The sky is falling
During an interview over lunch in the U.S. Senate cafeteria one wintry February
afternoon after he presented the latest on mercury research to staffers for
the 108th Congress, Krabbenhoft remembers how he began investigating mercury.
It was 1988 and mercury was all over the newspapers in Wisconsin, his home state.
The taxpayers in Wisconsin wanted to know why our northern Wisconsin pristine
lakes had so much mercury. Where was it all coming from? he says. So,
on his first day at USGS, he wrote a proposal to study mercury in temperate
lakes. It would be the first of many such studies.
Mercury was long thought to be a point-source pollutant from industrial and mining wastes. Inspiration for Lewis Carrolls character the Mad Hatter in Alices Adventures in Wonderland came from the strange behaviors displayed in the 1800s by people working in the felt hat industry. Constant exposure to mercury nitrate eventually caused the hatters to develop mercury poisoning and neurological disorders. Although mercury is no longer part of the felting process, its industrial legacy remains in the environment.
Once in sediment, mercury becomes very stable, locked up with sulfur and immobile in the absence of dredging or some large erosive event. One of the largest legacy sources, Krabbenhoft says, is from the California Gold Rush of 1849. Gold miners used mercury to extract gold from the ore, leaching mercury into the environment where it has remained trapped in sediments. But even this mercury is slowly mobilizing out of the Sierra Nevada Mountains.
Still, the mercury of the moment, whats on most peoples minds, comes from the air.
Fifteen years ago, no one had any idea that the mercury in the Wisconsin lakes was all coming from the atmosphere, Krabbenhoft says. Thats all changed, he says, with a number of multi-investigator studies, including Krabbenhofts in Wisconsin. Now, researchers and the public know that mercury also rains down from a number of places; Wisconsins taxpayers know that the mercury in their water originally came from the air.
Natural sources including volcanoes, geothermal springs and geologic deposits emit mercury to the atmosphere, as do human-related sources, such as coal combustion, waste incineration, industrial uses and mining all totaling about 7,260 tons per year of mercury.
An ice core collected from the Freemont Glacier in Wyoming shows wide variations in mercury emissions over the past 275 years, Krabbenhoft says. But overall, mercury releases since the industrial revolution dominate the core history, accounting for 70 percent of the deposited mercury in the past 100 years of the record. Save a volcanic eruption in Tambora, Indonesia, in 1815, mercury concentrations in the ice core for the last 100 years far exceed the prior 100 years. The last ten years of the ice core record have shown a distinct decline in atmospheric mercury, but, Krabbenhoft says, researchers are still unsure why (read sidebar).
David Krabbenhoft, left, and Marc Amyot examine the geochemical behavior of mercury in rain after
delivery to lake water. Photo by Cindy Gilmour.
The path mercury takes from a source to eventual deposition is complex. Three different species of mercury travel through the atmosphere at different time scales. Elemental mercury (Hg0) circulates for up to year before depositing on land or in water, while ionic (Hg(II)) and particulate (Hg(p)) mercury deposit within a relatively short distance to their sources. After deposition, the mercury works its way into the sediments. Once the ionic form of mercury settles in sediments, sulfate-reducing bacteria can take the mercury, which has formed a complex with sulfur, and convert it into methylmercury. Thats the potent neurotoxin that accumulates in fish tissues and continues through the food chain, until it reaches humans and other animals that eat fish.
What is still uncertain are all the steps, on the small scale, that need to occur for the mercury to both get into the sediments and to then become methylmercury. Also uncertain is the timing of those reactions and how the rate of reaction varies between newly deposited mercury and legacy sources.
What researchers are learning, Krabbenhoft says, is how quickly the newer mercury reacts in the environment. You can hardly propose something to investigate related to mercury and think back to 15 years ago compared to where were at now and not come to the conclusion wow, mercury is just super-reactive in the environment!
If it is the current sources of mercury that are reacting in the environment and not the legacy sources, Krabbenhoft says, reducing mercury emissions now could potentially reduce the amount of mercury accumulating in fish.
