In 1989, following
a conference on mercury in Sweden, Steve Lindberg was riding on a train
from Stockholm when he noticed a familiar face. It was John Huckabee,
whom Lindberg had first met in 1974 after he joined ORNL's Environmental
Sciences Division (ESD). Huckabee had led an ORNL study of the ecological
effects of mercury from the world's largest mercury mine, located in
Almadén, Spain.
Lindberg worked
with Huckabee and others in the 1970s on a National Science Foundation
project that measured mercury concentrations in the North Fork Holston
River and Cherokee Reservoir in Virginia and Tennessee. Also in the
mid-70s, Huckabee, Lindberg, and Danny Jackson discovered that green
plants absorbed mercury from Almadén's mercury-enriched atmosphere
through their leaves. (In 1995, Lindberg and ESD's Paul Hanson discovered
that plants also emit mercury from their leaves if their internal mercury
levels exceed background air concentrations.)
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Steve
Lindberg measures fluxes of stable mercury isotopes from soils
in Canada's Experimental Lakes Area.
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On the train Lindberg
struck up a conversation with Huckabee, then head of the Environment
Division at the Electric Power Research Institute (EPRI), about atmospheric
mercury. Mercury is a heavy liquid metal, but this toxic material can
float through the air as a gas. Lindberg was one of the first scientists
to suggest that airborne global mercury could be a source of mercury
to land and surface water. This hypothesis helped explain why some pristine
lakes far from industrial sources of mercury are contaminated with the
toxic metal, which can be converted from its elemental form to methyl-mercury
by bacteria. Methylmercury is readily accumulated by fish. Humans exposed
to excessive amounts of methylmercury from contaminated fish can develop
neurological and other health problems.
At the conference in Sweden,
Lindberg had described his computer model for quantifying mercury emissions
and deposition, including the two-way flow between the earth's surface
(e.g., soils, lakes, and forests) and the atmosphere. Lindberg told
Huckabee that he hoped to collect data and develop models that would
lead to a better understanding of the exchange of mercury between the
atmosphere and forests and lakes.
Huckabee was interested
because the energy production sector is the nation’s largest point source
of mercury. Coal-burning power plants release more than 40 tons of mercury
a year, about a third of the total entering the environment. Power plants
emit more mercury vapor than either mercury mines or chlor-alkali plants
(which produce chlorine). Lindberg had studied all three industrial
sources of mercury. In 1980 he and ESD's Jay Story measured mercury
emissions from the Tennessee Valley Authority's coal-fired power plant
at Cumberland, Tennessee. They found that 99% of the mercury in the
feed coal was discharged to the atmosphere as gases (92%) and particles
(7%) and that only 1% was captured by pollution control equipment.
On the train Huckabee
helped Lindberg outline a proposal to EPRI. EPRI managers knew well
that mercury was the next environmental challenge for U.S. coal-fired
power plants, which were installing expensive scrubbers to reduce their
sulfur emissions to meet new U.S. Environmental Protection Agency (EPA)
regulations. As the research arm of U.S. electric utilities, EPRI wanted
to know more about the environmental effects of mercury emissions.
As a result of the "train-ride"
proposal, Lindberg's group received EPRI funding to make new field measurements,
and the Department of Energy supported methodology and instrument development.
Collaborators in this work have been Ki Kim, Jim Owens, and Paul Hanson,
all of ESD; Ralph Turner, formerly of ESD; and Tilden Meyers of the
Atmospheric Turbulence and Diffusion Division of the National Oceanic
and Atmospheric Administration (NOAA) in Oak Ridge.
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Using
an approach developed at ORNL, Jim Owens employs an automated
mercury sampling system to measure dissolved gaseous mercury concentrations
in alligator-filled waters in Florida’s Everglades. Mercury vapor
is purged from a water sample and is collected in quartz glass
tubes that contain metallic gold. When air is drawn through tubes
at a prescribed flow rate, mercury vapor amalgamates with the
gold. By heating the gold, researchers can precisely measure the
amount of mercury vapor released at a level of about a picogram
using atomic fluorescence spectroscopy.
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During the 1990s,
Lindberg and his colleagues greatly advanced the understanding of atmospheric
mercury in collaboration with internationally known mercury research
laboratories, such as the Swedish Environmental Research Institute and
the German Hydrophysics Institute (where Lindberg spent two sabbaticals
in 1994–1996). Many early tests of his method were performed at
East Fork Poplar Creek (EFPC), where tons of elemental mercury were
discharged as a result of a lithium separation process at the Oak Ridge
Y-12 Plant in support of the development of the hydrogen bomb. This
"manipulated backyard" research site proved invaluable for method demonstrations,
which have since led to almost 20 new research projects in ESD on mercury
cycling. The results were interesting, too: Using their new micrometeorological
gradient technique, Lindberg and his colleagues found that more mercury
was released to the air from EFPC floodplain soils than to groundwater.
After a series of hearings and much scientific evaluation involving
many ORNL researchers, the most contaminated floodplain soil was replaced
with clean soil.
"While local officials
and scientists were focusing on mercury problems at the Y-12 Plant and
on the floodplain," Lindberg says, "my colleagues and I were making
plans to study mercury beyond Oak Ridge."
