The United States Geological Survey has determined that high mercury levels in the Great Lakes' ecosystem — enough to catalyze a chain of events leading to human consumption of the dangerous chemical — is largely due to remnants of pollution from old industries and other chemical-rich sources that no longer exist.  

Researchers drew this conclusion using an emerging technique called mercury fingerprinting, which allows scientists to understand the origins and detect movement of mercury in ecosystems. Their large-scale study, published March 13 in Science of the Total Environment, showed that mercury found in the Great Lakes is because of legacy contamination, or pollutants from sources that aren't there anymore, like major chemical accidents or broken-down factories. 

The authors hope their work can help inform restoration strategies, especially because heavy potency of the chemical in freshwater ecosystems significantly increases the risk of humans encountering mercury in their food. Ingestion can lead to serious health consequences, such as neurological deficits.

"Mercury is an extremely serious contaminant of concern across the country, but particularly in the Great Lakes, where it's actually designated as [such] for the U.S. as well as Canada," Sarah E. Janssen, a postdoctoral researcher at the USGS' Mercury Research Lab and the lead author of the study, told The Academic Times. 

Her team's survey — the largest ever to use mercury fingerprinting — also enumerated where mercury is most concentrated in the freshwater ecosystem of the Great Lakes. The researchers interestingly found that mercury contamination in the St. Louis River, which flows into Lake Superior, is much higher than that of the actual lake. 

"The fish from the St. Louis River have two to three times the amount of mercury in their tissue when compared to fish captured in Lake Superior and the other Great Lakes," Janssen said. "The question was, is that due to just contemporary sources ... or is it really being driven by this legacy contamination leftover in the river?"

The team used mercury fingerprinting to dissect where the mercury was coming from. That involved studying the isotopes of mercury found in various fish and other animals in the river, then categorizing the ones that matched. Isotopes are variations of the same atom; the difference resides in the number of neutrons it contains, but this doesn't really change the chemical properties of the atom. Two different mercury isotopes are still considered mercury to the same degree.

Mercury of the same isotope likely came from the same location, and mercury from different isotopes, separate locations. The technique revealed that the chemical with the isotope that indicates legacy contamination was most prevalent.

"Mercury that comes from legacy sources is much different than mercury that comes from atmospheric sources," Janssen said. "And that's how we're able to see the mercury in the sediment or fish from whichever source."

Because mercury is a known neurotoxin, human consumption of the chemical in large amounts can trigger defects and neurological deficits, particularly among children and young adults.

"When you see those fishing consumption advisories, a lot of times those are directed towards women of childbearing age, children or even teenagers under the age of 15," Janssen explained. "There can be developmental effects on humans."

One of the most famous cases of excessive mercury consumption is the emergence of Minamata disease in Japan. Because people ingested extremely mercury-rich shellfish and fish, they gradually developed grave symptoms such as tremors and sensory disturbances.

Although the study focuses on how mercury is being transported through the ecosystem, Janssen stressed that the ultimate issue with this distribution is the process by which the neurotoxin accumulates in fish tissue, and ultimately becomes present in people's food.

The chemical collects and grows exponentially within the food chain, eventually making its way to humans. Mercury diffuses into some fishes' tissue, then larger fish eat many contaminated fish, doubling the amount of mercury in their own tissue. People eat all these fish, and often the latter sort, which contain multiplied levels of the hazardous chemical.

Janssen relayed that the most contaminated fish are large, because they eat more, and older, because they have had more time to compound their mercury consumption. Purely through this process of accumulation, the higher it is in the food chain, the more mercury an organism contains. 

"It magnifies each step, so you can get 10 or 1,000 times the mercury concentration by the time it goes all the way up the food web," she said. "You can start at very low concentrations in the water column, but then with every subsequent step up the food web, you can get more and more."

And the contamination doesn't stop at the Great Lakes, Janssen suggested.

"The Great Lakes are like a lot of other ecosystems across the country and across the globe, where they have mercury exposure from emissions, generally," she said. "It's not just the Great Lakes' issue."

Citing the many efforts to remedy the problem of mercury poisoning, Janssen hopes her team's study could potentially lead to other resource managers and organizations keeping mercury fingerprinting in their pocket as an effective method to detect mercury origination, too. 

If it is found that mercury comes about from legacy contamination sources, it could change the approach and increase urgency surrounding cleansing freshwater ecosystems of the chemical, for example, by shifting the focus from current industrial locations to past ones. 

The paper, "Examining historical mercury sources in the Saint Louis River estuary: How legacy contamination influences biological mercury levels in Great Lakes coastal regions," was published March 13 in Science of the Total Environment. It was authored by Sarah E. Janssen, David P. Krabbenhoft, Jacob M. Ogorek, John F. DeWild and Michael T. Tate, U.S. Geological Survey Upper Midwest Water Science Center; Ryan F. Lepak, Joel C. Hoffman, Greg Peterson, Anne Cotter and Mark Pearson, Center for Computational Toxicology and Exposure; David Walters, U.S. Geological Survey Columbia Environmental Research Center; Collin A. Eagles-Smith, U.S. Geological Survey Forest and Rangeland Ecosystem Science Center; and Roger B. Yeardley Jr. and Marc A. Mills, U.S. Environmental Protection Agency Office of Research and Development.