Small-scale and "artisanal" gold mining is now the biggest source of current mercury pollution. Credit: Gold nuggets image via Shutterstock View full size image

If, as Robert Frost wrote, "nothing gold can stay," then mercury sticks around forever.

Mercury has an uncanny ability to bind to precious metals, and for millennia, people have used it to mine gold and silver. Small-scale, or "artisanal," mining — which makes use of mercury in this way — has recently become the leading source of mercury pollution, several recent studies show.

Mining releases mercury into the air when it is burned off to isolate gold from a chunk of rock or slurry; it also seeps into the soil and rivers from water used in the process and runoff from rainwater, contaminated by materials left behind from mining operations.

Many of these miners operate illegally in developing countries like Peru, where there is little or no regulation, making the practice difficult to quash, said Dave Krabbenhoft, a researcher at the U.S. Geological Survey in Middleton, Wis.

The heavy metal also sticks around for centuries, and is re-emitted from the soil and the water into the atmosphere, and vice versa. As a volatile element, mercury can evaporate at relatively low temperatures, and can then be deposited out of the atmosphere through chemical reactions back to the soil or bodies of water. In fact, most mercury that arrives in the ocean — after falling out of the air or being washed there by rivers — is "legacy" mercury that was already present in the environment, much of it spewed from smokestacks or leeched from gold mines hundreds of years ago, Krabbenhoft told LiveScience.