Wild fig tree rings offer a cheap method for tracking toxic atmospheric mercury, a byproduct of gold mining in the Global South, according to a study conducted in the Peruvian Amazon and published April 8 in the journal Frontiers in Environmental Science.
Computer models suggest that atmospheric mercury can potentially travel across the globe, to be deposited back in landscapes, though more study is needed to verify how far it spreads. When it falls to the ground or in water, it can accumulate in organisms such as fish and other food sources, where it acts as a neurotoxin to both humans and wildlife.
Peruvian assistants help to core a tree in the Peruvian Amazon.
Environmentalists and scientists may now establish biomonitoring networks with wild fig trees (Ficus insipida) in order to better understand how mercury spreads over time and space.
"We're trying to reduce emissions, especially from gold mining, as part of the United Nations Minamata Convention on Mercury, and in order to do that, we need to be able to measure it, to see the impact over time," said Jacqueline Gerson, the study's corresponding author and assistant professor of environmental and biological engineering in the College of Agriculture and Life Sciences. "This really offers a method that can be employed throughout the Global South to understand changes in mercury over time, as well as spatial indicators of mercury."
Artisanal and small-scale gold mining accounts for about 20% of all the gold produced worldwide, and is the biggest single source of mercury pollution. Liquid elemental mercury is used to separate gold from ore; mercury can then enter the environment either when ore residues are dumped onto the landscape or when it is burned off, a practice employed in some 70 countries worldwide. Dumped ore residues have a localized impact, while burning off mercury can spread farther afield.
The study revealed that from 1941 to 2019, in two study sites near mining towns where mercury-gold mixtures were burned, mercury concentrations were higher after the year 2000.
"This is likely due to the expansion of gold mining activities around this time period," Gerson said.
Also, Gerson said, a strong relationship existed between the average concentration of mercury across cores taken from trees and atmospheric levels, especially in the Peruvian dry season (May-October), when most mining occurs. Passive air samplers confirmed that mercury concentrations in tree rings mirrored atmospheric levels.
Previous studies have used tree rings to track mercury levels from coal combustion, particularly in Canada, but the method had not been used in the tropics for measuring mercury from gold mining.
"While the technique itself is not new," Gerson said, "we wanted to test its application in places where it's really hard to put out monitors for atmospheric concentrations, because they're costly and require energy or need to be changed a lot."
Currently, expensive active mercury monitors pump air through a device to collect mercury and require electricity, while passive air samplers use activated charcoal to collect ambient mercury, and are good for remote areas but cost up to $100 each.
The study included five different sites. One was near a gold mine, but ore was not burned there; two sites were near mines where burning also occurred; and two remote, unimpacted sites in the Peruvian Amazon. The researchers drilled cores (small enough to fit in a straw) from trees and chemically analyzed them for mercury concentrations in the lab.
Co-authors include Igor Lehnherr and Trevor Porter, both at the University of Toronto.
Funders include Duke University; the Forest History Society; the Geological Society of America; the Lewis and Clark Fund; and the National Science Foundation.
