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Mining Is Main Source of SF Bay Mercury

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Human activity injects 2,000 tons of mercury into the global environment annually, according to the University of Michigan. Enough of that goes into San Francisco Bay to prompt periodic seafood-consumption advisories for seafood lovers in that region of northern California.

Now researchers at the University of Michigan and other institutions have found a way to “fingerprint” mercury to determine its origin. An analysis of the mercury in bay floor sediments revealed that small fish near the bottom of the food web acquire their mercury from those sediments, the said the university, whose team also included researchers from the University of California, Davis, and the San Francisco Estuary Institute.

And the answer is: The miners did it.

"Our analysis of the sediments showed that it's most likely coming from either two or three dominant sources," said graduate student Gretchen Gehrke, who was the lead author on the paper, in a university press release. "There's one distinct fingerprint coming from historic mercury mines to the south and a different fingerprint coming from historic gold mines to the north. We see intermediate values in sediments in the middle of the bay, which could represent either mixing of the two or possibly a separate third source, so we can't say for sure whether it's two or three sources. But the fact that we see at least two separate fingerprints and a strong spatial gradient instead of a hodgepodge of many different fingerprints tells us that the mercury is coming from a small number of large sources rather than a lot of localized sources like a power plant here, a refinery there."

The fingerprint analysis looks at mercury isotopes. A similar technique was used in another study that analyzed the level of mercury in murre eggs in the Arctic and found a correlation between ice-pack cover and lower mercury levels. However that study did not cover seafood.

"This is the first study to track mercury directly from source to sediment to food web," said Joel Blum, who is the John D. MacArthur Professor of Geological Sciences and a professor of ecology at the University of Michigan, in the release.

The findings were outlined in two journals: The journal Geochemica et Cosmochimica Acta published one in its February 1 issue, and the Environmental Science & Technology published another paper on the matter on January 21.