Monday, October 20, 2014
PLACERVILLE, CALIFORNIA
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Rock doc: Mercury contamination from the good old days

By
December 14, 2013 |

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.
No one was actively mining the small area where the springs are found. There just wasn’t enough ore to make the project economic. But the rocks of the location had small veins of chalcedony, calcite and other minerals that had elevated values of both gold and mercury in them. Working with a couple of colleagues, I took samples of the spring waters, the gases bubbling out of the springs, the precipitates forming around them, and anything that looked interesting in the nearby rocks.
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 heat stroke. Another problem was that the rattlesnakes were numerous and big.
I spent a lot of time in the laboratory back East analyzing the waters of the springs. They were transporting gold, and the question was how. Gold is normally quite insoluble. That’s why it can be used to crown a tooth. Even in an environment rich in warm spit and sips of hot coffee, a golden tooth won’t dissolve away because gold is quite insoluble under most conditions. But clearly the hot springs were different. In the end, I concluded that sulfur in the spring waters was keeping the gold in solution until the waters broke to the surface and the gold precipitated out as temperature and gas concentrations changed.
There were some other interesting things about the strange springs, too. Some of the cooler ones had the larval stage of an insect living in them. I took samples of the wiggling little creatures and gave them to a biologist to identify. The insect normally lives around the ocean in salt-marshes, but it was making use of the salty springs even though they were well inland.
The area where I worked in California hadn’t played 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 old days, 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 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 of the mercury stayed behind, in the sediments.
New research is highlighting the environmental challenges those old mining techniques continue to create for us today. 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 is totally artificial, created by humans,” Michael Singer of the University of St. Andrews said to Inside Science.
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 there is a substantial flood that kicks loose materials that then move downhill toward the lowlands — which include agricultural areas like California’s 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.
Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University.

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