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. Levels  of mercury in the sediment were up to hundreds of times higher than background  levels.

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.