Environmental Engineering Reference
In-Depth Information
is deposited in estuarine regions and oceanic shelves and
does not reach the open ocean. Also, because wetlands can
be strong producers of MMHg, the outfl ow from wetlands
can be signifi cant sources to downstream water bodies
(St. Louis et al., 1996). One study of fate and transport
in a boreal wetland found that over the course of several
months, signifi cant fractions of a newly deposited isotope
(
202
Hg) were converted to MMHg and were transported
below the water table and toward a neighboring lake via
groundwater (Branfi reun et al., 2005).
estimate that coastal benthic MMHg production rates and
diffusional effl ux are suffi cient that they are likely a major
source to nearby ecosystems and potentially the open ocean.
Atmospheric Transport
DISPERSION OF POINT AND AREA SOURCE PLUMES
The majority of direct anthropogenic mercury emissions
are from point sources such as coal-fi red power plants,
municipal waste incinerators, metal refi neries, and chlor-
alkali plants. The dispersion of these types of plumes has
been widely studied, and a range of tools exist to describe
the fate of chemicals in those plumes (e.g., Mossio et al.,
2001). GEM emitted from these sources is largely unreac-
tive and has been suggested to be useful tracer of sources
(Friedli et al., 2004; Jaffe et al., 2005). RGM and PHg have
much different fates. RGM is rapidly deposited to particles
and surfaces, and can be sequestered by cloud and rain
drops (Schroeder and Munthe, 1998). Particulate mercury
can also settle out of the atmosphere or become incorpo-
rated into rain and cloud drops. Depending on the ambient
conditions and the chemistry of the emitted plume, some
of the RGM may be reduced to GEM (Lohman et al., 2006).
Thus, if an airmass remains in contact with the surface,
a large fraction of the RGM and PHg will be lost to the
surface within hours to days of emission.
Ocean Settling and Transport
The world's oceans play an important role in transporting
and redistributing heat throughout the globe, but they do
not play as prominent a role for mercury. This is primarily
due to the much shorter intrahemispheric mixing time of
the atmosphere (~10-20 days [Jacob, 1999]) as compared to
the oceans (10s to 1000s of years [Sunderland and Mason,
2007]). Also, the short lifetime of Hg in the surface oceans
against reemission (~0.6 year [Selin et al., 2008]) means
that the surface ocean and atmosphere are in steady
state on an annual time scale. Thus, the atmosphere will
act to damp local perturbations from the surface ocean.
Noteworthy oceanic transport processes include up/down-
welling, interhemispheric transport, particle settling, and
transport of MMHg from coastal sediments.
Mason et al. (1994) estimated that upwelling of thermo-
cline waters in the equatorial Pacifi c is similar to the net
atmospheric input to the surface, and in some places it may
be greater. This, along with observations of elevated mercury
in subsurface waters (Mason and Fitzgerald, 1993; Mason and
Sullivan, 1999; Horvat et al., 2003) suggests that mercury-
enriched water masses may sink from the surface and be
transported as a record of historical deposition. Nonetheless,
because of the relatively slow exchange rate of the world's
oceans, the fl uxes in and out of intermediate and deep reser-
voirs are a very small fraction of their overall burden.
Oceanic transport also likely contributes to interhemi-
spheric transport. The lifetime of mercury in deep ocean
compartments is much longer, 10s to 1000s of years
(Sunderland and Mason, 2007) than surface reservoirs and
would allow for interhemispheric transport. The impact of
this mercury on atmospheric concentrations and deposi-
tion to land, however, would depend on the deep waters
reaching the surface. The settling of particulate mercury,
from the surface to deep waters and the ocean fl oor, is an
important component in the oceanic cycle, particularly
in the North Atlantic. In the surface waters, carbon-rich
biomass or waste matter produced by phytoplankton or
zooplankton sequesters mercury and falls to the interme-
diate and deep ocean. It is believed that about 50% of the
sinking mercury is buried on the ocean fl oor and is lost to
deep reservoirs (Sunderland and Mason, 2007).
The transport of MMHg from coastal sediments is also an
important form of oceanic transport. Fitzgerald et al. (2007)
Continental Export and Long-Range Transport
Large, polluted airmasses can be exported from their source
region, and then transported and dispersed over thousands
of kilometers in the jet stream through their interaction
with a midlatitude cyclone. Because the lifetime of GEM in
the atmosphere is about 1 year (Selin et al., 2007), in some
cases plumes have been obser ved for 7-10 days or more af ter
emission (Jaffe et al., 2005; Slemr et al., 2006; Ebinghaus
et al., 2007; Weiss-Penzias et al., 2007; Swartzendruber
et al., 2008). In these studies, the mercury source region was
identifi ed through a combination of trajectory and synop-
tic analysis, and the calculation of enhancement ratios of
copollutants. The enhancement ratios refl ect the emission
ratio (under certain assumptions) and, because of a signifi -
cant difference in emission ratios between the major emit-
ters (Jaffe et al., 2005; Weiss-Penzias et al., 2007), can sug-
gest a source type. The suggested source type and backward
air-parcel trajectories can then be compared with emissions
maps. This approach has been successful in identifying
intercontinental transport of Hg from sources in East Asia,
Europe, and Africa. Modeling studies have confi rmed the
long-range transport of Hg seen in observations (e.g., Selin
et al., 2007; Jaffe and Strode, 2008; Strode et al., 2008).
Global Transport
Once polluted airmasses are lofted into the jet stream, they can
circle the northern hemisphere in as little as 7-10 days and will
