Mercury, image from Wikipedia
Special to USA Today
There really is gold in them thar hills, but there's something else in California's ridges, and in its valleys as well: mercury, a toxic legacy of the Gold Rush that will last for thousands of years.
Scientists have found high levels of mercury, which miners used to extract gold, in the foothills and valleys below former gold mines in California's Sierra Nevada range. Once in the valleys, the mercury can enter the food chain and could eventually make its way to San Francisco Bay. There's evidence that the poisonous chemical continues to ooze out of the Earth and travel to vulnerable ecosystems.
Other experts thought mercury-laced soils were staying put in the mountains, but "our research suggests that is not the case," says research leader Michael Singer, a geomorphologist at the University of St. Andrews in Scotland and the University of California, Santa Barbara. "There are massive amounts of mercury-laden sediment making its way down to the lowlands."
Pollution prevention wasn't a high priority in the 1850s, when the thirst for gold drove prospectors to adopt an environmentally devastating technique called hydraulic mining. Hillsides were blasted with water cannons, yielding a muddy slurry that was channeled through trough-like sluices. To separate the gold from the sand and gravel, workers dumped hundreds of pounds of mercury, which can cause severe neurological damage, into the slurry.
The traditional view held that mercury-laden sediment from the mines had all worked its way down to the bay and the sea by the early 1900s, Singer says. But there have also been signs the mercury is still flowing. So Singer and his team went looking for the stuff. They started at the Yuba Fan, a gigantic buildup of mining debris in the valley of California's Yuba River, which runs out of the Sierra and down into the lowlands. Not far upstream from the fan lie several abandoned gold mines.
The team's survey of 105 sites along the Yuba and two rivers nearby found relatively fresh mercury deposits. So it's clear that mercury is still percolating out of the ground and fouling waterways. For instance, huge amounts of mercury washed downstream from the Yuba Fan during major floods in 1986, 1997 and 2006, according to estimates by Singer and his colleagues. The scientists think there's still enough mercury in the foothills to contaminate the valley below for more than 10,000 years, they report in this week's Proceedings of the National Academy of Sciences.
The team didn't investigate whether the mercury that hits the lowlands goes all the way to the San Francisco Bay Delta, the network of bays and rivers that stretches from California's inland valley to the Pacific Ocean. But there's plenty of evidence that mercury from mining does reach the bay, says Duke University geochemist Gretchen Gehrke, who was not connected to the study. She says the new study is valuable for estimating how much mercury is reaching the lowlands, which is important for both humans and wildlife. The valley below the Sierra is a stopover for millions of migrating birds, and already several Bay-Delta fish species eaten by humans contain unsafe mercury levels.
And the amount of mercury washing into the lowlands could rise, Singer says, if climate change leads to longer floods as predicted.
There's a "romantic view of the Gold Rush, which is old guys roaming around with pans of gold. It was really an industrialized operation run by engineers," Singer says. More than a century later, the result is a "problem ... much, much bigger than (many others) are suspecting it is."