DULUTH -- For decades now, health officials in Minnesota and Wisconsin have warned people to limit their meals of walleyes from the St. Louis River estuary in Duluth and Superior because of unsafe levels of toxic mercury.

The fish consumption advisories are among the most severe in the Northland region of Minnesota: Men should eat only one meal of large walleye per month while women under 50 and children should not eat any large walleye, 20 inches or longer, from the river. Ever.

The amount of mercury in St. Louis River estuary walleyes is among the highest in either state. That matters because mercury is a potent neurotoxin that can cause severe brain and neurological disorders, especially in children and fetuses.

But it’s never been clear where all that extra mercury is coming from. Until now.

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After years of measuring mercury in the estuary muck, in the fish and in the rain falling from the sky, scientists now say they know the culprit: Huge amounts of decades-old “legacy” mercury pollution stored in the sediment is still getting into the fish.

In a study published in the journal Science of the Total Environment, published online March 13, scientists from the Environmental Protection Agency’s Duluth laboratory, the U.S. Geological Survey and several other institutions used isotope “fingerprinting’’ to trace mercury in the fish all the way back to its source, differentiating old legacy mercury from new mercury falling from the sky.

“There are different fingerprints for old mercury and new mercury,’’ said Sarah Janssen, a Wisconsin-based environmental chemist for the USGS and lead author of the study.

It’s that legacy mercury making fish in the St. Louis River estuary have much higher mercury than those in nearby lakes, or even the same river upstream, or than Lake Superior just downstream.

UMD researchers gather a sample of St. Louis River estuary sediment to be tested for mercury. Scientists from multiple universities and agencies gathered data for years to determine why so much mercury was showing up in walleyes in the estuary. The study concluded that much of the mercury has been in the sediment for decades. (John Myers / News Tribune / 2016 file)
UMD researchers gather a sample of St. Louis River estuary sediment to be tested for mercury. Scientists from multiple universities and agencies gathered data for years to determine why so much mercury was showing up in walleyes in the estuary. The study concluded that much of the mercury has been in the sediment for decades. (John Myers / News Tribune / 2016 file)

The original sources of the old mercury may never be known. Mercury was ubiquitous in both household and industrial uses in the 1800s and 1900s, said Joel Hoffman, a research biologist and co-author of the study who is chief of the Ecosystems Services Branch of the EPA's Duluth laboratory. Those sources likely included paper mills, lumber mills, steel mills, shipbuilding sites, manufacturing facilities and other sources that once dominated the river and Twin Ports harbor shoreline.

“It was probably everywhere, sitting around in big barrels for that matter, and there wasn’t the awareness that there is now of the (health) dangers or how to dispose of it,’’ Hoffman noted.

Much if it was probably dumped into the river. And that’s also where city sewage went, often laced with mercury from everyday uses like dental fillings, thermostats, thermometers and batteries. School science classes used to “play’’ with mercury to show its properties, and then often throw the stuff down the drain.

“Remember that the sewer systems back then discharged into the river with little or no treatment,’’ Hoffman said.

And with the unique nature of the estuary, where so-called seiche currents can push water upstream as well as the usual downstream movement, more old mercury became trapped in wetlands and bays throughout the estuary, not just near heavy industrial sites.

The research helped rule out other potential sources of mercury, like higher local doses falling from the sky. Some groups had speculated that the estuary’s elevated mercury might have been coming out of the smokestacks from Minnesota taconite iron ore processing plants.

10 times the mercury

The new study found mercury in the St. Louis River estuary sediment was on average 10 times higher than sediment from the Bad River, which scientists used for comparison in the study. Less than 100 miles away and subject to the same airborne mercury deposition patterns for new mercury, but not affected by historic onsite industrial or urban mercury sources, the Bad River served as the control for the research.

Scientists also found mercury levels in the walleye from the St. Louis River estuary were on average twice as high as those in Bad River fish.

Moreover, 60% of the walleye and 50% of the northern pike tested from the St. Louis River had levels of mercury higher than federal guidelines for human consumption.

A big St. Louis River walleye, considered unsafe for women and children to eat. A new study has found that the primary reason St. Louis River estuary walleyes carry so much toxic mercury is because of decades-old legacy mercury in the sediment. Efforts to clean up that sediment may help lower the mercury levels in fish. (Steve Kuchera / News Tribune / 2019 file)
A big St. Louis River walleye, considered unsafe for women and children to eat. A new study has found that the primary reason St. Louis River estuary walleyes carry so much toxic mercury is because of decades-old legacy mercury in the sediment. Efforts to clean up that sediment may help lower the mercury levels in fish. (Steve Kuchera / News Tribune / 2019 file)

And it’s clear not everyone is abiding by the fish consumption advisories. According to a study by the Minnesota Department of Health released in 2012, one of every 10 babies born in the Lake Superior region of Minnesota had unsafe levels of toxic mercury in his or her bloodstream, likely because their mothers ate too much contaminated fish while pregnant.

No matter what the source of the original mercury, the water in the St. Louis River is especially good at converting it to the toxic form, called methylation, because it has so much dissolved oxygen. Tea-colored waters and those with ample wetlands have been known for years to allow more mercury to be converted to the toxic form and build up in fish.

And the new study confirmed that fish that spend more time in the river had more mercury while fish that migrated out of the estuary, spending more time in Lake Superior where far less legacy mercury exists, had much less mercury in their tissue.

