do any good when the mercury getting into the food web may actually be coming from the atmosphere,’’ Krabbenhoft said.
The Madison-based scientist also is starting a project to determine where Lake Superior region mothers and their babies are picking up those high mercury levels. It’s likely from eating high-mercury fish, experts say, but it’s not clear if that means Lake Superior fish, inland lake fish or ocean fish from the supermarket.