Climate change brings new urgency to threats posed by abandoned California gold mines

By Leah Campbell

On the western slopes of the Sierra Nevada mountain range in California sits the small town of Nevada City, a historic Gold Rush community where prospectors once mined for riches in the creek that ran alongside town. Today, it’s a popular tourist destination known for its easy access to the picturesque Yuba River and the region’s famed ski resorts.

Belying the welcoming charm of Nevada City, just a few miles to the south lies a different sort of legacy – a 30-acre plot of toxic desolation known as the Lava Cap Mine where arsenic-laden waste from decades of gold and silver mining has contaminated area soils and water. In 1997, a log dam built to contain the debris collapsed in a winter storm and roughly 10,000 cubic yards of mine waste – enough to cover almost five football fields – washed through creeks and into a nearby lake.

For more than two decades now, the site has been on the Environmental Protection Agency’s (EPA) Superfund list, one of thousands of locations designated as posing “unacceptable risks to human health and the environment.” Clean-up efforts are still in progress at Lava Cap.

Lava Cap is among many abandoned hardrock mines strewn through what Californians call “Gold Country.” State and federal officials estimate that there are almost 50,000 such mines in California and more than 160,000 overall across the western United States. Not all are contaminated, but those that are continue to leach out toxic substances, experts say.

Cleanup of these sites take years and cost millions of dollars. But now, climate change is creating new urgency amid fears that more frequent and volatile flooding and out-of-control wildfires will further spread the toxins.

“There’s been decades of research about remobilizing of mine waste by floods,” said Sheila Murphy, a hydrologist with the US Geological Survey (USGS) who has studied the impacts of extreme weather on mine-contaminated watersheds. “One of the issues is when these big floods then move the sediment into streams or reservoirs,” she said. “It’s just sitting there vulnerable to being remobilized over time.”

Murphy is just as concerned with fire as she is with floods. Changes to the soil, ash buildup, and the burn-off of vegetation after a fire means that fire-scarred lands are more prone to floods and mudslides, not just for a single season, but potentially for several years after a fire.

When it comes to the remobilization of contaminated mine waste because of flooding, “fire puts that on steroids,” Murphy said.