the researchers discovered that trees growing near mining towns showed elevated levels of mercury in their rings. “The tree is just reflecting what the atmosphere is doing,” Gerson notes.

but three were within roughly 3.1 miles of mining towns previously known to rely on amalgam burning. One site was also adjacent to protected forest lands. Get the Popular Science daily newsletter ...

Most notably, the exact distance to mining towns was unknown due to the illegal nature of these operations. This likely influenced the concentrations of Hg found in the trunk wood.