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Mercury contamination in California to last 10,000 years

Toxic remnants of gold rush will seep into San Francisco area waterways for millennia

MERCURY RUSH Banks that line Northern California’s Yuba River contain mercury left over from the California Gold Rush. A new study predicts that the toxic metal will continue to contaminate the region’s waterways for more than 10,000 years. 

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California’s gold rush ended more than a century ago, but the contamination it caused will last thousands of years, a new analysis shows.

Some hydraulic gold mining processes use the toxic metal mercury to separate gold from gravel. In the mid-1800s, gold mining released more than a cubic kilometer of mercury-laden sediments into Northern California’s Sierra Nevada foothills. The sediments fanned out and inundated rivers that flow into the San Francisco Bay. Researchers estimate that 90 percent of the mercury is still trapped within the sediments.

To understand how flooding and erosion may trigger future releases of the poison, researchers led by Michael Bliss Singer of the University of St. Andrews in Scotland measured mercury levels in sediments at 105 locations upstream of the bay.

Drawing on historical flood data to predict sediment flow, the team reports October 28 in the Proceedings of the National Academy of Sciences that the mining sediments will continue to release mercury into waterways over at least the next 10,000 years. As climate change intensifies the area’s rainstorms, the researchers predict, the flood-driven discharges should become more frequent. 

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