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The Gold Rush Left Behind Mercury That’s Still Contaminating California

Leftover mercury will continue to flush through the environment, eventually making its way into the San Francisco bay, for the next 10,000 years

Damming and dredging a California river for gold (Harper’s Weekly magazine)

More than a century ago, around 300,000 hopeful people rushed to California with the aim of striking it rich. From 1848 to 1855, at the height of the Gold Rush, miners tore up the countryside in pursuit of that precious mineral. But even forty-niners who didn’t strike it rich left a legacy of rare metal behind—namely, mercury, which still contaminates California’s soil and waterways.

Miners didn’t employ only the quaint panning methods normally associated with the Gold Rush; they used powerful hoses to spray away entire hillsides.

Blasting away the hillside with water jets. Photo: Denver Public Library

Discovery News reports:

The sediment was then run through “sluice boxes,” where mercury was added to bind to gold. But large quantities of the heavy metal made their way into sediment downstream. This destructive mining filled valleys with sediments that caused flooding in California’s Central Valley, and in 1884, the federal government shut down much of this gold-mining activity.

Sluice running down from the hillsides. Photo: Denver Public Library

According to new research, that leftover mercury will continue to flush through the environment, eventually making its way into the San Francisco Bay, for the next 10,000 years or so. And because it’s in the water and soil, it also inevitably makes its way into living organisms.

When the mercury reaches the lowlands of the Sacramento-San Joaquin River Delta, where the Yuba River and other streams that flow out of the Sierra Nevada end up, it can be converted to methylmercury by microbes. Methylmercury is the organic form of the heavy metal, which can accumulate in animals and make its way up the food chain.

As the mercury concentrates in larger and larger organisms, Discovery points out, it eventually makes its way onto human dinner plates in the form fish like salmon and bass.

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