Seeing is believing
Distinguishing the recently deposited atmospheric mercury from the legacy mercury is key to understanding how a reduction in mercury emissions would change the risks posed by consuming fish. Thats what METAALICUS, the Mercury Experiment to Assess Atmospheric Loading in Canada and the United States, sets out to do. Adding stable isotopes of mercury to an experimental lake in Ontario, U.S. and Canadian researchers from several universities and research institutions have been able to follow mercurys path through the system.
The Canadian government allowed the study in the Experimental Lakes Area (ELA) providing 30 years of expertise and baseline data on the local ecosystem. There may be no place on earth that this study could be done other than the ELA, says Cindy Gilmour, a biogeochemist and microbial ecologist at the Academy of Natural Sciences, Estuarine Research Center in Maryland. The ELA has a history of large lake-manipulation experiments, such as the phosphorous addition experiments of the 1970s that turned one-half of an experimental lake green. The experiment showed the concept of eutrophication, or over-fertilization. ELA experiments in the 1980s largely focused on acid rain, and then the area moved on to metal studies, including mercury and METAALICUS.
METAALICUS investigators add different mercury isotopes to different parts of the watershed of Experimental Lakes Area Lake 658 in Ontario, putting mercury-202 in the lake, mercury-200 in the uplands and mercury-198 in the wetland. By simulating natural mercury loading to a North American watershed, they are able to follow each isotope as it moves through the system. Image by Condy Gilmour.
The basic concept that we want to demonstrate is very simple: If you change the load of mercury to a watershed, then you change the amount of mercury in the fish, says Gilmour, who began work on METAALICUS in 1999 with Krabbenhoft and others. But embedded in that basic concept is a whole bunch of basic research on mercury cycling this big picture demonstration and then moving to the next step of understanding how mercury works in the environment.
The power of demonstration is huge, particularly in politics, Gilmour says. The political reason for doing it is the thought by both U.S. and Canadian governments that they want to reduce mercury emissions, she says. If we pay the bill, and decrease the amount of mercury that were sending up out of coal-fired plants, then whats the payoff going to be and how long does it take to get that payoff?
After a couple of years of preliminary studies, the METAALICUS investigators began adding different mercury isotopes to different parts of the watershed of ELA Lake 658 mercury-202 in the lake, mercury-200 in the uplands and mercury-198 in the wetlands to simulate natural mercury loading to a North American watershed. For the purposes of the study, all other forms of mercury detected in the ecosystem are from old sources. Its a three-year experiment, with the final addition this year. The total amount added barely fills a teaspoon yet it costs half a million dollars to acquire and is enough to change the load to the ecosystem by a factor of four to five, Gilmour says showing the incredible toxicity and bioaccumulation potential of the metal.
Mercury deposition generally follows patterns of acid rain deposition, so the highest rates of mercury deposition in the United States are in the heartland and in the southeast and northeast. With the stable isotopes, were basically increasing the deposition of the system from what they currently get in northwest Ontario to what we get here on the East Coast, she says.
Methods for putting the isotopes down onto the system vary for each part of the ecosystem. In the lake, they just divvy up the isotope into 15 different bottles and dribble it out slowly about half a meter down, to reduce escape to the atmosphere. They add it at night so that the mercury can mix down before sunrise the next day. For the uplands and wetlands, the story is different. There, a professional spray pilot from the Canadian Forest Service comes out in May and waits until the perfect, rainy conditions. The reason the isotopes go on in rain is because the suns not out and it comes down in rain anyway so that mimics natural conditions. And also, the rain helps to wash it through the canopy so that were not spraying a very fine mist on the leaves thats going to evade the upland, Gilmour explains. No matter what, some will escape the system about a 30 percent loss from both the lake and the uplands.
Canadian law requires the research team to monitor the water quality
of Lake Winnange, which is about a thousand times bigger than the experimental
lake that feeds into it. Fortunately the lake is a slow-moving system, where
it takes about three years for a particle of water to leave; most mercury is
retained in the sediment. We couldnt do it in a fast-moving lake,
The results of the experiment have been diverse, and are still in the works. So far, the mercury in the upland and wetlands has stayed put, with less than 1 percent of the mercury from the upland making its way to the lake. Theres this time lag between when the mercury comes into the ecosystem and when its mobilized into the lake that we never would have known without the isotopes, she says. Theres at least a two-year lag time before the mercury that rains down on the canopy gets to the lake. They do not know yet just how long, because the isotopes have yet to show up. In this cold, arboreal forest, the summer is short, and it could be that the leaves have to rot before the mercury begins moving through the system, Gilmour explains.
