The heavy metal tends to be loosened during major floods that occur about once a decade and likely will continue unless something is done to prevent mercury-laden sediment from eroding and ending up in the state's agricultural heartland, according to the study published online Monday in the Proceedings of the National Academy of Sciences.
"The problem is very serious indeed," said lead author Michael Singer of the University of St. Andrews in Scotland.
During the 19th century gold rush, miners used mercury to process gold throughout the West. An estimated 26 million pounds of mercury was used for gold recovery in the Sierra Nevada and Klamath-Trinity Mountains.
Scientists have long known that mercury contamination from long-ago mining ends up in sediments and in stream channels. The metal sometimes converts into a more potent form that can be consumed by fish, amphibians and waterfowl. Numerous waterways in Northern California downstream of mining activities post warnings about fish consumption.
In the latest study, a group of U.K. and U.S. researchers studied soil from a Sierra Nevada mining deposit and points downstream.
Then the team analyzed topographic maps, streamflow data and satellite images, and used computer modeling to show that the mercury in the Sacramento-San Joaquin River Delta and San Francisco Bay came from historic gold mining that was released during episodic floods.
Charles Alpers, a research chemist at the U.S. Geological Survey's Sacramento office, said the findings show how important floods are in transporting mercury.
"This problem isn't going to go away anytime soon," said Alpers, who had no role in the study.