ORNL scientists solve the mystery of mercury

Oak Ridge has a unique relationship to the science of mercury contamination. In the 1950s and ’60s, the city’s Y-12 nuclear weapons plant released around 350,000 pounds of mercury into Upper East Fork Poplar Creek and other local waterways, making the region a de facto laboratory for mercury in the environment.

In the decades since, mercury has become a pollution concern worldwide. Releases come from natural processes such as volcanic eruptions and from industrial activities such as the burning of coal. Mercury can cause significant neurological harm to animals and humans. Of particular concern, women who eat mercury-contaminated fish are at greatly increased risk of having children with birth defects.

Today, four Oak Ridge National Laboratory scientists are collaborating on landmark mercury research that gives policymakers and people across the world a better understanding of mercury’s insidious behavior in the environment—and how to clean up contaminated water, soil and infrastructure.

Stumping scientists

“To get an idea of how much mercury was released near ORNL during the 1950s and 1960s, imagine a moving truck full of mercury,” says Eric Pierce, an ORNL geochemist and expert in the migratory patterns of mercury and other metal pollutants. “About half of that moving truck—say, two bedrooms worth of stuff—has been lost to the environment.” Tracking down the lost mercury in the Oak Ridge environment provides ORNL scientists an outdoor laboratory in which to study the metal’s still-puzzling behavior.

As it encounters streams, soil and organisms, inorganic mercury transforms to a more toxic, organic form called methylmercury. Organisms can readily absorb this methylated form of the contaminant, causing it to linger in the environment. 

Pierce hopes to limit this lingering effect by preventing methylmercury from moving or seeping deeper into the earth. The trickiest issue, he says, is that the contaminant’s change between forms can affect its movement—making a blanket solution nearly impossible.

Instead, Pierce looks at promising combinations of on-site technologies to restrain mercury. 

One method, for instance, encapsulates mercury by mixing a stabilizing compound with tainted soil. The technique could be used alongside tools such as reactive caps, or fabric layers interspersed with a material that chemically traps mercury. One prime candidate is Upper East Fork Poplar Creek on the Y-12 site, which took the brunt of mercury releases.

“Ultimately, we’re figuring out the right combination of on-site treatment technologies that keep Upper East Fork Poplar Creek and other local ecosystems thriving,” Pierce says. 

Mercury in the food chain

Helping to understand the role of methylmercury in ecosystems is Terry Mathews, an aquatic ecologist in ORNL’s Environmental Sciences Division. She’s part of the Biological Monitoring and Abatement Program, leading studies on the concentration levels of mercury in water-based ecosystems.

Mathews is interested in how food-chain interactions affect metal behavior at contaminated sites.

“For over 30 years, we’ve been gathering data, testing more than 40 sites along the watersheds and aquatic bodies in East Tennessee,” she says. “My team and I monitor the diversity and abundance of regional aquatic species to understand when and how mercury is transferred from contaminated water to aquatic microorganisms to the top rung of the food chain.” 

Fortunately, Oak Ridge is home to an impressive diversity of freshwater organisms, ranging from periphyton—a mish-mash of algae, bacteria and microbes at the bottom of the food chain—to the brightly colored redbreast sunfish near the top. It’s provided researchers with a wealth of data in their virtual outdoor laboratory.

Using this resource, Mathews and her team made a puzzling discovery.

“It’s actually counterintuitive,” she said. “Even though we have drastic reductions in aqueous mercury concentrations, methylmercury levels in fish have remained elevated. Downstream sites in the creeks are further from the contamination points and have higher species richness and diversity, more complex food webs, and a greater opportunity for mercury biomagnification.” 

A little goes a long way

The key to understanding how species amplify mercury levels is smaller than you’d think, according to Dwayne Elias, a microbiologist in ORNL’s Biosciences Division.

Zooming in on microbes and tiny creatures like periphyton and algae at the base of the food chain, Elias is part of a team that studies the biological processes that underlie mercury’s transformation into its neurotoxin form.

The team uncovered the genetic basis dictating how microorganisms convert inorganic mercury to the harmful methylated version, a long-awaited discovery in microorganism metabolism.

“For the last 40 years, people have been trying to identify the genes and proteins that control the methylation process to create this very toxic compound,” says Elias. “ORNL accomplished this in five years.”

Elias recently helped identify unexpected locations for methylmercury production far beyond East Tennessee. He analyzed genetic material from all around the world, finding hints of the gene “switch” in nearly 1,500 global environments.

“We found traces of the dual-gene cluster required for the methylation process in samples from the ocean floor to the Arctic permafrost,” says Elias.

Mercury in our own neighborhood

ORNL geochemist Scott Brooks isn’t worried about microbes in frosty locales. He’s focusing on the contaminant and organisms in ORNL’s own backyard along East Fork Poplar Creek.

“I identify where in the ecosystem methylmercury is generated, its characteristics, and how to use that information to change how it’s formed, or how we remediate it,” Brooks says. 

Brooks leads a team that studies the activity of mercury at specific sites along East Fork Poplar Creek in Oak Ridge. He samples directly from the gritty sediment where local creeks brush against the banks, identifying chemical compounds interacting with mercury in the ground.

“Mercury has a high affinity with surfaces,” Brooks said. “It can permeate through concrete or latch onto suspended aquatic particles and decomposed organic matter in the soil.”

Underground are subsurfaces, or zones where mercury density varies wildly, requiring a nuanced treatment approach. It’s a huge problem and has been for decades since people realized the hazards in mercury exposure. 

“To be fair, the world has been looking for a solution to remediate mercury contamination for a long time,” Brooks said. 

ORNL’s collaborative culture plays a big role in the researchers’ successful understanding of the metal pollutant.

 “I can take the same aquatic sample to Dwayne or Scott or other mercury experts outside my field and get more data from it,” says Mathews. “Without the partnerships we have at ORNL, I couldn’t conduct my research as effectively.”  

Pierce agrees. “Mercury contamination is really a global issue,” he says. “We are working together to understand how it works and how to remediate it.”