Study Shows Forests Play Greater Role In Depositing Toxic Mercury Across Globe

Authors say modelers can use results to improve understanding of how mercury cycles through the environment.

LOWELL, Mass. — Mercury measurements in a Massachusetts forest indicate the toxic element is deposited in forests across the globe in much greater quantities than previously understood, as per research led by the University of Massachusetts Lowell.

The study’s findings were published in the journal Proceedings of the National Academy of Sciences.

The results underscore a concern for the health and well-being of people, wildlife, and waterways, as mercury accumulating in forests ultimately runs off into streams and rivers, ending up in lakes and oceans, as per Daniel Obrist, chair of the university’s Department of Environmental, Earth and Atmospheric Sciences. 

Mercury is a highly toxic pollutant that threatens fish, birds, mammals, and humans. Hundreds of tons of it are released into the atmosphere each year by coal-burning power plants.

Gold mining and other industrial processes, winds, and currents across the globe distribute the pollutant.

Long-term exposure to mercury, or consuming food containing high levels of the pollutant, can lead to reproductive, immune, neurological, and cardiovascular problems, as per Obrist.

“The study is the first that examines a full picture of how mercury in the atmosphere is deposited at any rural forest in the world, including the deposition of mercury in its gaseous form, which most previous studies do not address,” said Obrist. 

“Trees take up gaseous mercury from the atmosphere through their leaves, and as plants shed their leaves or die off, they basically transfer that atmospheric mercury to the ecosystems,” he said.

UMass Lowell student Eric Roy, a double-major in meteorology and mathematics, is also among the study’s co-authors.

“This study allowed us to quantify how much mercury is being accumulated in this type of forest,” Roy said. 

“Modelers can use these results to improve their understanding of how mercury cycles through the environment on a global scale and how that might change in the future.”

For the past 16 months, the team has measured how mercury in the atmosphere gets deposited at Harvard Forest in Petersham, a nearly 4,000-acre site that includes hardwood deciduous broadleaf trees such as red oak and red maple that shed their leaves every year. 

A set of measurement systems placed at various heights on the forest’s 100-foot-tall research tower assessed the site’s gaseous mercury deposition from the tree canopy to the forest floor.

“76 percent of the mercury deposition at this forest comes from gaseous atmospheric mercury,” Obrist said. 

“It’s five times greater than mercury deposited by rain and snow and three times greater than mercury that gets deposited through litterfall, which is mercury transferred by leaves falling to the ground and which has previously been used by other researchers as a proxy for estimating gaseous mercury deposition in forests.”

“Our study suggests that mercury loading in forests has been underestimated by a factor of about two and that forests worldwide may be a much larger global absorber and collector of gaseous mercury than currently assumed,” he said. 

This larger-than-anticipated accumulation may explain surprisingly high mercury levels observed in soils across rural forests. 

Plants seem to dominate as a source of mercury on land, accounting for 54-94 percent of the deposits in soils across North America, as per Obrist. 

The total global amount of mercury deposited to land is estimated at 1,500-1,800 metric tons per year. Still, it may be more than double if other forests show similar levels of deposition.

The researchers are continuing their work at the second forest in Howland in northern Maine.

Howland Forest, a nearly 600-acre research site full of evergreens that retain their leaves year-round, offers a distinctly different habitat than the deciduous forest in Petersham. 

“Assessing both forests will allow researchers to examine differences in mercury accumulation between different forest types,” Obrist said.

(With inputs from ANI)

(Edited by Amrita Das and Pallavi Mehra)