Since 1994 the
ORNL group has published more than 50 journal articles on the development
and application of several new methods for measuring airborne mercury.
These ORNL-developed methods are now being used by a dozen other mercury
research groups around the world.
The wealth of information
obtained by these researchers has prompted EPA to take action. By December
15, 2000, as a result of a July 11 recommendation by a panel of 10 experts
convened by the National Research Council, EPA is expected to announce
regulations that will require coal-fired power plants to include some
level of mercury emission control, which could be costlier than sulfur
emission controls. Already EPA has issued guidelines and regulations
restricting mercury emissions from municipal incinerators and limiting
the use of mercury in fluorescent light bulbs and batteries, to reduce
the amount of waste mercury in landfills.
Mercury in Waste
Lindberg was also one
of the first scientists to suggest that wastes are a source of mercury
to the air. In 1975 he and former ESD geochemist Ralph Turner were involved
in a study to help pinpoint the sources of mercury to the contaminated
Holston River. In the 1970s there was considerable concern about mercury-contaminated
waterways because of the disaster at Minamata City, Japan. From 1953
through 1965, the Chisso Corporation's acetaldehyde factory discharged
methylmercury and inorganic mercury into Minamata Bay. As a result,
52 Japanese died and more than 1200 became ill from eating bay fish
contaminated with methylmercury. Awareness of this disaster was partly
responsible for the passage of the Clean Water Act of 1970.
Lindberg and Turner were
asked to study a mercury-contaminated waste storage site at a Saltville,
Virginia, chlor-alkali plant on the Holston River. The plant was permanently
closed, and the waste site was being considered for a trailer park.
The researchers measured the waste site's discharges of mercury to the
river as a result of runoff, as well as its emissions of mercury vapor
to the air.
"We were the first
to propose that there is an atmospheric route for mercury to get from
the waste to the local environment," Lindberg says. "Our measurements
showed that atmospheric emissions from the waste site were equal to
the direct runoff to the river. We also found that the total amount
of mercury lost from the passive waste storage site was higher than
that from some active chlor-alkali plants that have emission controls."
In 1977, Lindberg
and Turner published their research on chlor-alkali plant waste in Nature.
The paper concluded that waste storage sites could remain a source of
mercury to the air and the environment for hundreds of years. The Saltville
site was later remediated at a cost of millions of dollars.
Today Lindberg's
group includes several ESD scientists working on projects throughout
North America: Hong Zhang, George Southworth, Weijin Dong, Lala Chambers,
Todd Kuiken, and Mary Anna Bogle are involved in studies from Florida's
Everglades to Point Barrow, Alaska. One project involves measuring mercury
emissions from landfills.
"People dump garbage
that contains mercury, such as thermometers, batteries, electrical switches,
fluorescent light bulbs, and yard waste—mainly leaves and grass,"
Lindberg says. "Bacteria working on this waste form methane, which is
often used to generate 'green power.' Unfortunately, other bacteria
also convert the waste mercury into dimethyl mercury, a highly toxic
organic compound that is volatile and is released in landfill gas."
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For
a 1997 EPRI-sponsored intercomparison study of mercury fluxes
from natural sources, mercury emissions from geologically enriched
soils to the air were measured by various approaches near Steamboat
Springs, Nevada. Ten different groups from around the world compared
measurements using techniques originally developed by ORNL and
NOAA researchers, such as the micrometeorological modified Bowen
ratio gradient method illustrated here.
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The Oak Ridge
researchers also have been measuring emissions of airborne mercury from
natural sources, such as vegetation in the Everglades National Park;
surface waters of a forested lake site in south-central Sweden; soils
in hydrothermal areas in California and Nevada; soils at Walker Branch
Watershed on the Oak Ridge Reservation; and surfaces in the Arctic regions
of Siberia, Alaska, and Canada. Mercury from natural sources must also
be measured to predict correctly the effects of pollution controls.
A Sticky Gas
In the early 1980s, Lindberg
and others began thinking about the cycling of global mercury. They
knew that increasing amounts of mercury vapors were being discharged
to the atmosphere from burning fossil fuels, mining, manufacturing,
and incinerating waste. They knew that the elemental form of mercury
is highly volatile, that its atmospheric residence time is six months
to a year, and that it is barely soluble in water.
Lindberg was studying
air pollutants and acid rain at the time for the National Acidic Precipitation
Assessment Program (NAPAP), so he became interested in their role in
promoting the deposition of airborne mercury on the earth's surface.
This deposition was evidenced by the increasing number of reports of
mercury-contaminated fish in lakes far from industrial sources of mercury.
"We hypothesized that
ozone and other pollutant oxidants could react with elemental mercury
to form a reactive divalent mercury compound, which is much more soluble
in water and more likely to be deposited to the ground and on lakes,"
Lindberg says. Among the air pollutants Lindberg studied for NAPAP during
this period were nitrogen oxides (NOx), which are present
in automobile and coal plant emissions. He found that NOx
emissions when reacted with photo-oxidants resulted in the formation
of nitric acid vapor in the atmosphere. "We called it a 'sticky' gas,"
he says, "because nitric acid was rapidly deposited from the air to
vegetation and soil. It 'stuck' to everything."
