Step 1 in curbing mercury emissions: Find their source.
Unique ‘fingerprints’ of coal beds will help scientists track airborne toxin – and agree on controls.
Some amount of the element mercury, which scientists say is toxic to humans and animals, occurs naturally on earth’s surface. Since the Industrial Revolution began in 1750, the rate at which mercury is deposited into the environment has tripled.
The government gives it third-place priority in its list of 275 hazardous substances, right after arsenic and lead. Forty-four states have one or more mercury advisories on fish. And the US Centers for Disease Control says that 5.6 percent of women of child-bearing age have mercury levels nearing those deemed dangerous by the Environmental Protection Agency (EPA).
“It’s a real problem in the US,” says Russ Flegal, a professor of environmental toxicology at the University of California, Santa Cruz. “And it’s becoming increasingly problematic because we’re continuing to add mercury to the environment.”
Coal-fired power plants are the greatest single source of airborne mercury in the US. But mercury emissions from power plants remain unregulated. One reason: A debate on where this highly reactive and far-traveling metal originates has delayed what many consider long-overdue controls. One camp says mercury pollution is a global problem: Tamping down emissions in the United States won’t help much if the mercury deposited here originates outside the country.
The opposing camp says that while the mercury problem certainly is global, it’s also clearly regional and local. Areas nearest to mercury emitters are disproportionately contaminated. And if emissions from nearby sources are controlled, mercury in the nearby food chain also diminishes measurably.
Now, a team of scientists is developing a way to trace mercury in the environment back to its origin by identifying a unique chemical signature and matching it to a known source. The technique promises to change – and perhaps end – the debate over how to control mercury.
“It’s very important to be able to identify the source.… That’s probably the kind of direct evidence that is needed to implement change,” says Celia Chen, a research professor at Dartmouth College in Hanover, N.H. “If it’s just this global pool, nobody has to be held accountable for it.”
The new method relies on the ratio of mercury isotopes – mercury atoms with differing numbers of neutrons – to find where the mercury originates. Scientists gather coal from fields around the world, burn it, capture the escaping mercury, and determine its unique isotopic “fingerprint,” says Joel Blum, a professor of geological sciences at the University of Michigan, Ann Arbor, and co-author of a paper explaining the technique in the journal Environmental Science & Technology in October. Scientists can then assemble a library of these mercury signatures from coal. When that fingerprint is again detected in the environment, it can be easily matched to coal beds in the US, China, Kazakhstan, and elsewhere. The method is maybe five years from full deployment, says Blum.
Scientists currently infer mercury’s origin by measuring other elements nearby, and by using computer models. For example, the presence of iron or magnesium implicates an iron or steel mill, says Matthew Landis, an EPA research scientist in Research Triangle Park, N.C. But the new technique lets scientists determine precisely where the mercury came from, geologically speaking.
“It would give us essentially a one-and-only tool to look at global-scale source apportionment,” says Dr. Landis.
With China reportedly bringing two new coal-fired power plants on line every week and energy use growing elsewhere, knowing the source of mercury pollution with precision will be increasingly important, say experts. Between 1990 and 2002, mercury from human sources increased by 15 percent, much of it from the developing world. Some 2,000 tons are now released yearly worldwide. Half the mercury falling in rain in California comes from Asia, according to studies by Professor Flegal and others. Perhaps one-third of all mercury in the US comes from elsewhere – just as US mercury emissions go elsewhere.
Now “everybody wants to point the finger at the other guy,” says Charles Driscoll, a professor of civil and environmental engineering at Syracuse University in New York. Fingerprinting will provide “the framework where you can really think about managing this.”
With certainty about sources, treaties to control emissions could be hammered out among nations, says Robert Percival, director of the environmental law program at the University of Maryland School of Law in Baltimore. It may also change how, or if, people sue polluters for damages in court.
In the 19th century, people sued copper smelters that had devastated the surrounding landscape, he says. But as industrialization spread, pinpointing who was spewing what became nearly impossible, so this kind of lawsuit became less common. Mercury fingerprinting could reverse that trend, and more cases could strengthen the incentive not to pollute, says Percival.
When airborne mercury precipitates out of the atmosphere into waterways, it becomes methylated, and more easily absorbed by living organisms. From there, it moves up the food chain, where it concentrates as it moves higher.
The larger and longer-lived the animal – tuna, for example – the greater the potential concentration of mercury and the greater the risk to predators.
Scientists have long known that mercury “bio-accumulates” quickly in aquatic food webs. But they’re finding concentrations in terrestrial wildlife, like spiders, songbirds, and bats as well.
“We understand the need to reduce emissions,” says Dan Riedinger, speaking for Edison Electrical Institute in Washington, an association of electric companies. “It’s just a question of how it’s going to be done.” His industry wants a cap-and-trade approach to mercury, but expects the next administration to institute controls based on best-available technology.
Cleaning up waterways contaminated with mercury is tricky. Dredging sediment and dumping it at toxic-waste sites is the typical remedy. But when sediments are disturbed, they tend to release whatever mercury they contain into the water. Some argue that it’s best simply to leave mercury-laden sediments alone.
Now, an team of scientists has developed a way to remove mercury much more efficiently. It employs a combination of sound waves and genetically engineered algae. Sonic waves aimed at the sediments create microscopic bubbles in the water. They burst with such force that they shake loose the mercury. Algae kept behind a membrane porous enough to let mercury in but not porous enough to let algae out, suck up the mercury. The algae are genetically modified to enhance an already natural capacity to uptake the heavy metal – which is how mercury usually enters the aquatic food web.
From there, it's just a matter of changing the water chemistry to leach out the mercury, says Linda Weavers, a professor of civil and environmental engineering at Ohio State University, Columbus. That's easily done. The only real obstacle to making the technique commercially viable, she says, is lack of interest. Most funding now flows to global-warming research.