An Agate Original
It was a glorious day to be out on the St. Louis River. Sarah Janssen and Jacob Ogorek carefully stowed their equipment in the small research boat captained by Mark Pearson of the EPA’s Toxicology and Ecology lab, a Duluth institution. They had driven up from Madison, Wisconsin, a few days ago, and they’d had one rainy day and one sweltering day, but this one promised fair weather and just enough warmth.
Janssen and Ogorek are research scientists with the U.S. Geological Survey, an agency in the U.S. Department of the Interior that studies natural resource conditions and problems. They were going out onto the river in search of mercury—different isotopes of mercury, to be more precise. Isotopes are forms of elements that have the same number of protons but differing numbers of neutrons. They’re the same chemically, but they can exhibit different physical behaviors.
Janssen uses the various isotopes to learn where the mercury came from. “They’re a unique fingerprint that shows the different processes that a given bit of mercury has undergone before it enters the environment,” she explains.
Nature produces mercury in volcanic eruptions, forest fires, and evaporation from the ocean. Human-caused sources include fuel combustion, industrial processes, and mercury-containing products. Before the industrial revolution, there was one-third the amount of mercury on the earth than there is today. Mercury readily cycles between the atmosphere, land, and water. Air currents carry it around the globe. About half the mercury found in the open waters of Lakes Superior, Michigan, and Huron comes from airborne sources. Lakes Erie and Ontario have greater inputs from watershed and industrial sources.
Some of us remember mercury as a slippery, silvery metal to play with in high school science classes. Even in that form, mercury is toxic. But now we know that when it’s turned into an organic form, methylmercury, it can build up in the food chain and become a potent neurotoxin, particularly among infants and children. A Minnesota Department of Health study published in 2011 found ten percent of infants born in the Lake Superior region have levels of mercury in their bodies that can cause brain damage and other health problems.
Concern about mercury is the main reason for the fish consumption advisories so commonly issued by state health departments. In the St. Louis River estuary, pregnant women, women who could become pregnant and children under 15 should not eat more than one yellow perch or crappie per month.
Methylmercury is created in a complex process that starts when certain types of bacteria combine it with carbon, converting it to an organic form. Tiny invertebrates living in or near the sediment eat those bacteria, introducing the methylmercury into the food web. It magnifies, or bio-accumulates, as it moves from one animal to another.
Janssen and her team use advanced techniques developed over the last fifteen years to identify the sources of mercury. It’s called isotope fingerprinting, and it clearly differentiates mercury coming from industrial sources from mercury that falls from the air. “Industrial processing of mercury has undergone a lot of different steps compared to something that just comes from atmospheric deposition or from the watershed as runoff,” she says.
Earlier research on mercury in Great Lakes fish was conducted in open water, and these fingerprinting techniques showed that most of the mercury had come from the atmosphere. It wasn’t known how much historic pollution could also be methylated. But since 2017, Janssen’s team has been exploring the 19-square-mile St. Louis River estuary. They’ve collected samples of water, sediment, tiny invertebrate species, a species of spider that lives near water (Tetragnathidae), and forage fish as well as two predators, northern pike and walleye. They developed exacting techniques to avoid contaminating the samples, which is notoriously difficult when working with mercury.
In a paper published early this year, they reported finding that the historical contaminants can indeed be methylated and turn into the bio-accumulating, health-threatening kind. Further, they found this legacy mercury in water and sediments all over the St. Louis River estuary, not just in known industrial sites. And migrating fish like walleye carry it out of the river into Lake Superior. (Interestingly, though, migrating fish tend not to bioaccumulate as much as fish that stay in the river. The Lake has less mercury, less animal life, and fewer places for the methylating bacteria to live.)
They even found it upstream from the sections of the river that were directly impacted by industry.
“That might be due to flooding, or the motion of the river,” Janssen says. “We have seiche activity (when water sloshes from one end of the lake to the other, often with a tide-like regularity) and there’s just been lot of mixing over time. It’s not surprising that over many years this has gotten into a lot of other areas that may be removed from just the direct harbor influence.”
Those 2017 samples were taken mainly in the central channel of the St. Louis River. Today the crew is sampling from inlets, bays, and near-shore areas. “We’re targeting areas where we think methylation is occurring and trying to use these stable isotope fingerprints of mercury to look at what source is being methylated and getting into the fish,” Janssen says.
Ogorek and Janssen will take the samples they collect back to their lab in Madison and use a mass spectrometer the size of a small bus to separate the isotopes. “It’s not your average mass spectrometer,” says Ogorek. Rather, it’s a Multicollector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS). “It’s like two mass spectrometers smashed together,” says Janssen. “It allows you to get a really fine resolution.” Only in the last dozen years or so have scientists used this machine to study mercury.
Janssen is hoping her deep study of this place will help support clean-up work in other lakes and rivers.
The St. Louis River estuary is just one of more than three dozen heavily polluted sites around the Great Lakes identified in the 1980s as “Areas of Concern.” Intensive remediation work is underway at those sites. In the Duluth-Superior area, a multi-agency effort is about half-way through dozens of projects designed to improve the ecological health of the water.
Leading the work is the Minnesota Pollution Control Agency’s LaRae Lehto, who says Janssen’s findings are consistent with earlier research. “We’re getting at the hotspot sites where we have known legacy contamination,” Lehto says. Sadly, this work won’t solve the mercury problem, but it contributes to better understanding of the scope of the challenge. Knowing that the legacy mercury still contributes to health risks should give us pause as we plan new projects that will produce their own types of pollution.
No doubt future scientists and regulators will have plenty of work to do. The United Nations estimates annual global mercury emissions from anthropogenic sources are about 2,220 metric tons per year; the U.S. EPA says this country emitted thirty-thousand pounds from stacks in 2019; and the Minnesota Pollution Control Agency says as coal-fired power plants shut down, about half of mercury emissions generated in the state are now coming from taconite mining operations. And however successful clean-up efforts might be, it doesn’t help that 90% of the mercury in Minnesota comes from other states and countries in that cycling of mercury that distributes it around the globe.
The Clean Air and Water Acts have made vast improvements, and so has the hard work of cleaning up historic industrial sites. We’ll need a comparable effort to deal with contemporary and future pollution.