A study found that historic sources of mercury pollution are driving higher concentrations of the contaminant in sport fish that live and feed in the Duluth-Superior harbor.
According to Wisconsin Public Radio, these results show contamination from long ago can still create challenges for coastal areas of the Great Lakes.
The coastal areas experience industrial pollution from contaminants like mercury. These contaminants threaten fish and wetland habitats in coastal areas including the largest port on the Great Lakes, reported Wisconsin Public Radio.
As a result, fish consumption advisories in the St. Louis River estuary that runs between Duluth and Superior has been put into place.
According to the study’s lead author Sarah Janssen, a research chemist with the U.S. Geological Survey in Middleton, it was previously unclear whether older industrial contamination or modern-day mercury pollution from atmospheric sources led to the higher concentrations in fish.
The recent study set to be published in the journal Science of the Total Environment, relies on a new method to fingerprint mercury within fish that is coming from historical contamination in the Duluth-Superior harbor, according to Janssen, reported Wisconsin Public Radio.
"The game fish, specifically our walleye – those were really our target sport fish because they're targeted by anglers," said Janssen to Wisconsin Public Radio. "Those walleye tend to have two to three times the amount of mercury in their tissue than fish that are migrating and feeding into Lake Superior. This higher mercury in walleye residents of the river corresponds with those mercury source indicators of legacy industrial sources in the harbor."
Researchers paired the fingerprints with around 500 samples of organisms, sediments and water from the lower St. Louis River to understand where fish were feeding and picking up mercury, reported Wisconsin Public Radio. The findings in the harbor were compared to sediment and fish samples collected from the Bad River on Lake Superior, which has no known sources of industrial contamination.
The results found that mercury concentrations in sediments within the St. Louis River estuary were on average 10 times higher than the Bad River, reported Wisconsin Public Radio. The mercury contamination was most severe within Howards Bay in Superior and a multi-million dollar effort is underway to clean up contamination in the bay.
According to Janssen, the findings show that historical contamination is cycling through the river’s food web, which can extend out into Lake Superior.
This is the largest study so far using the new source indicator method and Janssen hopes the findings can be used to help natural resource managers remediate contamination and restore the St. Louis River, which is designated as Areas of Concern (AOC) on the Great Lakes.
Those working to restore the river received roughly $125,000 through the Great Lakes Restoration Initiative this year to conduct additional mercury sampling in 2021 at six locations on the river, according to Wisconsin Public Radio.
More than 50% of remedial actions to clean up the river are complete and the remaining restoration work on the river is intended to be complete by the end of 2024.
