Environmental Engineering Reference
In-Depth Information
Sources of Mercury in Water
fi sh, which were a staple of the local diet. In the following
years, more than 100 people died and more than 1000 were
permanently disabled from the resulting methylmercury
poisoning, which consequently bears the name, Minimata
Disease (Clarkson, 1997).
It is also important to note that, by defi nition, the
increased deposition (of RGM and particulate-bound Hg)
to oceans and lakes throughout the globe as compared with
preindustrial times is ultimately attributable to anthropo-
genic activities. Though the input occurs after transport
through the atmosphere, it is nonetheless of anthropogenic
origin.
Atmospheric Wet and Dry Deposition
The largest and most important source of mercury in water is
wet and dry deposition from the atmosphere. This includes
both a natural and anthropogenic component. Globally,
about 1% of the total oceanic burden is deposited and emit-
ted each year (Mason and Sheu, 2002; Sunderland and
Mason, 2007). Most of the mercury deposited to oceans in
precipitation is either bound to particles or is dissolved in an
ionic state, Hg(II). Over the oceans, the overwhelming por-
tion that is dry deposited is RGM, which has been produced
near the water surface from photochemically driven oxida-
tion by halogens (Laurier et al., 2003; Sprovieri et al., 2003;
Laurier and Mason, 2007; Holmes et al., 2009). A portion
(~10%) of the RGM that is deposited to the ocean is reduced
to elemental mercury in the surface waters, either directly by
sunlight, or through biologic activity (Fitzgerald et al., 2007;
Strode et al., 2007). This tends to make the surface waters
supersaturated with respect to dissolved elemental mercury
(Schroeder and Munthe, 1998). Therefore, (gaseous) elemen-
tal mercury is generally evading from the ocean surface.
Lakes and wetlands are more variable in their interaction
with GEM. Some studies have reported supersaturations in
surface waters with net evasion (Poissant et al., 2000, 2004),
and others have observed slow net deposition or near equi-
librium with the air (Zhang and Lindberg, 2000) or diurnal
cycles (Marsik et al., 2005).
Mercury that is bound to particles and bound to soluble
organic complexes can also be incorporated into the hydro-
logic cycle as a part of runoff after rain or fl ooding events
and through the movement of subsurface pore water.
Subsurface geothermal and hydrothermal vents are also
sources of mercury in the ocean, although the magnitude
of these inputs is believed to be negligible as compared
with the total oceanic burden. Subsurface vents may, none-
theless, be important in enhancing the concentrations in
ambient waters and sediments near the vent site (Stoffers
et al., 1999; King et al., 2006; Lamborg et al., 2006). Some
mercury is also thought to enter (or perhaps reenter) the
hydrologic cycle from diagenetic reactions, which are phys-
ical, chemical, or biologic changes that occur as sediment
(settled particulate matter that contains mercury) is depos-
ited, compressed, and transformed to rock.
Mining Runoff
An additional important source of mercury to aquatic
systems is runoff or leaching resulting from mining
activities. Large-scale mercury mines often produce large
quantities of tailings or leave mining passages open
after operations cease (e.g., Sulfur Bank Mercury Mine,
California [Engle et al., 2007]; Almadén, Spain [Gray et al.,
2004]; Idrija, Slovenia [Hines et al., 2006]). Early gold and
silver mining also produced large quantities of Hg-enriched
tailings because Hg was added to crushed ore in order to
amalgamate and extract the gold and silver (e.g., Bonzongo
et al., 1996). The wastes, along with the open mine passages,
allow rain and groundwater to leach and mobilize mercury.
Even smaller-scale mining activities, in particular artisanal
mining practiced in China, Indonesia, South America, and
Africa (reviewed by Veiga et al., 2006) produce signifi cant
amounts of waste matter that is enriched in mercury. This
is often dumped into streams or lakes, or otherwise allowed
to leach in an uncontrolled manner.
Methylated Species
Perhaps the most crucial process in the global cycling of
mercury, at least from the standpoint of toxicity, is the con-
centration and accumulation of MMHg or DMHg up the
food chain (called “bio-accumulation”). Most of the DMHg
and MMHg that is bio-accumulated is produced in situ in
natural waters or near the sediment-water interface. The
production of MMHg and DMHg from dissolved Hg(II)
(methylation) occurs primarily in sulfate-reducing bacteria
in anoxic environments and has been hypothesized to be
a cellular detoxifi cation mechanism. A limited number of
other methylation mechanisms have been proposed, but
bacteria appear to be the largest producers in lakes and
wetlands (Rudd, 1995; Fitzgerald et al., 2007).
Little DMHg and MMHg is found in the surface waters
of the open ocean. Higher concentrations of DMHg can be
found below the mixed layer of the open ocean and periph-
eral seas, while MMHg has been unambiguously detected
only in coastal zones, peripheral seas, and the Equatorial
Pacifi c Ocean (Fitzgerald et al., 2007).
Industrial Point Sources
Industrial waste is an important source of Hg to some
watersheds. The most prominent example of anthropo-
genic input of mercury to aquatic system occurred in Mini-
mata, Japan, in the early 1950s. Throughout this period, the
Chisso Corporation dumped more than 20 tons of mercury
that had been used as a catalyst in the production of acet-
aldehyde, into Minamata Bay. The mercury contaminated
the sediments, water, and ecosystem and ultimately the
