Mercury from Gold Mining Still Impacts the Environment in California

When California's gold rush began in 1848, prospectors rushed to the state in order to mine for the precious metal. A part of this mining, though, was mercury; now, scientists have discovered that even after all of these years, mercury is impacting the environment. In fact, sediment-absorbed mercury is being transported by major floods from the Sierra Nevada mountains to Central Valley lowlands.

The scientists actually weren't looking for this particular flow of mercury. Instead, they were working in California's Central Valley as they studied how quickly floodplains filled up with sediment. That's when they came across Burma-Shave signs that said "Sand."

"We thought that was quite strange because the floodplains around us were so much finer--composed of silt and clay materials," said Michael Singer, one of the researchers, in a news release. "So we followed the signs and ended up in a huge sand mine. They were mining sands by the truckload for the construction industry and said they would be doing so for at least the next several decades."

It turns out that the sand was carried by a massive flood in 1986 to the Yuba River Basin. The upstream Yuba was actually the biggest gold-mining drainage of all the Sierra drainages used in the 19^th century, which means that it was contaminated with mercury.

"People know there was gold mining in the Sierra Nevada and they know that there was mercury mining in the Coast Ranges, but they're not really sure of the modern-day impact, especially when the contaminant sources are not directly by the bay," said Singer in a news release. "People want to know what is causing contamination of the food webs of the Central Valley."

In order to take a closer look at mercury contamination, the researchers documented flood-driven fan erosion, sediment redistribution and a process called progradation, the growth of a sedimentary deposit farther out into the valley over time. They then compared gold rush data with modern topographic data sets. This showed that the Yuba River progressively cut through the sediment and in the process left behind massive contaminated terraces.

"There is a lot of sediment left in the system that is highly contaminated and readily available to be remobilized and sent downstream just because it's sitting in unconsolidated sediments along the margins of a river that can become very big during a storm," said Singer in a news release. "That susceptibility, coupled with projections for climate change in the region indicating more massive storms in the future, means that there is a dangerous synergy."

The findings are published in the journal Proceedings of the National Academy of Sciences.