Mercury-laden soil from the 1848 California gold rush is contaminating fish that often end up on humans' dinner tables.
"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," Michael Singer, associate researcher at UC Santa Barbara's Earth Research Institute said in a Past Horizons news release.
The mercury hitches a ride in floodwaters from the Sierra Nevada Mountains to the Central Valley lowlands. The contaminant was carried by the Yuba River and other nearby streams during "10 year-flood events" in 1986 and 1997.
The team used environmental models and independent data to demonstrate how 150-year-old mercury deposits could make their way to the California lowlands.
Singer and his team were analyzing how flood plains filled with sediment when they found signs from Burma-Shave (an a shaving cream company known for short advertisements) 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. "So we followed the signs and ended up in a huge sand mine. They were mining sand by the truckload for the construction industry and said they would be doing so for at least the next several decades."
A 1986 flood buried a rice field with sand; the land was then leased to sand-mining operation. The
The Yuba River was considered to be the largest source of drainage during the gold rush. Mercury was present because the miners used it to separate their gold.
"They didn't just pan for gold," Singer said. "That's a romantic notion of gold mining. It was actually an industrial process whereby they sprayed giant high-pressure hoses, invented in 1852, at upland hillsides to wash the sediment downstream. Sides of mountains were washed away and sent downstream, and the sediment started filling in these confined river valleys, actually spreading all the way out to San Francisco Bay. This caused problems for steamboat operations and increased flooding on lowland farms. The U.S. government ultimately got involved and stopped the mining in 1884, which basically ended the gold rush overnight."
Singer believes the mercury contamination is a problem for affected areas such as the San Francisco Bay.
"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."
The problem may only get worse due to climate change, future storms could also exacerbate the contamiantion.
