The mercury in rainwater is not in itself a health threat, but mercury pollution is a significant problem in San Francisco Bay and other California waters because the toxic element builds up in the food chain. State regulatory agencies are looking for ways to reduce the amount of mercury entering the state's waters from various sources. The new study provides valuable information to guide those efforts, said Russell Flegal, professor and chair of environmental toxicology at UCSC and coauthor of the new paper.
"People are talking about things like removing dental fillings before cremating bodies, but our analyses indicate that this may be a trivial source of mercury compared to the inputs from industrial emissions in Asia," Flegal said.
Interestingly, it is not just the mercury itself but a whole cocktail of atmospheric pollutants that contribute to the deposition of mercury in rainfall. Elemental mercury behaves as a gas in the atmosphere and is not washed out in rain until it has been oxidized into a charged ionic form that can be captured by water droplets, said Douglas Steding, the paper's lead author.
Ozone, a major component of urban and industrial smog, plays a key role in this oxidation process, said Steding, who did the study as a graduate student at UCSC and is now studying environmental law at the University of Washington.
"There is a relatively large reservoir of mercury in the atmosphere, and it's the rate of oxidation that determines how much of it gets deposited in rainfall," he said.
Mercury is a trace contaminant of most coal, and emissions from coal-burning power plants are a major source of mercury pollution in many parts of the world. In the Pacific Basin, the main source of atmospheric mercury is coal combustion in China. China relies heavily on coal as a fuel and accounts for about 10 percent of the total global industrial emissions of mercury.
Air pollution in China also generates ozone, which peaks during the winter due to increased fuel consumption for heating. Air loaded with mercury and ozone moves off the continent into the Western Pacific, where it is incorporated into developing storms.
"The mercury we measured in rainwater results from a combination of mercury emissions and ozone production, as well as meteorological factors--the storm tracks that transport the pollutants across the Pacific," Steding said.
Steding collected rainwater samples at two sites in central California: on the coast at UCSC's Long Marine Laboratory and at Moffett Field near San Jose, on the inland side of the Santa Cruz Mountains. For each rainfall event, the researchers used air mass trajectories calculated by a national climate lab to trace the movement of the storms across the Pacific from Asia.
Rainwater collected at the coastal site showed the background concentrations of mercury in storms as they arrived directly off the Pacific Ocean. Those measurements were about three times higher than estimates of the natural, preindustrial level, Steding said.
Rainwater from the inland site showed mercury concentrations 44 percent higher than at the coastal site. Steding tentatively attributed the difference between the two sites mostly to ozone in Bay Area smog, rather than local emissions of mercury.
"There is a local influence of urban smog on the mercury oxidation rate. We see a background signal of mercury blowing off the Pacific, then a local enrichment that's probably due to urban smog," he said. "If we want to reduce mercury deposition, it's not enough to shut down local emissions of mercury, because other pollutants influence how much of the mercury in the atmosphere ends up in rainwater."
Steding emphasized that people should not worry about health effects from the mercury in rainwater, because the concentrations are very low. But the deposition in rain does add mercury to surface waters, where the toxin enters the food chain and builds up to high levels in certain kinds of fish. State health officials have issued advisories warning people not to eat fish from more than a dozen bodies of water in California, including San Francisco Bay.
Much of the mercury contamination in California is the result of historic mining operations. Large amounts of liquid mercury were used in gold mining operations during the Gold Rush, leaving contaminated sediments throughout the vast watershed that ultimately drains into San Francisco Bay. In addition, Flegal's lab has shown that historic mercury mine sites in the Coast Ranges are still leaching mercury into rivers and streams that drain into San Francisco Bay.
Prior to this new study, however, no one had looked at atmospheric transport of mercury from Asia as a source of mercury pollution on the U.S. West Coast, Flegal said.