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“The fish that (stayed mostly in the estuary) had mercury levels two or three times higher than those in Lake Superior,” Janssen said, because of the legacy mercury in the river sediment.

Hotspot cleanups can help

Hoffman said the study’s findings help determine how to reduce mercury levels in the fish.

Globally, mercury pollution has been declining in recent decades, thanks in large part to less mercury going up smokestacks at coal-burning power plants. Some states, like Minnesota, also have gone to great lengths to reduce mercury in the waste stream, including catching mercury leaving dental offices and banning mercury in batteries, switches and other household items.

But all those efforts were not reducing the mercury levels in St. Louis River fish because so much old mercury was still available in the ecosystem, sitting in the sediment waiting to be moved up the food chain from invertebrates to small fish, to big fish and then people.

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But efforts to remove that mercury-laced sediment, or cap it so it can’t get into the water, can help, Hoffman said, when they are aimed at the most toxic hotspots.

Tens of millions of dollars worth of projects in the works or already completed at the Munger Landing, the ponds behind Erie Pier, 21st Avenue West, the Azcon/DSPA Slip and Minnesota Slip in Duluth and Howards Bay in Superior — all will take or keep old mercury out of the estuary ecosystem.

“We believe that those projects, some of which already have happened or are underway, will make a difference’’ in reducing mercury in fish, Hoffman said.

The St. Louis River estuary near the Bong Bridge. A new study has found that unusually high levels of mercury in St. Louis River estuary walleyes, twice as high as in a nearby river, are due to decades-old legacy pollution still in the sediment. Scientists say efforts to clean up mercury-contaminated hotspots in the estuary and harbor could reduce mercury levels in fish. ( Bob King / News Tribune / 2011 file)
The St. Louis River estuary near the Bong Bridge. A new study has found that unusually high levels of mercury in St. Louis River estuary walleyes, twice as high as in a nearby river, are due to decades-old legacy pollution still in the sediment. Scientists say efforts to clean up mercury-contaminated hotspots in the estuary and harbor could reduce mercury levels in fish. ( Bob King / News Tribune / 2011 file)

Scientists will go back out on a regular basis and check the top layer of sediment, and test fish, to see if mercury numbers indeed are going down.

“We will want to see if the source fingerprint changes over time” after the sediment cleanup efforts, Hoffman said. Less of the mercury in fish should have the legacy fingerprint. And, most importantly for people who want to eat fish “does it change the total concentration of mercury in the fish? We think it will.”

Janssen said similar efforts to trace the source of mercury are already underway in the Fox River in Wisconsin as well as high-mercury sites in Alabama, California and Oregon.

Tracing the mercury fingerprint “is a very useful technique. It drives how you do the cleanup at each site,’’ she said.

Meanwhile the St. Louis River estuary, which is already a decade into a massive cleanup effort under the Great Lakes Restoration Initiative, also can act as an example.

“It’s a national demonstration project on how to do mercury cleanup,’’ Hoffman said.

Boats on the St. Louis River for the 2009 walleye fishing opener. The river is a popular destination for anglers, but state health officials warn people to limit their meals of walleye due to high mercury contamination. Men should limit their meals of large St. Lous River walleyes — 20 inches and longer — to just one per month while women under 50 and all children should not eat any walleye from the river 20 inches or longer. (Sam Cook/ News Tribune / file)
Boats on the St. Louis River for the 2009 walleye fishing opener. The river is a popular destination for anglers, but state health officials warn people to limit their meals of walleye due to high mercury contamination. Men should limit their meals of large St. Lous River walleyes — 20 inches and longer — to just one per month while women under 50 and all children should not eat any walleye from the river 20 inches or longer. (Sam Cook/ News Tribune / file)


Eating fish is good for you, if you eat the right fish

The Minnesota Department of Health says fish are an important part of a healthy, balanced diet. They provide a good source of protein and vitamins such as D and B2 (riboflavin). Fish are rich in calcium and phosphorus and are a great source of minerals, such as iron, zinc, iodine, magnesium and potassium.

Health officials generally favor store-purchased fish such as salmon for the most health benefits, such as omega-3 fatty acids, with the least contaminants. But fish caught in the Northland also can be good for you, especially if you eat smaller fish such as crappie, sunfish, stream trout and perch that haven’t bio-accumulated large amounts of mercury and other toxins. If you eat game fish like walleye and pike, choose smaller fish to eat.

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Here are some Minnesota Department of Health suggestions for women and children for safe fish consumption:

  • Eat up to two weekly meals of store-purchased catfish, cod, herring, Atlantic mackerel, pollock, ocean-caught or farmed salmon, sardines, shellfish and tilapia.
  • Weekly meals are OK of canned “light” tuna, halibut, locally caught bullhead, crappie, inland trout, Lake Superior herring (cisco), lake whitefish, sunfish (bluegill) and perch.
  • Up to one meal monthly is suggested for canned “white” (albacore) tuna, Chilean sea bass, grouper, marlin, tuna (steak or fillet) or locally caught bass, trout, pike or walleye — unless more restrictive local guidelines are in place, such as walleye from the St. Louis River or lake trout from Lake Superior.
  • Do not ever eat swordfish, shark, king mackerel, tilefish or musky.

For more information go to health.state.mn.us/communities/environment/fish/index.html.

Source: Minnesota Department of Health