Californias gold rush ended more than a century ago, but mercury used to mine the gold is still making its way into valleys below the mines and the contamination will probably last thousands of years, a new analysis shows.
Background:In the mid-1800s, gold mining released more than a cubic kilometer of mercury-laden sediments into Northern Californias Sierra Nevada foothills. The sediments fanned out and inundated rivers that flow into the San Francisco Bay. The mercury contamination resulted from the hydraulic mining processes of the era that used mercury to separate gold from gravel. An estimated 13000 tons of mercury was used during the 19th century gold rush in the Sierra Nevada and Klamath-Trinity Mountains. Researchers estimate that 90 percent of the mercury is still trapped within the sediments.
"They didn't just pan for gold,"
says Michael Singer. "That's a romantic notion of gold mining. It was
actually an industrial process whereby they sprayed giant high-pressure
hoses, invented in 1852, at upland hillsides to wash the sediment
downstream." (Credit: "Hydraulic mining at Rockerville, Dak.", Library
of Congress via Wikimedia Commons) Transport of contaminated sediments into the San Francisco Bay Delta has contaminated the food web and poses a huge risk to the lowland ecosystems and to the human population as well because a lot of people eat fish from this system. Within the aquatic environment, mercury may be transformed to methylmercury, a mercury species that accumulates in fish, amphibians and waterfowl. Numerous waterways in Northern California downstream of mining activities therefore post warnings about fish consumption.
The new studyUp to now, the processes by which Hg is delivered to lowlands and the patterns of its floodplain deposition were only poorly understood. To understand how flooding and erosion may trigger future releases of
the poison, the research group led by Michael Bliss Singer measured
mercury levels in sediments at 105 locations upstream of the bay. Levels
of mercury in the sediment were up to hundreds of times higher than
background levels.
This new study addresses a gap in the general theory of the evolution of toxic sediment emplaced by industrial mining, which enables anticipation, prediction, and management of contamination to food webs, says Michael Singer, associate researcher at University of California, Santa Barbaras Earth Research Institute.
The team analyzed topographic maps, streamflow data and satellite images, and used computer modeling to show that mercury stored in immense Sierran human-made sediment deposits is carried by the Yuba River and other nearby streams to the Central Valley lowlands. The heavy metal tends to be loosened during major floods that occur about once a decade, most recently in 1986 and 1997.
Drawing on historical flood data to predict sediment flow, the team reports that the mining sediments will continue to release mercury into waterways over at least the next 10,000 years unless something is done to prevent mercury-laden sediment from eroding and ending up in the state's agricultural heartland. As climate change intensifies the areas rainstorms, the researchers predict, the flood-driven discharges should become even more frequent.
"The problem is very serious indeed," said lead author Michael Singer of the University of St. Andrews in Scotland.
The analysis was undertaken at the University of St. Andrews in Scotland. The findings have been published in the journal Proceedings of the National Academy of Sciences.
While the source of contamination in California is mostly historic,
mercury pollution related to gold mining is ongoing in other regions of
the world. In 50 countries small‐scale miners recover gold by amalgamation process,
using excessive quantities of mercury. Fifteen million artisanal gold
miners apply this process and 100 million people might depend upon these
activities. Artisanal gold mining worldwide is responsible for one third
of all mercury released into the environment approximately 1000
tons/year.
Michael Sperling
The original study
Michael Bliss Singer, Rolf Aalto, L. Allan James, Nina E. Kilham, John L. Higson, and Subhajit Ghoshal,
Enduring legacy of toxic fan via episodic redistribution of California gold mining debris, Proc. Nat. Acad. Sci., Published online October 28, 2013.
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Related information (mercury pollution by gold mining) BRI/IPEN: Global Mercury Hotspots, BRI/IPEN, January 2013 Blacksmith Institute: Mercury pollution WorstPolluted.org: 2012 - Artisanal gold mining WorstPolluted.org: 2011 - Artisanal gold mining WorstPolluted.org: 2010 - Top six toxic threaths: Mercury UNEP: UNEP: Reducing Mercury in Artisanal and Small-Scale Gold Mining (ASGM)
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