Gold mining has been called “the gift that keeps on taking” by geographer Gary Brechin, and the damage to California will possibly last ten millennia. The LA Times and USA Today report that the hydraulic mining practices of gold rush-era miners in the mid-to-late 1800s deposited mercury-laced sediments throughout the Sacramento Valley and deep into the San Francisco Bay. Even though the practice stopped, the damage remained, and the mercury washed down from the Sierra Nevada mountains.

The paper, authored by Michael Bliss Singer, et al, from the Earth Research Institute at UC Santa Barbara and published in the Proceedings of the National Academy of Sciences, analyzed the process by which sediment deposits form downriver.

By studying the historical streamflow and topographical data of 105 sites along the Yuba River, they were able to run computer modeling to gauge sediment distribution. Their conclusion was grave: large stores of undiluted mining debris are still present in the floodplains, and any major flood could wash the pollutants downstream for the next 10,000 years.

Because the Yuba is downstream from several abandoned gold mines, the scientists felt it was an appropriate site choice, and they also noted that huge amounts of mercury were washed downstream in the huge floods that took place in 1986, 1997 and 2006.

“[We have 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 said. “[But over 100 years later,] the problem [is] much, much bigger than (many others) are suspecting it is.”

The scientists were unable to investigate how far downstream the mercury goes, but a Duke University geochemist not attached to the study, Gretchen Gehrke, said the information they uncovered is valuable for estimating how much mercury reaches the lowlands. She contended that evidence exists of mercury’s polluting effects on ecologically diverse areas that represent stopovers for millions of migrating bird species.

“Sediment-bound Hg has contaminated food webs of the San Francisco Bay-Delta, but the dominant geographical sources of Hg to downstream ecosystems in this and similar river basins are debated,” the Academy writes of the study’s significance. “This research addresses a gap in generic theory of postmining fan evolution that enables anticipation, prediction, and management of contamination risk to food webs.”

[Image via this YouTube video about the effects of gold mining on San Francisco]