There’s Gold In Them, Thar Hills -- And Toxic Mercury

When gold was discovered in California in 1849, the miners were confronted with a problem: there were huge amounts of the precious metal in the foothills of the Sierra and the only way to get it out was to blast it out of the soil with high-pressure hoses.
The resulting mud containing the gold was run through sluices and mixed with mercury so the gold would settle to the bottom. The remains of the 19th-century practice are still visible in the area, and the poisonous mercury is now slowly making its way toward the fruit and nut orchards, and the rice fields of California’s lush Central Valley, America’s food basket, according to new research by a team of British and American scientists.
Every time there is a big flood -- usually once a decade -- the mercury in the sediment moves farther down into the valley. It could take 10,000 years for it all to finally be released from the sediment and spread out, the researchers said.
http://www.insidescience.org/content/there%E2%80%99s-gold-them-thar-hills-and-toxic-mercury/1475