January 6, 2003
Mercury in California rainwater traced to industrial
emissions in Asia
By Tim Stephens
Industrial emissions in Asia are a major source of mercury in rainwater
that falls along the California coast, according to a new study by UCSC
Russell Flegal Photo: UCSC Photo
The researchers reported their findings in a paper published online
December 19 by the Journal of Geophysical Research - Atmospheres.
(The paper will appear in print in a subsequent issue of the journal.)
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
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 collected rainwater samples at two sites in central California:
on the coast at 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
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.
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