MN study traces mercury in St. Louis River fish back to source
For decades, health officials in Minnesota and Wisconsin have warned people to limit their meals of zander from the St. Louis River estuary in Duluth and Superior because of unsafe levels of toxic mercury.
Fish consumption recommendations are among the strictest in the Northland area of Minnesota: men should only eat one meal of large pikeperch per month, while women under 50 and children shouldn’t eat large pikeperch 20 inches or longer away from the river. Ever.
The amount of mercury in the pikeperch in the St. Louis River estuary is among the highest in either state. This is important because mercury is a powerful neurotoxin that can cause serious brain and neurological disorders, especially in children and fetuses.
But it was never clear where all that extra mercury was coming from. Until now.
After years of measuring mercury in muzzle dung, fish, and rain falling from the sky, scientists now say they know the culprit: Huge amounts of decades-old mercury pollution stored in sediment still end up in fish.
In a study published in the journal Science of the Total Environment and published online March 13, scientists from the Environmental Protection Agency’s Duluth Laboratory, the US Geological Survey, and several other institutions used isotope fingerprints to detect mercury in the To fully trace fish back to its source, distinguishing old old mercury from new mercury falling from the sky.
“There are different fingerprints for old and new mercury,” said Sarah Janssen, environmental chemist at USGS in Wisconsin and lead author of the study.
The fact is that old mercury that makes fish in the mouth of the St. Louis River contains much more mercury than in nearby lakes, or even in the same river upstream, or as Lake Superior just downstream.
The original sources of the ancient mercury may never be known. Mercury was ubiquitous in both households and industry in the 1800s and 1900s, said Joel Hoffman, a research biologist and co-author of the study who heads the ecosystem services division at the EPA’s Duluth laboratory. These sources likely included paper mills, lumber mills, steel mills, shipbuilding sites, manufacturing facilities, and other sources that once dominated the Twin Ports river and coastline.
“It was probably all over the place, sitting in big barrels, and there was no awareness that the (health) hazards are now or how to dispose of them,” noted Hoffman.
University of Minnesota Duluth researchers are collecting a sample of the St. Louis River estuary sediment to be tested for mercury in 2016. Scientists from several universities and agencies spent years collecting data to determine why so much mercury was found in Zander in the estuary near Duluth. Minn. The study concluded that much of the mercury has been in the sediment for decades. (John Myers / Duluth News Tribune)
A lot, if it’s likely to be thrown into the river. And that is where the municipal sewage came, which was often mixed with mercury from daily use such as dental fillings, thermostats, thermometers and batteries. School science classes “played” with mercury to demonstrate its properties and then often tossed the stuff down the drain.
“Remember, the sewers were discharged into the river back then with little or no treatment,” Hoffman said.
And with the unique nature of the estuary, where seiche currents can drive water upstream as well as the usual downstream movement, more ancient mercury has been trapped in wetlands and bays throughout the estuary, not just near heavy industrial sites.
The research helped rule out other potential sources of mercury, such as higher local doses falling from the sky. Some groups had speculated that the estuary’s increased mercury might have come from the chimneys of Taconite iron ore processing plants in Minnesota.
10 times the mercury
The new study found that mercury in the estuary sediment of the St. Louis River was, on average, ten times higher than the sediment from the Bad River, which scientists used to compare in the study. The Bad River is less than 100 miles away and is subject to the same airborne mercury deposition patterns for new mercury, but is not affected by historic local industrial or urban sources of mercury. It served as a control for research.
Scientists also found that the mercury levels in the pikeperch in the St. Louis River estuary averaged twice that in Bad River fish.
Additionally, 60 percent of the pikeperch and 50 percent of the pike tested by the St. Louis River had higher levels of mercury than federal guidelines for human consumption.
And it is clear that not everyone follows the advice on fish consumption. According to a 2012 study by the Minnesota Department of Health, one in 10 babies born in the Lake Superior area of Minnesota had unsafe levels of toxic mercury in their bloodstream, likely because their mothers ate too much contaminated fish during pregnancy.
Regardless of the source of the original mercury, the water in the St. Louis River is particularly good at converting it to the toxic form known as methylation because it contains so much dissolved oxygen. Tea-colored waters and those with large wetlands have been known for years to allow more mercury to be converted to the toxic form and accumulated in fish.
And the new study confirmed that fish that spend more time in the river have more mercury, while fish that migrate out of the estuary and spend more time in the Upper Lake, where there is far less old mercury, have far less mercury in their tissues to have.
“The fish that (mostly remained in the estuary) had two or three times higher mercury content than those in the Upper Lake,” said Janssen because of the old mercury in the river sediment.
Hotspot cleanups can help
Hoffman said the results of the study will help determine how to reduce mercury levels in the fish.
Globally, mercury pollution has decreased in the last few decades, in large part due to the fact that coal-fired power plants have less mercury in the chimneys. Some states, like Minnesota, have also gone to great lengths to reduce the amount of mercury in the waste stream, including trapping mercury when leaving dental offices and banning mercury in batteries, counters, and other household items.
But all of these efforts did not lower the mercury levels in St. Louis River fish because there was still so much old mercury available in the ecosystem that sat in the sediment waiting to go from feeding invertebrates to small fish, large fish, and fish in then people to be transported down the food chain.
But efforts to remove this mercury-laced sediment or cap it so it doesn’t get into the water can help, Hoffman said, if targeted at the most toxic hotspots.
Tens of millions of dollars worth of projects in progress or completed – at Munger Landing, the ponds behind Erie Pier, 21st Avenue West, Azcon / DSPA Slip, and Minnesota Slip in Duluth and Howards Bay in Superior – all utilize or keep old mercury out of the estuary ecosystem.
“We believe these projects, some of which have already taken place or are ongoing, will make a difference,” said Hoffman.
The scientists will return regularly and check the top layer of sediment and test the fish to see if the mercury levels are actually going down.
“We’ll want to see if the source’s fingerprint changes over time,” Hoffman said after the sediment cleanup effort. Less mercury in fish should have the old fingerprint. And especially for people who want to eat fish: “Does this change the total concentration of mercury in the fish? We think it will be. “
Janssen said similar efforts to trace the source of mercury are already being made in the Fox River in Wisconsin, as well as high-mercury sites in Alabama, California and Oregon.
Mercury fingerprint tracking is a very useful technique. It determines how you do the cleanup on each site, ”she said.
In the meantime, the mouth of the St. Louis River can serve as an example, which has already been massively rehabilitated for a decade as part of the Great Lakes Restoration Initiative.
“It’s a national mercury purification demonstration project,” Hoffman said.