The Fate of Mercury in Forest Fire Ash

For two weeks in November 2018, smoke from the Camp Fire near Chico, CA was blown about by prevailing winds to Berkeley Hills near San Francisco. This includes my home. Earlier, we were in the smoke plume of fires near Napa. During the gold rush, mercury was mined nearby. I’d wondered about the fate of mercury in the fires. Did incineration concentrate Hg in ash, or vaporize it? The Mad Hatter and the terrible fate of gold prospectors in Europe’s castles came to mind.

Martin TSZ-Ki Tsui of the Department of Biology, University of North Carolina at Greensboro, studied the ash residue from forest fires in California.1 Black ash is the residue from low-intensity combustion. White ash correlates with higher intensity (<500 °C). The fear was that the ash is mobile in run-off, which often occurs postfire. (See Figure 1 in Ref. 1, which shows the fire cycle common in California. During the summer, vegetation dries out. Late in the season, fires consume the vegetation. During the rainy season, groundwater washes the ash downstream, where mercury in many forms is converted to methylmercury, the most toxic form.)

Surprisingly, Tsui found that the ash samples contained mercury that is chemically unreactive, especially compared to unburned vegetation. Stream water mixed with ash for 4 or 12 weeks contained low levels of elemental and methyl mercury. Tsui proposes that the black carbon (aka charcoal) in the ash binds to mercury to keep it in a nonreactive form, even when leached with groundwater.

Personally, I’m relieved. I’m sure that the smoke plumes transported lots of mercury downwind. However, if the ash is sequestering it in unreactive forms, the impact on health may not be as bad as one might expect.

Reference

  1. Ku, P.; Tsz-Ki Tsui, M. et. al. Origin, reactivity and bioavailability of mercury in wildfire ash. Environ. Sci. Technol; doi: 10.1021/acs.est.8b03729.

Robert L. Stevenson, Ph.D., is Editor Emeritus, American Laboratory/Labcompare; e-mail: [email protected]

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