In 1993 Lindberg
proved that airborne mercury also consists partly of a "sticky" gas.
He and Wilmer J. Stratton, retired professor of chemistry at Earlham
College in Richmond, Indiana, who was conducting research at ORNL at
the time, were the first to measure reactive gaseous mercury (RGM) in
ambient air. To identify and measure RGM, they developed a method that
takes advantage of a "high-flow refluxing mist chamber" previously used
in NASA-sponsored, gas-chemistry studies in the Amazon River valley.
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ESD
researchers worked with visiting professor Wil Stratton to develop
a mist chamber approach to make the first-ever measurements of
reactive gaseous mercury in ambient air. A vacuum pump draws air
through the mist chamber from an inlet at the bottom causing a
mist to be sprayed into the chamber by aspiration. As the air
passes through to the top, the highly soluble reactive mercury
in the air is dissolved in the mist. In the field laboratory,
the mercury is then reduced to elemental mercury with tin chloride
(which adds the two missing electrons). The mercury is stripped
from the water droplets by purging with zero gas onto a gold trap,
followed by atomic fluorescence spectroscopic analysis.
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"Because RGM is
so soluble in water," Lindberg says, "it can be deposited quickly in
rain and snow, thus making it a possible source of mercury to lakes
far from mercury-discharging industrial plants. During dry weather,
RGM would also be rapidly dry deposited to vegetation where it may be
washed into soils and nearby streams."
In 1997 Lindberg and Stratton
published an article in Environmental Science and Technology
about their detection of a sticky RGM compound of either mercuric oxide
or mercuric chloride. According to their measurements, 2 to 4% of total
gaseous mercury in air is the highly water-soluble species of RGM, whereas
about 97% is elemental mercury vapor. They discovered that this small
fraction of airborne mercury strongly influences the deposition of mercury
to the earth's surface.
How does RGM get
into the air? It is formed in flue gas when coal or municipal waste
is burned. However, Lindberg suspects that it can also be formed in
air by chemical reactions with elemental mercury vapor. When inert elemental
mercury (Hg°), which has no electrical charge, is oxidized in air,
it loses two electrons, forming RGM (Hg+2). Lindberg believes
that in the troposphere Hg° is oxidized by reactive halogens, such
as bromine, in the presence of ultraviolet light.
In a 1999 paper
in Nature, Canadian scientist Bill Schroeder observed the disappearance
of airborne elemental mercury during his gold-trap measurements of atmospheric
mercury at Alert in the Northwest Territories near the North Pole. Most
of the year, the mercury levels averaged 1.5 nanograms/m3,
but after the polar sunrise and before snowmelt, elemental mercury levels
dropped to levels of 0.2 ng/m3.
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Steve
Brooks of NOAA takes a break from mercury measurements at Point
Barrow, Alaska, to enjoy the first sunlight in two months.
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After reading
Schroeder's paper, Lindberg's group proposed a companion study at Point
Barrow, Alaska, that involved Steve Brooks and others from the Oak Ridge
NOAA laboratory. Since 1998, portable Tekran-automated, mercury-speciation
units have been used to measure simultaneously near-real-time concentrations
of RGM and elemental mercury. Lindberg and Brooks found that when elemental
mercury levels fell to 0.5 ng/m3, their measurements of RGM
rose from 0 to 0.9 ng/m3. According to Lindberg, "The evidence
suggests that airborne elemental mercury is depleted when conditions
are right for converting it chemically to RGM, which is then deposited
to the Arctic snow." New analyses by Mary Anna Bogle and George Southworth
of snow collected from January through May 2000 confirm for the first
time that mercury is accumulating in Arctic snow at record levels.
Mercury Manipulation
Study
Lindberg's group is now
focused on a multi-million-dollar mercury manipulation study being planned
for the Experimental Lakes Area (ELA) in Northwest Ontario. It is called
the Mercury Experiment to Assess Atmospheric Loading in Canada and the
U.S. (METAALICUS). DOE is supporting the Oak Ridge group in this international
collaboration between the United States and Canada, which is designed
to answer this central question: "Are atmospheric emissions of mercury
largely responsible for the methylmercury contamination of fish in lakes
far from industrial sources of mercury?" The ELA has hundreds of remote
lakes that can be used safely for environmental experiments; in fact,
it was the site of pioneering lake acidification studies in the 1980s.
"We will use aircraft
to spray a different stable mercury isotope on a forest, a nearby lake,
and a nearby wetland," Lindberg says. "Using inductively coupled plasma
mass spectrometry, a University of Toronto lab will analyze the degree
to which mercury in the fish comes from the air, the lake, and runoff
from the forest and wetland, based on isotopic ratios."
The results of
the study will be reported at the October 2001 International Conference
on Mercury, which will be co-chaired by Lindberg. It will be held in
Minamata City, Japan. Interestingly, one source of the mercury isotopes
for the study is the stockpile of stable mercury at ORNL. Oak Ridge
mercury is being sent beyond Oak Ridge to help solve a global problem.
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