However, the mercury added to the lake is extremely dynamic, taking no time at all to methylate. We were out measuring methylation of isotope in sediments, and accumulation of methylated isotopic mercury in zooplankton three weeks after the first spike, Gilmour says. It lands on the lake, moves into sediments, gets methylated and into the food web in three weeks the newer mercury is at least five times more readily methylated than the pools of legacy mercury, she says. As the mercury ages, it becomes less and less available to microbes for methylation which means that the current emissions from coal-fired plants have a major effect on how much mercury is in the food supply now, Gilmour says.
Until more time has passed, though, the researchers will not know how much the mercury added to the uplands and wetlands affects the food web, or in what time frame, Krabbenhoft says. All I can say after two years is that we have hardly seen any of the mercury we put onto the uplands in the lake. Our hypothesis is that mercury landing on the uplands needs to mature, if you will, work its way deeper into the soil horizon before it becomes mobilized. But even then, the mercury might be pairing itself with dissolved organic carbon complexes, making it unavailable to the methylating microbes; but, Krabbenhoft says, they still dont know. The final year of the mercury-addition experiment and the next few years analyzing the resulting data will prove useful in helping researchers determine the analogs in a natural system.
Canadian experimental lake serves as a surrogate for watersheds everywhere,
Gilmour says, it is important to look at ecosystems elsewhere to extrapolate.
METAALICUS and another project in the Florida Everglades called ACME, which
looks at the mechanics of methylation and its relationship with sulfur and dissolved
organic carbon, are changing the face of mercury studies and giving researchers
views at both the small and large scale. Other projects are underway or starting
up in San Francisco, New York and Wisconsin.
Krabbenhoft is also involved in studying mercury gradients in urban areas, and the relationship between where the mercury comes from and where it deposits, looking at the footprints of individual power plants in any given area. These small-scale studies could hint at global geographic relationships.
The argument not to regulate mercury emissions could rest on either: theres so much mercury already out there that reducing emissions isnt going to make any difference, or that the United States only emits a relatively small amount of the global emissions that we wouldnt make a dent, Krabbenhoft says. Indeed, the United States only emits 6 percent of the total human-related mercury globally; Asia accounts for more than 48 percent.
Whether or not we reduce emissions in the United States, we need to know more about how our emissions contribute to our local contamination, he stresses. Its not as simple as totaling up world emissions and looking at which piece of the pie we represent. But, regardless of relative magnitude, local emissions do contribute substantially to the local problem, he says.
Georgia Riedel works with sediment cores in the wetland end of Lake 658 in June 2003. Photo by Cindy Gilmour.
While it seems nothing is simple in the world of mercury research, looking
back on the past two decades, some things are becoming abundantly clear, says
Krabbenhoft, finishing up his lunch at the Senate. If we more universally
in the world move toward reducing mercury emissions, we will see different positive
impacts among ecosystems, he says. Although some ecosystems will experience
more of a positive effect than others, and many of those details are still uncertain,
Krabbenhoft is certain that reducing mercury emissions will reduce the contamination
of fish in U.S. watersheds and its not going to take millennia
to get a response. Thats the message he hopes the congressional staffers
received from his presentation.
Decisions in the near term are going to need to rely on either the scientists gut instinct or go on blind faith, he says. Even if decision makers dont know the exact amounts by which to reduce emissions, at least they will know that a reduction of any amount can and probably will make a difference.
drop in atmospheric mercury
An ice core from the Freemont Glacier in Wyoming reveals a marked decline
in atmospheric mercury over the past 15 to 20 years and researchers have
not known exactly why. Now, new research published in the May 22 Geophysical
Research Letters appears to support this trend, showing a sharp drop
in gaseous mercury in the atmosphere since its peak in the 1980s, a drop
three times higher than previously thought.