Rock Doc: Miners’ mercury still makes mark on environment

When I was a younger and more sprightly woman, I spent part of my life investigating unusual hot springs in rural California. They were salty and quite stinky springs out in the middle of nowhere, and several of them occurred right in the center of an old gold-laced mercury deposit.

The fieldwork had its challenges. In the afternoon it was routinely over 100 degrees, and the sun was relentless. One afternoon, I even flirted with heatstroke. Another problem was that the rattlesnakes were numerous and big.

But the springs were fascinating from a scientific point of view. I spent a lot of time in the laboratory back East analyzing the waters of the springs. They were transporting gold, and the important question was how. It was the sulfur in the spring water, I ultimately concluded, that made this unusual trick possible. In short, the stinky aroma of the springs was key to their ability to transport gold up to the surface.

The area where I worked in California didn’t play a direct role in the Gold Rush of 1849. There just wasn’t enough gold around the hot springs to have caught the attention of the old-timers who made fortunes elsewhere in California. But the place where I worked had been mined for mercury, including back in the old days. That was because mercury was used to concentrate gold in materials miners elsewhere were processing.

In the past, miners worked with pans, hydraulic hoses and sluices to remove and concentrate gold-rich sediment. Because gold is attracted to mercury, the miners poured liquid mercury on the earthen material they had concentrated. The gold particles moved into the mercury. The miners could then heat the mercury and boil it away, leaving a concentrated “button” of gold behind.

There was a lot of mercury being slopped around in the old processes the miners used. Much of it went into the air when the miners heated the mercury-gold mixture, but some stayed behind in the sediments. That was an environmental hazard in the past, and it still is today.

New research highlights the environmental challenges those old mining techniques continue to create for us. As explained in a recent piece on the website Inside Science, one of the key places at issue is the Yuba Fan, a volume of sediment built up around the Yuba River, a tributary of the Sacramento River.

The Yuba Fan contains more than a billion cubic yards of sediment. Terraces in the fan act like small dams, keeping the material from moving downstream. But about once every 10 years, a substantial flood kicks loose materials that then move downhill toward the lowlands, which include agricultural areas like rice fields.

The recent research was published in the Proceedings of the National Academy of Sciences, which is a measure of its importance. In part because California’s agricultural bounty is a keystone to all of us who like to eat, I’m sure more follow-up research will be done.