Gold rush-era mercury flowing downstream

California's gold rush may long be over, but mercury-contaminated soil from mining activities in the foothills of the Sierra Nevada continues to flow downstream, a new study suggests.

"The problem is very serious indeed," said Michael Singer of the University of St Andrews in Scotland.

Singer is the lead author of a study published on Monday in the Proceedings of the National Academy of Sciences.

During the 19th century gold rush, miners used mercury to process gold throughout the west. An estimated 26 million pounds (11.7 million kg) of mercury was used for gold recovery in the Sierra Nevada and Klamath-Trinity Mountains.

The heavy metal tends to be loosened during major floods 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.

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 UK and US 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 analysed topographic maps, streamflow data and satellite images, and used computer modelling 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 US 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.