California’s gold rush ended more than a century ago, but its contamination will last thousands of years, say scientists!

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 scientists report 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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