Public healthMercury levels in rainfall are rising in parts of U.S.

An analysis of long-term trends in the amount of mercury in rainfall and other forms of precipitation in North America found recent increases at many sites, mostly in the center of the continent. At other sites, including those along the East Coast, mercury levels in rainfall have been trending steadily downward over the past twenty years.

The findings are consistent with increased emissions of mercury from coal-burning power plants in Asia and decreased emissions in North America, according to Peter Weiss-Penzias, an environmental toxicologist at UC Santa Cruz. Weiss-Penzias is first author of a paper on the findings published in the journal Science of the Total Environment.

Mercury is a toxic element released into the environment through a variety of human activities, including the burning of coal, as well as by natural processes. Rainfall washes mercury out of the atmosphere and into soils and surface waters. Bacteria convert elemental mercury into a more toxic form, methyl mercury, which becomes increasingly concentrated in organisms higher up the food chain. Mercury concentrations in some predatory fish are high enough to raise health concerns.

Monitoring network
UCSC reports that in the new study, researchers analyzed data from a network of monitoring sites in the United States and Canada that collect measurements of mercury in precipitation. The network has been collecting data since 1997, but monitoring sites were initially concentrated in the eastern United States, while sites in the Midwest, Rocky Mountain, and West Coast regions were added only relatively recently. The long-term dataset extending back to 1997, which represents mostly the eastern regions, showed an overall decline in mercury concentrations. But when the researchers looked at more recent time periods starting in 2007 or 2008, they found many sites with significant positive trends in mercury concentrations.

“This hadn’t been observed before, and the sites with positive trends were primarily concentrated in the Intermountain West and in the central part of the continent, in the Rocky Mountain, Plains, and Midwest regions,” Weiss-Penzias said. “On the Eastern Seaboard, the trend was still largely negative even for the shorter time periods.”

Coauthor Mark Brigham of the U.S. Geological Survey noted that the findings are broadly consistent with other recent reports of decreasing atmospheric mercury deposition in North America since about 1997, but with a new twist.

“By examining trends in more detailed time increments and more geographic detail, we saw that in recent years the long-term decreasing trend has leveled off,