Mercury Contamination: The Lesser-known Legacy of the California Gold Rush

New research shows that sediment-adsorbed mercury -- a lesser known legacy of California's historic gold rush -- is being transported by major floods from the Sierra Nevada mountains to Central Valley lowlands where it is threatening to infiltrate a series of food webs.

These are the findings of Michael Singer, an associate researcher at the University of California, Santa Barbara's Earth Resource Institute, writing in the Proceedings of the National Academy of Science.

His report reveals the Yuba River and other neighboring streams transfer mercury stored in Sierran man-made sediment deposits to the lowlands during 10-year flood events.

"This new study addresses a gap in the general theory of the evolution of toxic sediment emplaced by industrial mining, which enables anticipation, prediction and management of contamination to food webs," Singer said in a statement.

The discovery of the process was a fortunate one: Singer was working with a colleague in the state's Central valley when they discovered signs that read "SAND."

"We thought that was quite strange because the floodplains around us were so much finer -- composed of silt and clay materials," Singer said.

They learned that a massive 1986 flood in the Yuba River Basin carried with it enough sand to bury a major rice field. Knowing that the upstream Yuba was the biggest gold-mining drainage of all the Sierra drainages used in the 19th century, Singer decided to search for mercury given that gold rush miners used it to separate gold.

Centuries later, mercury remains a significant problem for individuals living in San Francisco Bay and the Delta.

"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," he said. "People want to know what is causing contamination of the food webs of the Central Valley."

By documenting flood-driven fan erson, sediment redistribution and the process by which a deposit grows out into a valley over time, the new study begins to answer this question, as well as raise a voice of warning of what may be yet to come.

"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," Singer said. "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."