Environmental Engineering Reference
In-Depth Information
it proved difficult to detect signs of impact and the cadmium source was believed to be re-
lated to local geology. A similar situation was found with the Finlayson caribou herd in the
Yukon. This raised concern for the indigenous people who harvest this herd, particularly
because their cadmium levels are already high due to cigarette smoking (the normal source
of most excessive human exposure to cadmium).
Mercury is the heavy metal of most concern in the circumpolar Arctic. Throughout
geological time, it has been naturally released to air and water via the erosion and weath-
ering of minerals, such as cinnabar, and volcanic processes. From these reservoirs, it has
recycled through other environmental compartments back into sedimentary rocks. Human-
kind has for many centuries added to this input before the industrial era, notably associated
with use of mercury in gold and silver mining. However, it was not until the mid-nine-
teenth century that anthropogenic emissions from sources, such as the burning of fossil
fuels (especially coal), metal smelting, waste disposal and waste incineration, led to signi-
ficant widespread increases of mercury to the environment. This occurred when the com-
bined natural and anthropogenic inputs exceeded the capacity of the natural cycle to move
mercury back into rock, leading to a growing “pool” of mercury in air, water, biota and un-
consolidated sediment.
The main form of mercury entering the air is gaseous elemental mercury (Hg
0
) that
has an atmospheric residence time of about one to two years - more than enough to ensure
rapid long-range transport around the globe. Elemental mercury is generally unavailable to
biota. In the atmosphere, it can be transformed into forms of ionic mercury (such as mer-
curic chloride) that may also occur as a primary emission. Within 100-1,000 kilometres
from a source, ionic mercury is usually deposited onto the surface, where it is strongly
bound to sediments and organic matter. This deposited mercury can then be reemitted into
the atmosphere as secondary emissions, which therefore include mercury that originated
from natural and anthropogenic sources. There have been many attempts to apportion the
relative significance of sources to the environment, but the one I summarize here is based
onthe2013UNEP
Global Mercury Assessment
.Thisestimates thatabout30%comesfrom
annual anthropogenic emissions of mercury to air, another 10% comes from natural geolo-
Search WWH ::
Custom Search