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Gold rush-era mercury flows downstream in Calif.

FRESNO, Calif. (AP) -- Though California's gold rush ended long ago, mercury-contaminated soil from mining activities in the foothills of the Sierra Nevada flows downstream during major floods.

That's according to a new study published Monday in the Proceedings of the National Academy of Sciences. Research finds the runoff will continue for thousands of years unless something is done to prevent mercury-laden sediment from eroding and ending up in the state's agricultural heartland.

Scientists have long known that mercury contamination from mining activities poses a potential risk to fish and the environment.

In the latest study, a group of U.K. and U.S. researchers analyzed 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.