New research has found that some tropical trees in the Peruvian Amazon can be used to monitor mercury pollution from gold mining, offering an alternative to expensive air monitors.

Roughly 16 million people worldwide engage in artisanal and small-scale gold mining, much of which is illegal due to environmental and human health concerns. In many tropical areas, including the Peruvian Amazon, miners use mercury to separate gold from its ore. The gold-mercury amalgam is then burned off, leaving the gold behind but exposing miners and the environment to toxic mercury fumes.

Previous research has mostly focused on how mercury from gold mining affects aquatic ecosystems, where it bioaccumulates up the food chain, posing risks to fish and the people who eat them. For this study, researchers wanted to understand mercury contamination on land.

The air monitors used to measure mercury on land are expensive, so the researchers explored the possibility of using tree cores instead to detect the presence of mercury and determine when it was emitted. However, in the absence of distinct seasons, most tropical trees don’t create growth rings, which can be counted to determine the year they were formed and analyzed for chemical content.

But the researchers found three species of tropical trees in their study area that were previously documented to create growth rings. So they took samples from all three but found only fig trees (Ficus insipida) actually created rings. The core samples were taken in 2019 and could be dated back to 1941.

Researchers identified five different study sites, each about 50 kilometers (30 miles) apart. Three were near mining activity and two were more distant. Four trees were sampled at each site and their mercury content compared to mercury levels recorded by nearby passive air samplers.

Researchers found a strong correlation between the two. “Where you see elevated mercury concentrations in the air, you’re seeing elevated concentrations reflected within the tree itself,” Jacqueline Gerson, study lead author and an assistant professor at Cornell University, U.S., told Mongabay by phone.

The trees closest to the mining sites had higher concentrations of mercury compared to trees farther away.  Also, at the most contaminated sites researchers could see mercury levels increase over time; in the more distant sites, a temporal change couldn’t be detected.

“The takeaway is that now we have a way to monitor Hg [mercury] in the air anywhere in the world, making a problem that was difficult and expensive cheap and accessible,” Miles Silman, a biology professor at Wake Forest University, U.S., who wasn’t involved in the study, told Mongabay by email.

Gerson added that using tree rings to measure mercury could be useful for monitoring progress toward the Minamata Convention on Mercury, an international agreement to address human-caused mercury contamination.

“I think it’s a really promising tool,” she said.

Banner image:A cross section of a tree showing its annual growth rings. Image by Bill Kasman via Flickr  (Public domain).